i^rOC ^ * fss~ /iffteoAZ ° f entry* /fY ■ygrr MBL/W HOI 0301 00M7fl. y.. 1 -.L-'i-# LONDON: SWAN SONNENSCHEIN AND CO., Ltd. NEW YORK: THE MACMILLAN CO. 1899. PREFACE. The present and third instalment of the translation of the Lehrbuck der Dergleichenden Entwicklungsge&chichte der wirbel- losen TMere contains the Arachnida and appended groups, the Onychophora, the Myriopoda, and the Insecta, thus completing the Arthropodan portion of this work. The remaining volume, which contains the Mollusca, Ascidia, and Cephalochorda, will, I hope, be published at the end of the year. In connection with the present volume, I have to thank Mr. II. I. Pocock for his valuable assistance in the Arachnidan part, and also I have to thank Mr. A. I). Michael, who kindly read through the chapter on the Acarini and corrected many errors in the same. Most of the suggestions made by these gentlemen have been added as editorial footnotes, but some matter, when the original was obviously at fault, has been placed in the text. The very important work by Brauer on the Ontogeny of the Scorpion renders some of the text relating to this genus out of date, and this work should certainly be consulted by those studying the Arachnida. His discovery of an additional segment between the thorax and abdomen is of especial importance in the interpretation of the Arachnidan body. In this connection it is interesting to note that this segment was correctly figured, though misinterpreted, by Metschnikoff in 1871, as a careful comparison of the figures of these two authors will show. Mil PREFACE. Iii the Onychophora, Willey's paper on Peripatus novae - hritanniae is of great importance, especially in connection with the invagination germ -band in the Insecta and the interpretation of the embryonic membranes. Among the numerous additions to the literature on the In- secta, Heymon's works, especially that on Lepisma, are worthy of careful study. Unfortunately, the interpretation of the ontogenetic processes in the Insecta is very difficult, and in consequence we still find a terrible confusion enshrouding the origin of some organs, especially that of the alimentary canal, which a number of recent authors maintain to be entirely ectodermal, a condition which, judging from what occurs in other Arthropods, seems extremely improbable. The germ-cells, as in the two previous volumes, are still treated of as mesodermal, whereas, as has been pointed out in the editorial notes to Vols. i. and ii., these cells are probably handed down from parent to offspring as distinct and con- tinuous structures, their identity being temporarily merged in the egg. In the present volume I have added more notes and literature and made more alterations in the text than in Vol. ii., and I hope that such alterations will tend to bring this volume more up to date. MAETIN F. WOODWARD. Royal College of Science, London. July, 1899. CONTENTS OF VOL. III. The lung-sacs The intestinal canal . The mesodermal derivatives Blood-vascular system and coelom The coxal glands The genital organs . Chapteb XXI. ARACHNIDA. By E. Korschelt I. Scorpiones .... 1. Cleavage aud formation of the germ-layer 2. The origin of the embryonic membranes and the development of the external form of the body . 3. The formation of the organs A. The nervous system and the eyes B. C. D. E. F. G. II. Pedipalpi III. Pseudoscorpiones .... IV. Opiliones V. Solifugae VI. Araneae ..... Oviposition and constitution of the egg 1. Cleavage and formation of germ-layers 2. Development of the external form of the body 3. The development of the organs A. The nervous system . B. The eyes .... C. Survey of the Arachnidan eyes D. The respiratory organs E. The spinning glands and the poison glands F. The intestinal canal and its appendages . The mesodermal structures . G. The blood-vascular system and the body-cavity H, The coxal glands /. The genital organs VII. Acarina ..... Oviposition ..... 1. Embryonic development 2. The formation of the larval integuments and the further course of development VIII. General considerations regarding the Arachlrida Literature .... PAGE 1 1 1 4 12 12 19 19 22 23 24 25 26 27 32 34 37 37 38 45 59 59 63 68 76 79 81 85 87 92 93 93 93 94 97 110 118 3d^f, X CONTENTS. By E. Kohschelt Chapter XXII. PENTASTOMIDAE. 1. Embryonic development 2. The larval development 3. General considerations Literature ..... Chapter XXIII. PANTOPODA. By E. Korschelt Oviposition and care of the brood 1. Cleavage and formation of the germ-layers 2. The further development of the embryo 3. The form of the larva and its transformation into the 4. General coi>siderations . Literature ..... Chapter XXIV. Literature TARDIGRADA. By E. Korschelt adult Chapter XXV. ONYCHOPHORA (Pekipatus). By E. Korschelt Structure of the egg .... 1. Cleavage and formation of the germ-layers Peripatus novae-zealandiae Peripatus capensis The American species 2. The development of the external form of the body 3. The formation of the organs The ectodermal structures The integument The nervous system and the ventral organs The eyes .... The slime- and the crural glands The alimentary canal The mesodermal structures The body-cavity and the blood-vascular system The musculature The nephridia The salivary glands The anal glands The genital organs Another account of the origin of the mesodermal structures General considerations Literature ..... Chapter XXVI. MYRIOPODA. By E. Korschelt Oviposition and the constitution of the egg 1. Cleavage and formation of the germ layers 2. The development of the external form of the body A. Chilopoda .... E. Diplopoda .... The first rudiment of the embryo Flexure of the germ-band The further development of the embryo The interpretation of the mouth-parts of the Myr Post-embryonic development . C. Symphyla and Pauropoda opod PAGE 129 129 130 136 137 139 139 139 144 148 156 160 162 163 164 164 165 166 169 170 175 188 188 188 189 194 195 195 198 201 204 204 206 207 208 209 211 216 218 218 220 223 223 229 229 229 231 232 236 238 CONTENTS. XI Chapter XXVI. MYRIOPODA— conlirvu d. page 3. The formation of the organs . ... 239 The nervous system . . ... 239 The eyes . . . ... 241 The tracheae . . . ... 243 The protective glands . . ... 243 The alimentary canal . . ... 244 The enteron . . . ... 244 The stomodaeum and the proctodaeum . . . 246 The mesodermal structures . ... 246 The body-cavity, the blood- vascular system, the fat-body, and the musculature . . ... 246 The salivary glands . . ... 251 The genital organs . . ... 252 General considerations . .' ... 253 Literature . . . . ... 257 Chapter XXVII. IXSECTA. By K. Heider . . . 260 I. Embryonic development . . ... 260 1. Oviposition and the structure of the ripe egg . . . 260 2. Cleavage and the formation of the blastoderm . . . 263 3. The formation of the embryonic rudiment and the embryonic integuments . . ... 268 A. General view of the germ-band and the germ-envelopes 268 B. The distinction between the superficial and the im- mersed germ-band . . ... 272 C. The distinction between the invaginated germ-band and the germ-band that has been overgrown by the membranes . . ... 274 D. Insects with invaginated germ-band . . . 276 E. Insects in which the germ-band is overgrown by the amniotic fold . . ... 282 F. Transition forms between the two types of development of the germ -band . . . 287 G. General considerations . ... 289 4 Development of the external form of the body . . . 290 A. Segmentation . . ... 290 B. Stomodaeum and proctodaeum. Labrum . . 293 C. Extremities . . ... 295 I). Xervous system and tracheal invaginations . . 300 E. Transition to the definitive form of body . . 302 5. Completion of the dorsal part of the embryo and degenera- tion of the embryonic envelopes . . . 302 A. Involution through the development of a continuous dorsal amnion-serosa sac . ... 304 B. Involution accompanied by dorsal withdrawal of the amnion only . . ... 307 C. Involution accompanied by dorsal withdrawal of the serosa and complete separation of the amnion . . 307 D. Involution accompanied by the amputation of both embryonic envelopes E. General considerations Xll CONTENTS. Chapter XXVII. INSECTA— continued. 6. The formation of the germ-layers . ... 7. Further development of the mesoderm Development of the body-cavity . ... 8. The formation of organs . . ... A. Outer integument . . ... B. Endo-skeleton . . ... C. The nervous system . • . . . . D. The sensory organs . . ... The ocelli . . ... E. The tracheal system . . ... F. The alimentary canal and intestinal glands G. Heart . . . ... H. The musculature, the connective tissue, and the fat-body /. Genital organs . . ... II. Metamorphosis . . . ... 1. The larval forms . . . ... A. Homomorpha . . ... B. Heteromorpha . . ... 2. Development of the imago . . ... A. Development of the external form of the body . B. Development of the internal organs of the imago Hypodermis . . ... Musculature . . ... Intestinal canal . . ... The tracheal system . ... The nervous system . ... The fat-body . . ... The ultimate fate of the phagocytes General considerations regarding the development of the imago in the pupa . ... III. Parthenogenesis, Paedogenesis, Heterogeny . ... IV. General considerations . . . ... Literature . . . . ... PAGE 309 318 318 322 322 323 323 329 329 334 336 338 341 342 355 355 356 359 367 370 378 379 381 382 385 386 386 386 387 388 390 396 Chapter XXVIII. GEXERAL CONSIDERATIONS ARTHROPODA Formation of the germ-layers . Appearance and further development of the organs Nervous system Eyes .... Respiratory organs r . Fore- and hind-guts . Development of the enteron Development of the mesoderm Germ-band Embryonic envelopes, metamorphosis Interpretation and relationships of the Arthropoda Literature .... ON THE Subjects Index Authors Index 411 412 413 413 414 420 421 422 423 424 425 426 431 433 437 CHAPTEE XXL ARACHNIDA. Systematic : — I. SCORPIONES. II. Pedipalpi. III. Palpigradi (Koenenia). IV. PsEUDOSCORPIONES. V. Opiliones. YI. SOLIFUGAE. VII. Araxeae. VIII. Acarina. I. Scorpiones. The Scorpiones are viviparous. The oval or spherical eggs, which are rich in yolk and are each surrounded by a thin membrane, lie in follicles that arise as outgrowths of the walls of the ovarian tubes. Fertilisation takes place either in the ovarian follicles (Euscorpivs and Scorpio, aIetschnikoff, Laurie), or when the egg has left the follicle and passed into the ovarian tube (Androdonus, Kowalevsky and Schulgin). In the former case the embryo remains in the follicle during the greater part of its development (Scorpio, Joh. Muller), or leaves it when the formation of the germ-band commences (Euscorpius italicus). Further development then takes place in the ovarian tubes or oviducts, which thus function as uteri. At birth the young resemble the adult in their general organisation. 1. Cleavage and Formation of the Germ-layer. The cleavage of the egg in Scorpions is discoidal. At the pole of the egg, which is directed from the follicle towards the ovarian tube, in the youngest stage as yet observed, there were found a number of cells which formed a small unilaminar cap on the yolk ; this is the germ-disc (Fig. 1). The blastoderm spreads gradually from this point, advancing very slowly over the yolk (Fig. 2 A and B). Long B ARACHNIDA, before it has grown round the latter, however, the rudiment of the germ-band has appeared, and the first differentiation of the latter takes place at the point where the blastoderm first began to form. A cleavage of the yolk, such as is met with in the eggs of the Araneae, does not occur in the Scorpiones. The discoidal cleavage of the Scorpiones might be compared with the Crusta- cean method of cleavage distinguished as Type IV., and might, like the latter, be traced back to superficial cleavage (Vol. ii., pp. 117 and 118). This would be the more permissible as superficial cleavage is, as a rule, widespread among the Arachnida also. In this respect the Scorpiones, as compared with the Araneae, must be considered as showing a modified condition, although they are in other respects more primitive. The development of the embryo within the body of the mother is a sufficient proof that modification in the primitive method of development has taken place. The Formation of the Germ-layers. The germ-disc does not long retain the character of a single layer of cells. A thickening appears at its centre, which, on the sur- face turned towards the yolk, appears as a swelling. This, according to Kowalevsky and Schulgix, has arisen by a down- sinking of the cells. If we bear in mind, in addition to this, the longitudinal furrow described by Metschxikoff on the surface of the now oval germ-disc (Fig. 4 A, p. 6), we are reminded of the long slit -like blastopore that occurs in Peripatus and in the Insecta, and which constitutes the longitudinal germinal groove. In any case, the differentiation of the inner and middle germ-layers starts from this point. Fig. 1. — Egg of Euscorpius italicus showing the germ-disc (after Metschnikoff, from Balfour's Text-book). Laurie (No. 23), in his recent work, does not actually deny the " down- sinking " of the cells and the presence of the longitudinal furrow, but being unable to convince himself, seems inclined to doubt their existence. This, indeed, cannot be considered as established, especially as the descriptions given of these processes are not very exact. Laurie derives the ento-mesoderm by delamination from the cell-mass of the germ-disc, in which no special regularity of structure is apparent. But he also finds a thickened point at the posterior end of the germ-disc, in which rapid increase of cells takes place (formation of the ento- mesoderm), and which could therefore be compared with an invagination (Fig. 3, CLEAVAGE AND FORMATION OF THE GERM-LAYER. 3 4,OPo2 A, e). The caudal prominence described by Metschnikoff (No. 24) is probably to be identified with this growing point ; the former projects into the yolk, and at a later period shifts into the caudal region of the embryo. Laukie compares the thickened part with the primitive streak in vertebrates, and we are again involuntarily reminded of the conditions found in Peripatw. In the latter the "point of ingrowth" lies at the posterior end of the long blastopore.* When the germ-disc, by the active increase of its elements, lias attained a thickness of several cells, these still appear but slightly differentiated into layers. The inner rf( surface of the germ- disc is now quite irregular, for single ■cells become de- tached from it, and shift into the yolk .C cj$. birth. This latter point was confirmed by Kowalevsky and Schulgin, who observed the gland both in its earlier slightly coiled stage and in its later more compact condition. [See Brauer, No. II.] The structure and position of the coxal glands in the youngest known stage render it highly probable that they are formed from the somatic mesoderm. They are assumed to be nephridia, a view which seems very probable. Considering the primitive character of the coclom in the /fi Scorpiones, we should , expect the nephridia to open into the body- cavity through funnels, and this is actually the case for a time. The further development of the inner terminations of the gland must de- pend essentially on the modifications undergone by the body-cavity, but this point is somewhat obscure. More thorough ontogenetic researches are required before it can be stated with certainty whether, as in Pcripatus and the Crustacea, a part of the body-cavity forms a capsule for the forma- tion of the terminal sac of the gland, or whether the mouth of the funnel is retained for a consider- able time in a wide secondary body - cavity. The most recent writer on this subject, Stxjiiany (Xo. 14) was not able to prove that the coxal glands in the Arachnida opened into the body- cavity, and he inclines to believe in the presence of a closed terminal sac, such as is found in the Crustacea, but here also we must demand actual proofs. G. The Genital Organs. The ontogeny of the genital organs has as yet been little investi- gated. They were first observed by Laurie at a late stage of development shortly before birth, in the first abdominal segment [second, Brauer], as tubular structures at first unconnected with Fig. 14. — Euseorpius italicits. Portions of sections through a newly-hatched Scorpion (A) and an advanced embryo (B) to show the coxal gland and the formation of the genital organs (after Laurie), a, efferent duct of the coxal gland ; ec, ectoderm ; g, efferent duct of the genital organ ; (/.op, genital operculum ; Ih, body-cavity ; in, external opening of the coxal gland ; mes, mesoderm ; n, ventral nerve-cord ; J'.-ii Pti bases of the third and fourth limbs ; so, somatic, sp, splanchnic layer of the mesoderm. 26 ARACHXIDA. the exterior (Fig. 14 B). Kowalevsky and Schulgix, who also noticed them, referred them, though with some hesitation, to the splanchnic layer of the mesoderm. Laurie's account would rather tend to show that they arise from the somatic layer, as do the coxal glands of the Scorpiones and the nephridia of the Annelida (Vol. i., Fig. 137, p. 297). The nephridial character of the efferent genital ducts seems to be confirmed by the fact that they open into the body- cavity in the form of a wide funnel (Kowalevsky and Schulgix [Brauer]). Laurie also believes that at least in part they are nephridial in origin. The ends of the canals which are directed outwards long remain closed, a fact which we do not regard as disproving the nephridial character of the efferent ducts, since even the Annelidan nephridia develop in a similar way. From Laurie's description we might imagine that the mesodermal efferent ducts become directly connected with the ectoderm at the points where the remains of the first pair of abdominal limbs lie in the form of ectodermal thickenings (Fig. 14 B, g.op), as is the case, according to Bergh, with the nephridia of the Annelida. Kowalevsky and Schulgix, however, speak of an ectodermal invagination, towards which the mesodermal efferent duct grows, so as to unite with it. This invagination, as far as can be made out from their short account, is small, and it appears very possible that such an ectodermal invagination might arise at the thickening which indicates the position of the abdominal limbs. An ectodermal termination has also repeatedly been assumed for the nephridia and the genital efferent ducts of the Annelida. It is, however, highly probable that the short unpaired portion is derived from a depression of the ectoderm. In the Pedipalpi this unpaired segment is much larger, and becomes a large cavity (No. 31). The genital glands arise, according to Kowalevsky and Schulgix, as cell- thickenings "apposed to the inner tube." This can only be understood to mean that a part of the peritoneum {i.e., of the secondary body-cavity) is concerned in the formation of the genital organs ; on this point, however, as well as on the differentiation of the mesodermal structures, we await further particulars.* II. Pedipalpi. f According to Bruce, who has made a few statements as to the ontogeny of Phryuas, the embryo here, as in the Scorpiones, lias an embryonic envelope. We may indeed make the general assumption that the course of development in the Pedipalpi resembles that in the Scorpiones. Bruce points out as specially remarkable the existence of a sensory organ at the base of the second ambulatory limb, consisting of columnar cells prolonged externally into filaments. The Pedipalpi are very closely related to the Scorpiones, and, like the latter, show in their organisation many points of agreement with * [See footnote, p. 3. — Ed.] t [The Pedipalpi are oviparous ; the eggs are carried in a gelatinous sac attached to the ventral surface of the mother. For chief ontogenetic features see App. Lit. Pedipalpi, Nos. I.— III., noting presence of reversion of germ-bands and unanimous conclusion that Pedipalpi are more nearly related to the Araneae than to the Scorpiones. — Ed.] PSEUDOSCORPIONES. 27 Limulus (Ray-Lankester, Bruce). Our knowledge of the ontogeny of the Pedipalpi is unfortunately very incomplete, and this may also be said of Koenenia mirabilis, a form discovered by Grassi (under stones in the plains of Catania), which shows great resemblance to the Pedipalpi, but has been placed by him in a separate order, the Microtelyphonidae, the Palpigradi of Thorell.* This form is said to have no special respiratory organs, and Grassi therefore sees in it a transitionary form between the Gigantostraca and the Arachnida, which has " already lost the gills, but has not yet developed respira- tory organs suited to a terrestial existence " ! We can hardly imagine A Fig. 15.— Embryos of Chelifer in their envelopes (after Metschnikoff, from Balfour's Text- book). A, early cleavage stage. B, stage in which the blastoderm (6?) has separated from the yolk-masses within. C, splitting of the blastoderm into two layers. The yolk-masses are seen within the egg. A cell-like albuminous tissue appears between the blastoderm such a transition, and would rather regard the absence of respiratory organs, if it actually occurs, as a degeneration, such as is met with in other air-inhabiting Arthropoda in cases where the body is dis- tinguished from related forms by its specially small size (e.g., in a few Mites, among the Arachnida, and in Pauropus among the Myriopoda). III. Pseudoscorpiones. The little that is as yet known of the ontogeny of the Pseudoscorpiones does not seem sufficiently well established to enable us to form a decisive judgment with regard to the extraordinary development of these forms. Metschnikoff's * [Hansen and Soiiensen (App. Lit. on Palpigradi, No. I.) give a very careful account of Koenenia, and correct many errors in Grassi's description. — Ed.] 28 ARACHXIDA. statements as to the development of Chdifcr np to the time of the formation of the blastoderm are, indeed, confirmed by Stecker with regard to Chthonius, but the description of the latter author is not calculated to inspire confidence. A more recent treatise by J. Barrois* on the ontogeny of Chelifcr is too short to supply many further details. The eggs of Chelifer and of Chthonius are spherical and crowded with yolk-spherules. Each is surrounded by a vitelline membrane, and again by a second envelope probably secreted by the oviduct. These eggs are carried by the mother on the ventral surface of the abdomen, where they pass through their development. The cleavage is at first complete, the egg dividing up into two, four, and eight equal blastomeres (Fig. 15 A). In the latter stage, i.e., when the egg is divided up into eight spheres, clear protoplasmic segments are said to appear on the surface of the yolk-laden spheres. The number of these clear cells soon greatly increases, until they form a layer surrounding a central mass of yolk (Fig. 15 B); this layer may be regarded as the blastoderm. The large yolk-segments with their nuclei can still be clearly seen within the egg.f The whole process must, no doubt, be thus explained : The few nuclei which enabled the yolk to break up into segments, by division, send off nuclei to the periphery, the nuclei which remain within corresponding to the yolk-nuclei of other Arthropod eggs. In the fact that the yolk itself remains segmented these forms are peculiar. As the segmentation of the yolk gradually disappears, the blasto- derm divides into an outer and an inner layer of cells (Metschnikoff, Fig. 15 C). About this time, large clear bodies appear between the blastoderm and the egg - integument ; these contain structures resembling nuclei, and therefore resemble cells (Fig. 15 C). Metschnikoff was reminded by them of an embryonic envelope, but could not convince himself that such a covering was actually present, and regarded these structures as disintegrated masses of albumen, a view also taken by Stecker. These cells recall those found beneath the cuticular envelopes in the Mites (Claparede's haemamoebae, Fig. 53, p. 99). * We have not heard of any more detailed work on this subject by Barrois ; Stecker's preliminary notice also seems not to have been followed by any larger treatise. [See Barrois (App. to Lit. on Pseudoscorpiones, Xo. I.).— Ed.] t [Barrois (App. to Lit. on Pseudoscorpiones, Xo. I.) has recently very fully investigated the development of Chelifcr; he finds that segmentation may be either total or partial, the latter condition predominating and resulting in a core of yolk with peripheral cells, some large, which form the blastoderm, others very small, which become applied to the vitelline membrane. A deep median ventral longitudinal groove appears, from the walls of which mesoderm-cells are pro- liferated off. "Origin of the entoderm obscure, nuclei appear in the yolk.— En.] PSEUDOSCORPIONES. 29 The further differentiation of the embryo is characterised by the early and pronounced development of the future anterior end of the body ; this appears as a great accumulation of cells belonging to the inner layer of the blastoderm. A pair of marked swellings appear on either side of this region, and from each of these a large truncated appendage soon arises (Fig. 16 A). These processes are the rudiments of the pedipalps which are here, as in Scorpio, the first limbs to appear. They are still in a very primitive condition, the inner yolk-mass extending far into them (Fig. 16 A and B). In front of the limbs, towards the ventral surface, there is a swelling which, even at this early stage, is distinguished by its strong muscu- lature, and consequently has a striped appearance (Figs. 16 A and B, r, 17 .4). This is the rudiment of a provisional organ — a kind of sucking proboscis (Fig. 16 C) which serves for attachment and for db Fig. 10. — A and B, larva of Cliclifer ; C, provisional proboscis of an older stage (after Metsch- nikoff). A, ventral aspect ; B and C, from the side, ah, abdomen ; d, yolk ; (j, brain ; p, the four limbs ; pd, pedipalps ; )•, proboscis (provisional larval organ). taking in food. The embryo leaves the egg at this stage, having previously undergone a larval ecdysis. A fine cuticle, which occupies a peculiar position between the bases of the two limbs, becomes detached from the embryo. The larva, when hatched, at the youngest stage shown in Fig. 16 A, has the muscular proboscis, the truncated pedipalps, and the rudiment of the abdomen directed forward. The proboscis, which is regarded as a modified upper lip, already seems to function as a sucker, for the larva attaches itself by means of this organ to the ventral surface of the mother. The proboscis lengthens considerably at a later stage, and becomes applied to the ventral surface of the larva, lying between the limbs (Fig. 16 B). Barrois has described a provisional oral aperture situated between the pedipalps. There are also, according to Barrois, 30 ARACHNIDA. chitinous structures in the proboscis. There is no mention of an external aperture to the proboscis; Metschnikoff could not find one, although he assumes that the larva obtains its nourishment by sucking the blood of the mother. Soon after becoming attached to the body of the mother, it swells considerably, and becomes filled with a clear fluid (cf. Fig. 17 A and B). If this fluid comes from outside, we must certainly assume that an intestinal epithelium has already developed round the inner yolk-mass, although no such differentiation has been recognised. ° c o on -J.oO°°^ °A< Fig. 17. — Embryo and larvae of Chclifcr (after Metschnikoff, from Balfour). A, embryo in the egg-integument ; B and C, larvae taken from the ventral surface of the mother. «f>, abdomen with the provisional appendages; un.i, anal invagination; ch, chelicerae ; pd, pedipalps ; between the last two {ch and yd) the upper lip is visible in C. Above the pedi- palps are seen, in A the rudiment, in B the base, and in C the last vestige of the proboscis. In l! the rudiment of the oesophageal ganglion can be recognised, lying dorsally to the proboscis. The pedipalps are followed posteriorly by the four limbs, and, in 11, by the rudimentary abdominal appendages. C represents the larva just undergoing ecdysis. The larval integument is partly loosened (noticeably on the ventral side) ; the remains of the proboscis are attached to it. The later stages (Figs. 16 and 17 B) differ from the youngest larvae (Fig. 16 A) in external form chiefly in the swollen nature of the dorsal region, brought about by the presence of the clear fluid mentioned above. Other modifications have also taken place, the rudiments of the first pair of limbs having budded out behind the pedipalps, and these are followed by the three other pairs PSEUDOSCORPIONES. 31 (Fig. 17 B). On the abdomen, which is bent ventrally, four pairs of limb-rudiments appear (Fig. 17 B), which, however, soon com- pletely degenerate. The Pseudoscorpiones agree in this respect with other Arachnida. The most anterior pair of limbs is still wanting, but a paired thickening is found dorsally, above the base of the proboscis ; this has apparently arisen from an invagination, and is the rudiment of the supra-oesophageal ganglion (Fig. 16 B, g). This recalls the cephalic pits of the Scorpiones and Araneae (pp. 12, 53). The larva continues to approach the adult in form, segmentation appearing both in the limbs and in the abdomen, but the cephalo- thorax remains unsegmented. The chelicerae have, in the meantime, appeared in front of the pedipalps. The true upper lip arises between them, some way from and altogether independent of the larval proboscis (Fig. 17 C). The proboscis degenerates, the last vestige of it being lost when the larva moults, at the stage depicted in Fig. 17 C. It is then still found attached by a delicate thread to a point behind the future mouth, until it is cast off with the larval integument (Barrois). A large mass of yolk can still be seen within the body, enclosed in the enteron, which opens exter- nally through the proctodaeum at the posterior end of the body (Fig. 17 C, an.i). The oesophagus is probably also formed by an ectodermal invagination (Metschnikoff). General Considerations. The ontogeny of the Pseudoscorpiones is remarkable on account of the embryo leaving the egg-membrane with a much simpler structure and at a much earlier stage than in other Arachnida. Further, the larvae, in their half parasitic life on the body of the mother, have developed a provisional sucking organ which at first lies in front of the first pair of limbs, but shifts back later, in consecpiience of processes of growth, on to the ventral surface (Figs. 16 and 17); this organ, however, cannot be compared to a pair of limbs. No homologue has so far been discovered among the Arachnida for this proboscis, which must therefore be regarded as an organ acquired by the Pseudoscorpiones through their peculiar method of development. The difference between the ontogeny of the Pseudoscorpiones and that of the Scorpiones, to which they are perhaps most nearly related, is very striking. The cleavage, the formation of the blastoderm, and the first rudiment of the embryo in the two forms can hardly be compared. They also differ in impor- tant points of their organisation. The absence of the taildike abdomen, the disappearance of the abdominal ganglia (Ckoneberg), the position of the genital apertures (in the second abdominal segment), and, not least, their tracheal respiration, remove the Chernetidae from the true Scorpiones so far that the 32 ARACHNIDA. variations in their method of development appear comparatively unimportant. Attempts have been made to connect the Pseudoscorpiones with other divisions of the Arachnida, especially with the Opiliones, but these have not been sufficiently based on the organisation of the two groups. We must therefore, according to a recent investigator of the anatomy of the Chernetidae (Cuonebekg), leave the systematic position of the Pseudoscorpiones undecided, since their ontogeny, so far as it is yet known, throws no light upon the subject. IV. Opiliones. The spherical eggs of the Opiliones are surrounded by two membranes. The inner membrane is secreted by the egg, the outer by the epithelium of the genital duct ; they represent the vitelline membrane and the chorion. The eggs, glued together so as to form a large ball, are deposited in a hole in the ground (Henking). The first ontogenetic processes have been closely studied in Opilio and Leiobunvm, by Henking, but we are unable to accept his view of the origin of" the cleavage-nuclei through free nuclear formation, since it contradicts what is known of other Arthropoda.* Accord- ing to Fatjssek, the egg of Phalangium divides up into a number of large spherical cells filled with yolk-spherules, each cell contain- ing a central nucleus. Cleavage is therefore total. These cells might be compared to the yolk-pyramids in the eggs of the Araneae, but, in the subsequent processes, these cells in the Opiliones seem to differ from those structures. A cleavage-cavity does not appear. The formation of the blastoderm occurs by the separation and more rapid division of some of the peripheral cells. Not all the cells, indeed, not even the majority of them, rise to the surface to form the blastoderm, a large proportion of them remain within the egg as yolk-cells (Henking, Faussek). The formation of the blastoderm takes place more rapidly in one half of the egg than in the other, a condition similar to that observed in the Araneae. Active increase in number of the blastomeres in one region of the blastoderm leads to the formation of a thickening in it; this is the germ-disc. According to Faussek, immigration of cells into the yolk-mass from the disc does not take place ; the entoderm being possibly represented by the cells which remain in the yolk, and from them, at a later stage, the epithelium of the enteron arises. The origin of the entoderm from cells which, from the first, remain behind in the yolk, has been assumed for the Araneae (Schimkewitsch), but the forma- tion of the germ-layers in the Opiliones has not yet been observed sufficiently * [Most cytologists do not believe in the existence of the process termed free nuclear formation ; all modern research tends to prove that every nucleus has originated directly from a pre-existing one. — En.] OPILIONES. 33 closely for us to decide whether this is also the case in them. Faussek found in embryos in which the segmentation of the germ-band is commencing, an accumulation of cells at the posterior end of the band, which strongly resembles the point of ingrowth in the germ-band of the Scorpiones. The statements hitherto made as to the nature of this structure are, however, so contradictory that it is impossible to gain any clear idea of it. Faussek derives these cells, which appear like a thickening of the blastoderm, from a deposit of yolk-cells on the blastoderm. At first he derived the genital glands from this deposit, i.e., from yolk-cells, but he afterwards traced them to a thickening of the blastoderm which appeared at a very early stage. A more exact account of the partly contradictory statements on this subject may be expected in Faussek's larger work [App. to Lit. on Opiliones, Nos. III. and IV.] The mesoderm, so far as we can gather from the few statements on the subject, splits into a somatic and a splanchnic layer, so that in this respect also there is resemblance with the Scorpiones and the Araneae. The enteron seems to form as in the Araneae, apart from the origin of the entoderm, which arises differently according to Faussek. The yolk is directly surrounded by the splanchnic layer of the mesoderm, and the yolk-cells now become applied to this layer, eventually giving rise to the continuous epithelium of the enteron. This process commences in the anterior part of the body. We have only a few isolated statements as to the further develop- ment of the Opiliones. Metschnikoff (No. 34, p. 520) traces the origin of the abdominal limbs, and Balbiani describes a few of the later ontogenetic stages. It appears that the cephalo-thoracic seg- ments to which the four pairs of limbs belong are distinctly marked off from one another in the embryo, but this segmentation disappears during the further course of development, and is not recognisable in the adult. Between the eyes and the bases of the chelicerae lies an unpaired, spine-like structure, which, like similar structures in the Araneae, and especially in the Myriopoda (Chilognatha), we shall call the egg-tooth (p. 58, and cf. the chapter on the Myriopoda). The little that is known of the ontogeny of the Opiliones is in harmony with that of the Arachnida generally. An important feature which is still recognisable in the adult, seems, according to Balbiani, to be very marked in the embryo. This is the occurrence of masticatory ridges on the pedipalps and on the two anterior pairs of limbs. Herein we find a striking resemblance to the Scorpiones. The Opiliones further resemble other Arachnida in the number and position of the limbs, and in the presence of a coxal gland \MacLeod), homologous with the synonymous organ in other Arach- nids. Whereas, however, in other groups, this gland is merely D u ARACHNIDA. provisional, and degenerates in the adult (Scorpiones, Araneae), in the Opiliones it is a well-developed organ, still functional in the adult, and consisting of a large coiled canal, a wide, sac-like reser- voir, and an efferent duct ; the latter opening externally at the base of the third ambulatory limb (Loman, No. 9).* V. Solifugae. f Of the ontogeny of the Solifugae, like that of all the Arachnida already considered, so far as we are aware, very little is known. The little that we do know is in connection with Galeodes araneoides, some of the later ontogenetic stages of which have been described by Croneberg. % B. Km. IS. — A, embryo, and II, newly-hatched young form of Galeodes araneoides (after Crone- berg). a, anus ; ch, chelicerae ; peel, pedipalps ; p, limbs ; r, rostrum. The first embryo discovered by Croneberg was already in an advanced stage, not far from hatching. In Fig. 18, A, it is seen to be very like the embryo of an Araneid. As in the latter, the spherical abdomen, probably well filled with yolk, forms the chief * [Leredinsky (App. to Lit. on Opiliones, No. V.) describes this gland in Phalangium opilio as arising entirely from the mesoderm, the ectoderm only sharing in the formation of the external aperture. He expresses his belief that the coxal glands of Arachnids, the antennae, shell, and coxal glands of Crustacea and Limulus, are all nephridia and thoroughly hemodynamic, but perhaps not thoroughly homologous, some being derived from the primary and others from the secondary coelom. See also, Faussek (App. to Lit. on Opiliones, No. IV). — Ed.] f [See Bernard, App. to Lit. on Solifugae. No. I.] X [BiRULA (App. to Lit. on Solifugae, No. II.) finds that the ova of Galeodes develop within the cavities of the ovaries ; there are no embryonic membranes ; the thoracic and abdominal segments are visible before the appendages. A flexure-reversal occurs as in the Araneae. Hutton states that the Solifugae are oviparous. — Ed. ] SOLIFUGAB. 35 part of the body. The broad and flattened cephalo-thorax seems closely pressed against the ventral surface of the abdomen. The rudiments of the limbs are seen on the cephalo-thorax ; the chelicerae are bent towards the rostrum (Fig. 18, A), the latter being approxi- mated to the slit-like anal aperture. After the embryo is hatched, the abdomen appears longer, and shows a few slight constrictions, which no doubt correspond to seg- ments (Fig. 18, B). It carries two rows of dorsal setae, six in each row. These are the only traces of the hairy covering which is so profuse in the adult. The chitinous integument of the young is thus only provisional. The young probably remain for some time after hatching in a pupa-like condition, resembling in this respect the Araneae (p. 58), which after leaving the egg remain quiescent surrounded by a cuticular envelope, which is not cast off for some time. This fact explains why the limbs (now bent backwards) up to this time show no traces of segmentation (Fig. 18, B, Croneberg), and are also devoid of claws. No abdominal limbs were found in the young animal, nor was their presence to be expected at so late a stage. A very remarkable structure, not occurring in the adult,* is a pair of wing-like appendages, which arise dorsally between the points of insertion of the first and second pairs of limbs. These outgrowths consist of a double layer of cells, invested with a cuticle, and thus represent integumental folds ; no nerves or tracheae extend into them, and they are also devoid of muscles. The significance of these -wing-like appendages is not understood. Croneberg compares them to the paired appendages of the Asellus embryo (Vol. ii., p. 151), which are to be regarded as vestiges of the shell, but lays no special stress on this comparison. f The Solifagae are distinguished from the other Arachnida by a few important features, in which they seem more nearly to approach the Insecta. The most anterior pair of limbs with the segment to which it belongs enters into close relation with the preceding (cephalic) segments, and is marked off from the posterior (thoracic) segments, so that a separate cephalic region with three pairs of limbs ai-ises. This has been compared to the head of the Insecta and the next region, which now consists only of three segments, each with a pair of limbs, to the thorax of the Insecta. The resemblance is increased * Croneberg examined adults of the same species, and found that this structure was altogether wanting in them. t [It is now generally agreed that these structures are embryonic sensory organs, and similar to those found in Phrynus, See Bruce (Lit. on Pedipalpi, 26) and Laurie (App. to Lit. on Pedipalpi, No. I.). — Ed.] 36 ARACHNIDA. by the fact that the abdomen consists of ten segments visible externally. It is a striking fact that the Solifugae, which breathe by means of dendriform tracheae, possess at least three pairs of stigmata; the first opens on the fourth segment of the body, viz., the second thoracic (i.e., the first free thoracic) segment; the second pair opens on the second abdominal, and the third pair, which are closely approximated, open on the third abdominal segment. A fourth opening may be present as a median stigma on the fourth abdominal somite.* We cannot agree with those who find actual relationship to the Insecta implied in the very striking features we have mentioned, and regard the Solifugae as a connecting link between the two stocks of air- breathing Arthropoda. The value of a division of the anterior body into head and thorax, in which the three anterior pairs of limbs would have to be considered as the equivalents of the three pairs of oral limbs in the Insecta, is diminished by the fact that one pair is still wanting, i.e., there is in the Solifugae no homologue for the antennae of the Insecta. The most difficult point to explain is the position of the pair of stigmata on the cephalo-thorax ; we can only assume that it was acquired later. The assumption gains in proba- bility when we find that stigmata appear on the cephalo-thorax in the Acarina also, on the legs in Opiliones (Hansen), and on the head in Scolopendrella and Sminthurus (?). The presence of a spiral filament in the tracheae of the Solifugae is no proof of their relationship to the Insecta, since it occurs also in other Arachnida. In spite of the external division of the body into three parts, the Solifugae agree so closely with the Arachnida in outer and inner organisation, that we are not justified in separating them from that class. The shape of the chelicerae, the possession of a coxal gland, like that which is found in the Arachnida (MacLeod, No. 44), the hepatic tubules derived from the enteron,f the position of the genital aperture on the first abdominal segment, and other less striking features favour the Arachnid character of the Solifugae. We there- fore regard them as a branch of the Arachnid stock developed in a special direction, a view which corresponds to that of Ray Lankester (No. 45) and other writers on this subject. The slight * [Beunakd, op. cit.] t With regard to the liver, it should lie mentioned that a more recent observer (Biiujla, No. 42) has found certain differences in structure between this organ in the Solifugae and the Arachnida in general. He also, however, describes the liver as a well-developed organ filling up the interstices between the other organs, a description which applies to the liver of an Arachnid, but not to that of an Insect. ARANEAE. 37 data that are afforded by ontogeny confirm our view, the embryo of Galeodes closely resembling an Araneid embryo. More accurate data as to the development of the Solifugae are very desirable. „ ,. VI. Araneae.* systematic : A. Tetrapneumones. Avicularia (Mygale), Atypus. B. Dipneumones. Epeira, Theridium, Agalena, Lycosa, and all the other Araneae mentioned. Oviposition and the Constitution of the Egg. The Araneae build nests or prepare cocoons for their eggs, and usually watch over them. In many cases the cocoons are carried about by the mother, held by the chelicerae {e.g., Dolomedes, Pisaura) or attached to the abdomen {e.g., Lycosa, Tarantula). The eggs, which are rich in yolk, are surrounded by a vitelline membrane as well as by an external envelope, probably secreted by the oviduct, the latter being described as the chorion. A thin protoplasmic layer (the periplasm or blastem) covers the yolk, which in turn surrounds a central mass of protoplasm (the centroplasm), within which the nucleus is situated ; from this central mass fine protoplasmic strands extend to the surface, thus breaking up the yolk into columns. Besides the nucleus, a remarkable structure is found in the eggs of Araneae, and called the yolk-nucleus, but this is not yet sufficiently understood. It consists of a compact accumulation of spherules ; occasionally it is quite a complicated structure, composed of several concentric layers. When the egg matures the yolk-nucleus usually disappears, but it appears sometimes to be still retained, and is said to be still found near the nucleus in one of the yolk -complexes in the two- and four-celled stages of cleavage (Kishinouye). According to Ludwig (No. 66), the external envelope is marked out into polygonal areas, but this has recently been referred to the breaking up of the periplasm into polygonal divisions, Sabatier (No. 70) and Locy (No. 64), these writers thus agreeing with older statements made by Balbiani (No. 46). This polygonal marking must not be confounded with blastoderm-formation (which only occurs later) ; the former is said to appear even before cleavage * [Pocock divides the Araneae into two groups — A. Mesothelae, comprising one genus, viz., Liphistius, with a segmented abdomen. ■d r\ ■ ii u i ( Mygalomorphae. B. Opisthothelae J A / a ° chn0U10r p hae ._ ED . ] 38 ARACHN1DA. takes place. Locy, with whom Kishinouye agrees in the main, explains these markings hy contractions of the egg after it is laid, drawing the periplasm closer to the yolk. The columns of yolk-granules can be separately recognised at the periphery as prominences, and this causes the polygonal markings on the surface. Some of Bai.biani's numerous figures that bear on this point seem to confirm this view, while others contradict it. In these figures, besides the original division of the periplasm into areas, another and true division is shown, caused by the presence of blastoderm cells. Since the egg is said to contract, there might be a regular folding of the vitelline membrane (in the form of polygonal areas), such as is said to occur in Cetochilus (Grobben), but this possibility seems to be excluded, as Locy mentions a perivitelline fluid which appears when the egg contracts, between its surface and the vitelline membrane. 1. Cleavage and Formation of Germ-Layers. Cleavage may here at first be described as total, but passes later into a superficial form. The central nucleus divides, the two daughter nuclei still lying near the centre of the egg (Fig. 21 ^4). Fig. 19. — Three ontogenetic stages of Philodromus limbatus (after H. Ludwig, from Balfour's Text-book). Although there is no furrow dividing the egg into two, a complete division is indicated, at first, however, only in the yolk. The yolk- granules become arranged radially one behind another in the form of cylindrical columns (Ludwig, Figs. 19 and 21 ^4). These columns, radiating from the centre, become divided into two groups by the division of the nucleus into two (Fig. 19 B). Between them lies formative yolk. As nuclear division proceeds, the two groups of columns, which Ludwig described as rosettes, again divide, and yield four rosettes (Fig. 19 C), which then divide further into eight, sixteen, and thirty-two rosettes, following the usual course of total CLEAVAGE AND FORMATION OF GERM-LAYERS. 39 and equal cleavage. Each rosette, which has now become a simple column, has a nucleus. In the further course of cleavage (Fig. 20 A) the nuclei shift to the periphery, accompanied by the formative protoplasm belonging to them. These, together with the periplasm already present, separate from the yolk to form a peripheral layer, which now contains the nuclei, and must thus be described as the blastoderm (Fig. 20 B, bl). The yolk-columns, or rather pyramids, may still be present at this time. Even earlier a cavity appears at the centre, the cleavage-cavity (Fig. 20, B), the central yolk-mass being withdrawn into the blastoineres as they develop, and pressing further towards the periphery. The yolk-rosettes do not seem, as a rule, to be so distinct as Ludwig found them in Philodromus. Yolk-pyramids have also been seeninAgalena, Titer idium, ^ B Epeira, Phol- cus, and other forms, but the groups formed by them (the rosettes of Phi- lodromus) lie closer to one another (Fig. 21 ^4). A stage in which there are eight such groups closely resembles an egg that has undergone total and equal cleavage, and that has a small cleavage-cavity (Fig. 21 B). Each group of yolk-columns with its nucleus corresponds to a blastomere. The blastoineres here also divide further, as in a case of equal cleavage, and when, after repeated division, a large number of blastomeres (about 128) have been formed, the nuclei, which have meantime shifted to the periphery, with their protoplasm, separate from the yolk below them, and thus give rise to the blastoderm (Fig. 21 C and D). The cleavage -cavity, which may be fairly large (Figs. 20 B and 21 G), becomes again filled with yolk, and the regular arrangement of the latter is gradually lost (Fig. 21 D and E). The formation of the blastoderm seems to take place more rapidly in the one half of the egg than in the other (Fig. 21 E), (Salensky, Ludwig, Locy, Morin, Schimkewitsch). The former is Fig. 20.— Superficial aspect and optical section of a later stage in the cleavage of Philodromus limoatus (after Ludwig, from Balfour's Text-book). U, blastoderm ; yk, yolk-pyramids. In the space between the vitelline membrane and the blastoderm the perivitelline fluid is found (11). 40 ARACHXIDA. the region in which the germ-band appears later, and may possibly correspond with the germ-disc from which the blastoderm spreads in Scorpio. The method of cleavage of the Araneid egg agrees closely with that of Crustacean eggs classed under type II. (Vol. ii., p. 109). If, as appears probable to us, a cleavage-cavity does not occur in all Araneid eggs, the centre in some cases remaining filled with an unsegmented mass of yolk, these latter cases would probably be referable to that type which was described, in connection with the Crustacea, as total cleavage with subsequent transition to superficial cleavage. Fio. 21.— Sections through the egg of Theridium mmmlatum in different stages of cleavage and blastoderm-formation (after Morin). bl, blastoderm; d, yolk ; dp, yolk-pyramids ; dz, yolk- cells ; fh, cleavage-cavity ; p, periplasm. [The blastomeres are, for the most part, flattened (Fig. 21 JE), but at one spot the cells become spherical and multiply rapidly, conse- quently a large accumulation of blastoderm-cells, several deep, forms at this spot. By reflected light this spot appears as a round whitish area. Shortly after this a second thickening, and consequently a second white area, appears. These two thickenings herald the formation of the germ-disc] CLEAVAGE AND FORMATION OF GERM-LAYERS. 41 There is little agreement among authors concerning the onto- genetic processes which follow the formation of the blastoderm, some ascribing great significance to the prominence, called by Claparede the primitive cumulus, which appears in the blastoderm by the thickening of the cell-layer * (Figs. 22 B and 23 A and B), others denying its importance. According to Morin, a thickening of the blastoderm arises in the region which corresponds to the later ventral surface, i.e., to the rudiment of the germ-band (Fig. 21 F) ; not only do the cells here increase in size, but some of them separate from the blastoderm, and form definite layers ; the blasto- derm thus becomes multilaminar. At the same time a few cells a. &.■ Fig. 22.— Sections through the egg of Pholeus plialangioides during the formation of the germ- layers (after Morin). c.p. primitive cumulus ; d, yolk ; dz, yolk-cells ; e, point of ingrowth. in this region become entirely disconnected from the rest, and migrate into the yolk (Fig. 21 F, dz). The three germ-layers may be now recognised. An outer layer, which constitutes the greater part of the blastoderm, is the ectoderm. Below this, at one pole of the egg, is the mesoderm, while the cells which have migrated into the yolk represent the entoderm. In the Araneids observed by Morin, the primitive cumulus arose only after the germ-layers had formed, if indeed it arose at all. It was wanting in Theridium, the form in which the origin of the germ-layers has been just * [Unfortunately there is a good deal of confusion surrounding the term "primitive cumulus." As stated above, there are two thickened white areas in the blastoderm ; and according to Kishinotjye, Claparede overlooked the first of these, and applied the term " primitive cumulus " to the second. The former author terms them the primary and secondary thickenings ; but Kingslet, while agreeing that Kishixouye may be right, nevertheless retains Clai'Arede's term " primitive cumulus" for the first thickening. — Ed.] 42 ARACHNIDA. described. It is, however, not impossible that those cases in which it is wanting are not primitive, but are specialised, and that it really is of greater importance than its late appearance in Pholcus and its entire absence in Theridium would lead us to believe. This last view is confirmed by the recently-published work of Kishixouye (p. 44). The primitive cumulus* arises as a thickening of the blastoderm (Fig. 22 B), and may project from it as a prominence of considerable size (e.g., in Tegenaria and Agalena, Fig. 23, A and B, p. 46). It has been found in most of the Araneae as yet examined. A depression is said to appear in front of it (Salensky, No. 71, Schimkewitsch, No. 72). We are tempted to regard the latter as the blastopore, at the posterior edge of which the ingrowth of cells is specially active, as in the Scorpiones (p. 2). Some of the statements as to the relation of the primitive cumulus to the germdayers in the process of formation (e.g., those of Bruce, No. 54, and Lendl, No. 63) must evidently be understood in this way. If we consider the primitive cumulus to lie at the posterior end of the embryo, we find ourselves in the position which was taken up by Balfour (No. 47). Although, since the time of this writer, the ontogeny of the Araneae has been investigated by several zoologists, very little further light has been thrown on this point. According to the above view, the primitive cumulus corresponded more or less to the future caudal end, the depression lying in front of it, and the cephalic lobes again in front of this (Balfour, Schimkewitsch, Lendl) ; according to another view, the caudal end arises at some distance from the primitive cumulus, the cephalic lobes lying nearer it (Balbiani, Locy). In inclining rather to the view that the primitive cumulus corresponds to the posterior end of the embryo, we are actuated chiefly by theoretical considera- tions. The figures given by Morin and Schimkewitsch seem also to support such a conclusion. It is, however, true that there is little convincing evidence for our assumption that the mesoderm arises from the primitive cumulus. There is, indeed, evidence of active proliferation of cells in the primitive cumulus, but in front of it also (in the region of the future germ-band) the blastoderm appears to be multilaminar (Fig. 22 B). It has already been mentioned that Morin entirely denies this significance of the cumulus. According to him, when such a prominence appears, it arises only after the development of the germ-layers. It cannot, however, be denied that Morin himself represents it as of considerable size (Fig. 22 B). It decreases later by giving off isolated mesoderm cells, and, by degrees, shifts dorsally. This displacement is also evident in Claparede'.s figures, if indeed the prominence seen in them actually corresponds to the primitive cumulus (Fig. 25 A and B, p. 48). That the blastopore, or the last * [The authors here use this term in the sense in which it was originally applied by Claparede, i.e., they apply it to the thickening which forms a projection from the surface of the blastoderm, which Kishinouye termed the secondary thickening. Kingsley, on the other hand, terms the primary thickening in Limulus the primitive cumulus. — Er>.] CLEAVAGE AND FORMATION OF GERM-LAYERS. 43 traces of it, should occupy such a position is from previous evidence improbable, unless we may assume that the proliferating area shifted as the posterior end developed, and thus attained a position which is apparently dorsal. Further ■ discussion of this point is unadvisable, as a glance at the figures of Clatakede, Balbiaxi, Salexsky, Balfour, Schimkewitsch, Locy, and Moein shows that they cannot be brought into agreement with one another. It is evident that the difficulty which attends investigation of this point is the cause of our uncertainty with regard to it. Orientation in the almost spherical egg is rendered still more difficult by the appearance of the different parts of the embryo (cephalic lobes and caudal end) simultaneously with the degeneration of the primitive cumulus. On this account one of the more recent investigators of the ontogeny of the Araneae, Kishixouye, was unable satisfactorily to decide the position of the primitive cumulus in the embryo. We must, for the present, accept with some hesitation the view that the depression which appears in the blastoderm of the Araneae and the primitive cumulus corresponds to gastrula- tion, although such an interpretation appears very probable, especially when comparison is made with the Scorpiones. This subject is not exhausted with the question as to whether the germ-layers originate in a region corresponding to the later ventral surface, in which the primitive cumulus represents an area of active cell -proliferation (perhaps a point of ingrowth), for there exists a different interpretation of the origin of the germ-layer. According to the view given above, it is to be assumed that the cleavage-cells shift to the periphery to form the blastoderm, and that the germ-layers originate there by an ingrowth of cells (Figs. 21, F, and 22, A and B). While the mesoderm remains as a compact accumulation on the ventral side, the cells of the entoderm become detached from it and shift into the yolk ; from these the enteron forms later. The origin and the fate of these yolk-cells is otherwise described by Balfour, Schimkewitsch, Locy (?). The most important point in these diverging views is the assumption that some of the cleavage-cells remain in the yolk. These cells, which are not utilised in the formation of the blastoderm, do not represent the entoderm alone, but some of them give rise to mesoderm-elements (Balfour, Schimkewitsch). According to Schimkewitsch, cleavage and the formation of the blastoderm take place in such a way that the egg breaks up into a large number of yolk- pyramids in the manner already described. Each of these pyramids contains a nucleus which at first lies at the centre. The nuclei shift to the periphery later, and there, with the protoplasm which surrounds them, become separated from the yolk. An outer cell-layer, the blastoderm, is thus formed. It appears, however, as if a further division of the nuclei had taken place previously, and a large number of nuclei had remained within the yolk ; at least, this is what we understand from Schimkewitsch's description of the cleavage-process.* During the development of the blastoderm there is a further increase in the number of the nuclei which remained within the yolk. Before following its further fate, we must mention a process which was observed by Schimkewitsch in Araneid eggs, and had been previously noticed by Salexsky. The blastoderm - cells which at first surround the egg, shift towards the ventral side, and there * The statements of Schimkewitsch as to the breaking up of the yolk- pyramids and the formation of uninuclear and multinuclear yolk-cells do not come within our scope, and also require corroboration. As a whole, his figures agree with the descriptions of earlier writers. Schimkeavitsch also found the central cleavage-cavity in a few forms ( Tegenaria, Epeira), and describes it as filled with masses of yolk, in the way described for Theridium (Fig. 21, C and D). •44 ARACHNIDA. form a thickening which, together with the later proliferation of cells at this spot, yields the rudiment of the germ-band. Morin's account also, as far as we can follow it, seems to confirm this, and the figures adopted from him (Fig. 21 D-F) show that an accumulation of blastoderm-cells at first lies on the dorsal side of the egg, while at a later stage only a few cells are perceptible in this region. According to Schimkewitsch, the dorsal side of the egg becomes completely denuded of blastoderm, which only later grows out towards it again. We were at first disposed to attribute the absence of blastoderm on the dorsal side rather to a belated advance of the nuclei out of the yolk, especially as authors state that the formation of the blastoderm progresses from the ventral to the dorsal side. There seemed here to be a distant resemblance to the cleavage and the formation of the blastoderm as observed in the eggs of Scorpio. Further investigation is needed to show whether this conjecture is correct, or whether such a marked redistribution of the blastoderm-cells as is shown in. the figures actually takes place. A similar crowding together of the blasto- derm-cells, though not nearly to such a great extent, has also been observed in other Arthropoda (Astacus, cf. Vol. ii., p. 128). According to Schimkewitsch, who on this point is essentially in accord with Balfoui:, the yolk-cells take part to no inconsiderable extent in the formation of the mesoderm, although the chief mass of them is to be described as entodermic. Schimkewitsch, like Balfouk, assumes a two-fold origin for the mesoderm, inasmuch as it is formed from the thickening of the ventrally situated blastoderm, especially from the primitive cumulus, and also by the addition of yolk-cells to this thickened region. Certain modifications here appear in individual forms (Tegenaria, Epeira, Lycosa) ; upon these, however, we shall not enter, as we are unable to agree with this view. Of the two opposed views, the one assuming the existence of yolk-cells giving origin to the entoderm and the mesoderm to some extent, the other deriving both the entoderm and the mesoderm from the blastoderm by a process comparable to gastrulation, the latter appears to us to be by far the more justifiable. This view is confirmed by Kishinouye's recent work (No. 62). This observer found no nuclei in the yolk after the formation of the blastoderm, but observed cells migrating into the yolk from the blastodermic thickening (Figs. 21 and 22). These cells, which become distributed through the yolk, form the entoderm. Further thickening of the ventral region of the blastoderm gives rise to the mesoderm, as was described above (p. 41). The ventral blastodermic thickening known to us as the primitive cumulus is in any case of significance in connection with the formation of these two germ-layers, for it, like the ventral plate (to be described later), appears before the differentiation of the germ -layers (Kishinouye), and not after it, as Monix assumed (p. 41). When we trace back the formation of the germ-layers to the blastoderm, we thereby imply that the yolk-cells also arise from the blastoderm. These latter, according to the unanimous opinion of authors, contain, in the Araneae, the rudiments of the whole entoderm, giving rise later to the epithelium of the enteron. If these cells were to remain in the yolk when cleavage takes place, the process of blastoderm-formation would have to be regarded as epibolic, but this is contradicted by what occurs in related forms. The germ-layers are moreover formed in the Scorpiones also by the DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 45 ingrowth of cells on the ventral side of the blastoderm, and the entoderm, when first appearing, occupies this position in both these divisions of the Arachnida. In the Scbrpiones, it forms a regular epithelium, so that it cannot fail to be recognised as a separate germ-layer, but here also isolated cells enter the yolk. All these facts confirm us in regarding the view of the formation of the germdayers adopted by Morin and Kishinouye (Fig. 21 F) as correct. It cannot, however, be denied that the figures given by Schimkewitsch, and especially those given by Balfour, show yolk- cells in earlier stages and further removed from the thickened part of the blastoderm, which might rather be assumed to have remained behind in the yolk at the time of cleavage, than to have become detached from the thickened part of the blastoderm. If this should be the case, the view here taken is not thereby contradicted; we have then to do merely with single cells which were not utilised in the formation of the blastoderm, and remained behind in the yolk. These cells, as vitellophags, perhaps render the absorption of the yolk possible. In that case we must assume that they do not later enter into the formation of the entoderm, but probably disintegrate during the gradual disappearance of the yolk, as is the case with corresponding (yolk) cells in the Insecta. 2. Development of the External Form of the Body. The development of the external form of the body has been repeatedly investigated more or less thoroughly in the cases of Agalena, Clubiona, Epeira, Theridium, Lycosa, and PJtoIcus, and has been found to follow a very uniform course. In spite of this fact, and although a large number of zoologists, among whom we may mention Herold, Claparede, Salensky, Balfour, Schimkewitsch, Locy, and Kishinouye, have investigated the subject, some points, especially in the earlier ontogenetic stages, still remain obscure. The chief difficulty is connected with the early rudiment of the embryo and the first appearance of segmentation. At a time when the blastoderm is either approaching completion or is fully developed, there appears (probably on the later ventral side) the prominence known to us as the primitive cumulus, the sig- nificance of which has already been discussed (p. 41, etc.) From this there extends forwards a band which is distinguished by its white colour from the rest of the egg, and is caused by a marked thickening of the blastoderm (Fig. 23 A, Claparede, Balfour). Herold mentions a comet-like structure which arises at an early stage on 4G AHACHXIDA. the surface of the Araneid egg, this comparison heing apparently suggested by the band just described, together with the primitive cumulus (Fig. 23). The band soon widens at the end furthest from the primitive cumulus, and it becomes still broader as the thickening of the blastoderm extends out laterally from this region. Such a lateral extension of the blastodermic thickening, starting from the band, implies that we regard the band itself, as well as the primitive cumulus, as thickenings of the blastoderm, which have arisen by active increase of cells at these points. According to Salensky, a depression appears in front of the primitive cumulus ; this soon closes again, and is regarded by him as the blastopore. We are disposed to attribute the same significance to that thickening of the blastoderm which was mentioned above in the description of the formation of the germ-layers. "We thus assume that the primitive ^S. **" A, Fig. 23. — Superficial aspect of three early stages in the development of an Araneid, showing the embryonic rudiment (A and B, Ayalena labyrinthica, after Balfour; C. Theridium, after Morin). c.pr, primitive cumulus; h, posterior; v, anterior. cumulus lies at the future posterior end, and that the band runs out from it anteriorly. Its position therefore indicates the ventral surface. The latter is clearly recognisable as such at a somewhat later stage, the blastodermic thickening extending further, and finally becoming evident on the surface of the egg as a region shaped somewhat like an isosceles triangle (Fig. 23 C). The basal part of this triangle seems to appear first (Fig. 23 B), and then by degrees the parts nearer the apex. The base of the triangle corresponds to the rudiment of the cephalic lobes, the point to the posterior end of the embryo. According to this account, the primitive cumulus would occupy the apex of the triangle, and must be looked for in the posterior region (Fig. 23 B), and the band which developed at first and proceeded from the primitive cumulus (Fig. 23 A) would DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 47 then indicate the longitudinal axis of the embryo. The whole of the triangle thus represents the germ-band or the so-called ventral plate. Embryonic envelopes, such as are found in the Scorpiones (p. 4), are wanting in the Araneae. The amniotic fold described by Bruce on the head of the Araneid embryo must without doubt be referred to the infolding which takes place during the formation of the brain. The formation of cuticular larval integuments will be again referred to later (p. 58). At about the time when the rudiment of the germ-band (the so-called ventral plate) first appears (Fig. 23 A-C), the egg is said to be flattened on this side, but at a slightly later stage the ventral surface of the embryo appears much arched (Figs. 24 and 25 A), either because this surface has become secondarily convex, or because this special region has not been affected. In Pholcus it appears to be the dorsal part that is flattened (Fig. 25 A and B), and Claparede mentions that in this way the anterior and posterior ends are approxi- mated. Fig. 24. — Young embryo of Clubiona incompta show- ing the commencement of segmentation of the germ-band (after Salensky). hi, cephalic lobe; si, caudal lobe ; between these are a few segments in the act of forming. The larger cells outside the region of the germ-band are said to repre- sent blastoderm -cells which are here less crowded (Salensky). The segmentation of the germ-band begins with the appearance of a few transverse furrows which mark off a large anterior and a posterior region, as well as several intermediate segments (Fig. 24). These segments at first appear very indistinct, the parts of the body to which they correspond being doubtful. In the youngest segmented stage, three segments were found besides the large anterior and posterior regions (Fig. 24, Salensky, Balfour, Locy, Lexdl). These seem to correspond to the first three thoracic segments. According to Locy, we must, however, assume that the three middle segments represent the second, third, and fourth thoracic segments. He believes that the segments develop in the following order ; fourth, third, second, first thoracic segments, then that bearing the pedipalps, and last of all that carrying the chelicerae. The differentiation of the segments would thus take place from behind forward, an exact reversal of the order usually met with in segmented animals. There is a general resemblance between this view and that adopted by Metschnikoff for the Scorpiones, according to which the embryos at first break up into three regions, the anterior corresponding to the cephalic region, 48 ARACHNIDA. the posterior to the telson, with the as yet undifferentiated segments of the post-abdomen, and the middle part giving rise to the other segments of the body (p. 6). It is well to bear in mind how extremely difficult it is to orient the regions of the body in these early stages of the Araneid embryo (cf. Figs. 23-25). This difficulty throws some doubt upon the correctness of the identification of the body-segments, given above on Salensky's authority, and consequently the whole order of formation of the thoracic somites may be at fault. Fig. 25.— Embryos showing varying degrees of segmentation, but not yet provided with limbs. A and Jl, Phokus opilionoides; C, Club'wna (after Claparede). A and B, lateral aspects ; C, ventral aspect, ch, cheliceral segment ; c.pr, primitive cumulus (?) ; eh, egg- integument; h, posterior, kl, cephalic lobes; ped, pedipalpal segment; I-IV, thoracic segments ; 1, first abdominal segment ; si, caudal lobe ; v, anterior. The segments of the pedipalps and chelicerae are said by almost all authors to appear later than the four thoracic segments. Just as the posterior region of the embryo contains in itself a number of segments, so also does the anterior lobe comprise, besides the cephalic part, the segments of the chelicerae and the pedipalps. A regular separation of segments from before backward, therefore, does not take place. At the stage in which there are six segments interposed between the cephalic and caudal lobes (Fig. 25 A and B) the four posterior segments are much better developed and more distinctly marked off than the two anterior segments. .Balfour, Schimke- DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 49 witsch, and Locy give figures of this stage in Agalena, in which the cheliceral segment is still united to the cephalic lobe, or is in the act of separating from it. We are unfortunately not able, from this description, to decide the order in which the thoracic segments become differentiated, but it seems as if the most posterior segment {the fourth) arose after the others. The abdominal segments separate from the caudal lobe in the usual order, i.e., from before backward. As the germ-band segments, it extends further over the egg; and not only do its anterior and posterior ends grow towards the dorsal side, but it extends laterally, and may thus, in a few forms {e.g. Pholcus), cover the greater part of the surface of the egg (Fig. 25 A). Seen from the ventral surface, the germ-band now appears broken up into segments, which extend transversely across the whole surface of the egg (Fig. 25 C). The segments appear somewhat narrow and as if separated by broad transverse furrows. The egg therefore somewhat resembles the dorsal surface of a rolled -up Isopod. This condition, however, is not long retained, a lateral •contraction of the germ-band taking place which causes it to draw back again on to the ventral surface (Fig. 25 B), and to lie there in the form of a segmented band. The cephalic and caudal lobes retain their positions unchanged during this process, and, owing to the dorsal extension of the anterior and posterior extremities of the germ-band, they appear closely approximated (Fig. 25 B). In those forms in which the germ-band does not extend so far over the egg in early stages {e.g. Agalena), the cephalic and caudal ends only approach one another on the dorsal surface at a later period. The shape of the germ-band becomes modified, the cephalic portion widening and assuming a bilateral, bilobed form ; the abdom- inal segments, further, become separated from the caudal lobe, which has also widened. There may be as many as twelve abdominal segments besides the telson (e.g., Pholcus, Schimkewitsch). The abdomen is thus richhj segmented in the Araneid embryo, in direct opposition to its condition in the adult. The complete segmentation of the abdomen does not take place till the later stages, other im- portant modifications in the germ-band preceding it. The first of these to be noted is the appearance of a longitudinal furrow in the ventral middle line (Fig. 28 A), which is caused by the division of the mesoderm lying on the ventral surface into two bands, these subsequently shifting to a more lateral position. The germ-band is in this way divided into two symmetrical halves (Figs. 28 A and E 50 ARACHNIDA. Long before d-~ B, and 26), which may lie so far apart that the yolk protrudes between them (e.g., Agalena, Balfour, Fig. 29, p. 53). Anteriorly, in the cephalic lobes and also at the caudal end, the two halves of the germ-bands remain united (Figs. 28 A and B, 26). the germ -band has divided to such an extent, the rudiments of the limbs- have appeared, the first to be seen being those of the four pairs of ambulatory limbs, as slight prominences a little re- moved from the median groove (Fig. 28 A, 3-6). These are followed by the rudiments of the pedipalps (2), and, a little later by those of the cheli- cerae (1). The rudiments of limbs arise in the same way on the first four abdominal segments (Figs. 28 A, a, 27), so that the abdomen of the embryo is not only much more fully segmented than that of the adult, but even has limbs on some of its segments. In this respect the Araneae re- semble the Scorpiones, which also have limbs on the ante- rior abdominal segments (p. 8). Further similarity is found in the fact that, in the former, the posterior part of the abdomen may be flexed forward ventrally like the post-abdomen of the Scorpion embryo. This is the case in Pholcus, as was pointed out by Claparede, and confirmed by Emerton, Schimkewitsch, and Morin. It is almost universally admitted that the first four abdominal segments carry provisional appendages (Balfour, Locy, etc.). Even Morin's researches, carried out by the help of the latest methods, yielded the same result, although Salensky had mentioned a first limbless segment, and Schimkewitsch had accepted this view. The statements of these two authors are supported by the notes and figures of Bruce, published after his death (No. 54). We were able easily to convince ourselves, by examination of an Araneid embryo Fig. 2ij. — Embryo of Phrfcus opilionoides, ideally unrolled (after Claparede). ch, chelicerae ; d, yolk ; kl, cephalic lobes ; ped, pedipalps ; Pi~Pt, first four pairs of limbs ; 1-3, first three abdominal segments ; pah, the posterior part of the abdomen flexed ventrally. DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 51 Ae.__ Fig. 45. — Transverse sections through embryos of Thendium muculatum (after Morin). In A, the embryo, which is curved round the yolk, is cut through twice ; the thoracic limbs and primitive segments can be recognised below, while the abdominal primitive segments are seen above. D, cross-section through the abdomen of an older embryo, in which the primi- tive segments have increased in size, hi, blood-corpuscles; d, yolk; cfr, yolk-cells; ex, limbs ; I, lung invaginations ; n, rudiment of the chain of ganglia ; us, primitive segments. the cephalo-thorax and also in the abdomen, as far as the latter possesses appendages, the primitive segments extend into the limbs ; indeed for the time they withdraw almost entirely into the limbs (Figs. 44 and 45 .4). The mesoderm-bands naturally also take part in the displacement undergone by the two halves of the germ-band in consequence of the pressing forward of the yolk-mass to the ventral side. Whereas they formerly lay near the ventral median line (Fig. 43 C) they now appear removed from it, and divided by the so-called yolk-sac (Fig. 29 A, p. 53). The segmental cavities THE BLOOD-VASCULAR SYSTEM AND THE BODY-CAVITY. 87 increase considerably in size, the primitive segments extending towards the dorsal side (Fig. 45 B). This process exactly corre- sponds to that we have already met with in the formation of the coelom in the Annelida (Vol. i., p. 289). The following are the derivatives of the primitive segments : — 1. The somatic layer gives rise to the body -musculature (as thickenings near the ventral middle line of the abdomen), the two strong longitudinal muscles being specially noteworthy, and also to the subcutaneous connective tissue. According to Schimkewitsch, .the endoskeleton also is derived from the somatic layer, but this statement Ave give with reserve. The covering of the parts arising through invagination of the ectoderm (stomodaeum and proctodaeum, lungs, glands), together with their musculature, thus the strong musculature of the stomodaeum, already mentioned, is also derived from the somatic layer. 2. The splanchnic layer gives rise to the covering of the enteron, the blood-vascular system, and the genital organs. The coxal glands are probably related to the mesoderm and coelom, as in the Scorpiones (pp. 24 and 92), [cf. Brauer and Purcell]. G-. The Blood-vascular System and the Body-cavity. The Blood-vascular System. At a time when the limbs have already developed, there appear, above the primitive segments, between the ectoderm and the yolk, large round cells (Fig. 45 A and B, bl), concerning whose origin there is considerable difference of opinion. Balfour derived these cells from the yolk-cells. To the latter he also traced the origin of the dorsal mesoderm (Fig. 29, p. 53). This last assumption was refuted by Schimkewitsch, Locy, and Morin, who agree in stating that the primitive segments extend to the dorsal middle line. The cells which, at later stages (Fig. 29), are found dorsally, as in Figs. 45 B and 46, therefore belong to the primitive segments. But, besides these, there are the large round cells mentioned above (Fig. 45 A and B, bl), and with regard to their origin, Schimkewitsch and Locv agree with Balfour, deriving them from the yolk-cells. Kishinouye has recently adopted the same view, which seems in accordance with the constitution of these cells. They are much larger than the cells of the primitive segments (Fig. 45 A and B) ; we might, nevertheless, like Morin, derive them from these, and assume that they had separated from the primitive segments in an early stage, when the cells of these segments were themselves larger. Better nourishment near the yolk as the cells increased in number would also determine increase in size. This view is further supported by the fact that they are found in the cavities of the primitive segments (Schimkewitsch). This latter author, indeed, thinks that they reach these cavities from the yolk by breaking through the wall of the segment, but this view seems improbable. 88 ARACHNIDA. SfJ.W. Fig. 46. — Cross-section through the abdomen of an embryo of Phohus phalangioides (after Morin). bg, ventral chain of ganglia ; W, blood-corpuscles; d, yolk ; dz, yolk cells; h, heart; so, somatic, sp, splanchnic mesoblast; sp.iu, spinning mammillae. So long as the origin of the isolated cells lying between the ectoderm and the yolk is not definitely established, we may regard them asmesoderm-cells, and we are especially inclined to consider them as derivatives of the yolk-cells from a comparison with simi- larly related cells found in the Vertebrata, which are there undoubtedly derived from the yolk. These isolated cells eventually become blood-corpuscles. They collect dorsally during the upward growth of the primitive segments (Fig. 45 B), and, as they press somewhat closely against one an- other, they form (especially in the abdomen) a compact strand of cells which prevents the junction of the primitive segments in the dorsal middle line (Fig. 46, U). Subsecptently the mesoderm grows between this strand and the ectoderm, and thus the two primitive segments meet to form a partial dorsal mesentery. At a later period, the walls of the primitive segments grow between the yolk and this strand of cells, and unite with one another below the latter (Figs. 46 and 47 ^4). This strand of cells has consequently become enclosed by a layer of mesoderm having the form of a longitudinally-placed tube, which is at first attached to the somatopleure above and the splanchnopleure below. The tube soon loses its connection with its parent mesoderm (Fig. 47 B), and we now find a continuous layer of mesoderm (somatopleure) lining the ectoderm, while another layer covers the yolk (splanchnopleure) ; between these two layers is the body-cavity, in which the mesodermal tube now lies freely. This tube is the heart, and, so far as can be judged, it is formed directly from the walls of the primitive segments (Schimkbwitsch, Loot, Morin (Fig. 47 A and B)). As a consequence of the development of the heart, the primary continuity of the cell-elements of the primitive segments becomes interrupted at this point (Fig. 47 B). THE BLOOD-VASCULAR SYSTEM AND THE BODY-CAVITY. 89 (Compare with the development of the heart in the Annelida and in the Mollusca.) The isolated cells which had hecome grouped together into a strand become blood-corpuscles. Their crowded condition and their extremely close connec- tion with the walls of the primitive segments suggested the idea that the heart was derived from a solid mesodermal strand extending along the dorsal middle line (Balfour), but this view cannot be verified ; the formation of the heart may be directly compared with the similar process in the Annelida. The cavity of the heart corresponds to apart of the primary body-cavity, enclosed on each side by the primitive segments. Fig. 4". — Transverse sections through the abdomen of embryos of Theridium maculaUua, showing the formation of the heart (after Moris), hi, blood-corpuscles ; c, coelomic cavity ; d, yolk ; dz, yolk-cells ; ec, ectoderm ; h, heart ; so, somatic, sp, splanchnic mesoblast. The heart lies in a depression of the yolk (Fig. 47 B). The latter is covered only by the splanchnic layer of the mesoderm, as the entodermic epithelium is still wanting. From this part of the splanchnopleure, a mesodermal lamella is said to separate and grow round the heart to form the pericardium (Schimkewitsch). The alary muscles of the heart are then formed from the somatic mesoblast. The pulmonary veins arise as outgrowths of the peri- 90 ARACHNIDA. eardrum, while the anterior and posterior aortae, as well as the lateral arteries, originate as prolongations of the heart or as out- growths from it (SchiiMKEWITSCh). While the cavity of the heart appears to be a part of the primary body-cavity, the pericardial space, according to Schimkewitsch, corresponds to a part of the secondary body-cavity. The pericardium in the Arachnida forms a tube, and is not comparable with the synonymous structure in the Insecta. But before we can make any definite statement as to the nature of the pericardium we must have a more exact account of its origin. The Body-cavity. In the Arachnida, as in other Arthropoda, the blood-vascular system is not separated from the body-cavity, but the latter is directly connected with the circulation of the blood. The method of development of the body-cavity in the Arachnida is, however, strikingly different from that in the Crustacea, Myriopoda, and Insecta. While, in these latter, the primitive segments are not large and soon undergo degeneration, in the Arachnida they are almost as largely developed as in the Annelida (Figs. 45 and 46). The primitive segments are also highly developed in Peripatus to begin with (Fig. 100), but this form resembles the Insecta in that the segments very soon cease growing, and after a rich growth of cells undergo early disintegration. The adult body-cavity forms (as a pseudocode) outside the primitive segments. In the Arachnida it forms somewhat differently; it is, however, difficult, from the statements before us, to arrive at a satisfactory judgment, since little stress has until now been laid upon this point. It is certain, however, that the primitive segments are of considerable size even at a someichat advanced stage of development (Bigs. 46 and 47). Between the somatic and splanchnic layers of each primitive segment there is a rather large cavity, and we must assume that when the union of the segmental cavities takes place this passes direct into the adult body-cavity. It is true that here, also, the body-cavity would not retain the coelomic epithelium up to the last, but the wall of the primitive segments would also break up (Figs. 47 A and B, 41, p. 80, 42, p. 82), yielding the muscidar and con- nective tissue elements, so that at last, in the Arachnida, a condition would be reached similar to that attained at a much earlier stage in the development of the Crustacea, Myriopoda, and Insecta. The segmentation of the mesoderm begins to disappear when the primitive segments have grown to a considerable size and the embryo itself is near the stage illustrated in Fig. 27. The segmental cavities unite in the cephalo-thorax and the dividing walls (dissepi- THE BLOOD-VASCULAR SYSTEM AND THE BODY-CAVITY. 91 ments) gradually disintegrate, the cells falling into the body-cavity (Schimkewitsch). These cells probably give rise to blood-corpuscles. The primitive segments of the cephalic lobes seem already to have fused with those of the cheliceral segment, at least Schimkewitsch speaks of a connection between the two which, however, he explains in another way. If we understand Schimkewitsch rightly, he assumes that the pair of primitive segments in the cheliceral segment arise by division from the pair in the head; we should be more inclined to assume the opposite of this, i.e., an extension of the first trunk-segment into the cephalic region. It, however, appears from the accounts and figures before us that the cephalic and cheliceral segments undoubtedly have separate primitive segments. A union of these two pairs of segments, like that described by Kleinenbeisg for Lumbricus, would then take place. The two segmental cavities of the head become united ; such a union of the cavities of the two sides must take place in the trunk also as a result of the processes described in connection with the formation of the heart (Fig. 47). This at least applies to the dorsal side ; on the ventral side, the primitive segments are at first still far apart (Fig. 46), but they shift gradually towards the middle line, so that they finally extend round the whole mass of yolk. In the abdomen the primitive segments remain separate longer, a fact which is in keeping with their later differentiation. Even when they are fused together, the mesoderm represents two extensive layers passing into one another — an outer or somatic layer and an inner or splanchnic layer ; between these is the secondary body- cavity (Schimkewitsch). From the splanchnic layer, the folds already mentioned in con- nection with the formation of the intestine grow into the yolk (Fig. 42, p. 82), in this way cutting off from it isolated masses which correspond to the later hepatic lobes. "We should like here to draw special attention to the important fact that the yolk is so long a time bounded solely by mesoderm (Figs. 46 and 47), and that the epithelium of the enteron develops very late (Fig. 41, p. 80); indeed, the mapping out of a large part of the enteron, that of the liver, seems to be commenced by the mesoderm. Whether the distribution of these folds corresponds to a true segmentation appears doubtful, although this might be indicated by the appearance of four lateral folds in the cephalo-thorax. It appears that these correspond to the thoracic caeca of the enteron (?), for in the abdomen also a number of folds occur, and it is these principally that give rise to the form of the liver (Mof.ix). The folds which penetrate the yolk not only come from the side, but from above and below, and thus represent oblique as well as longitudinal layers 92 ARACHNIDA. (Schimkewitsch), so that it is impossible to trace them back, as Balfour attempted to do, to the partition walls of the somites. A structure resembling the fat-body of the Insecta which is present in the body-cavity (Ray Lankester's lacunar blood-tissue) is, according to Schimkewitsch, formed, like some of the blood-corpuscles, out of the yolk- cells which immigrate into the body-cavity, and these cells also are said to become arranged into a " peritoneal " layer, which envelops the internal organs. In both cases we should, after what has already been said, feel disposed to derive these structures from the mesoderm, i.e., from the primitive segments, although such a derivation would have to be established by further researches. Where a peritoneum is present, it would be of interest to learn its relation to the primitive coelomic epithelium. rrv. Fig. 48. — Portions of transverse se.ctions of Pholcits phalangiddes (A) and Lycosa saccata (B and C), through different regions of the abdomens of embryos (after Schimkewitsch). bl, blood -corpuscles or detached mesoderm-cells ; e, ectodermal covering of the body; g (and gi?), portions of the genital glands descending to the ventral side ; hi, median portion of the rudiment of the ventral cord; ran, muscles; n, rudiment of the ventral cord; so, somatic, sp, splanchnic layer of the mesoderm. H. The Coxal Glands. The coxal glands, which we shall describe as they are found in the already hatched Araneid, show great resemblance to those of Scorpio, and no doubt arise in the same way as in that animal. An actual efferent duct has for the most part only been proved to exist in young Araneids, where it opens at the base of the fifth pair of appendages (Bertkau, No. 51). In the young of Atypus, Bertkau found, on the anterior pairs of limbs, i.e., third and fourth pairs of appendages, slits corresponding in appearance and position to the apertures of the coxal glands on the fifth pair, and this led him to conclude that there were originally several pairs of these glands. THE GENITAL ORGANS — ACARINA. 93 Kishinotjye's derivation of the coxal glands from an ectodermal invagination winch lengthens into a tube is not only incompatible with their origin in the Scorpion from the mesoderm (p. 24), but also with their relation to the body- cavity. According to Kishtnouye's own statement, the tubular rudiment of the coxal gland opens at its inner end in the shape of a funnel into the coelom, so that the accepted view that these glands are nephridia is confirmed, provided the accounts given are correct. If these glands are ectodermal in origin, then they must be regarded, not as coxal, but as crural glands, and we would expect them to end blindly. Sturany (No. 14), the most recent investigator of the coxal glands in the Arachnida, considers them to be nephridia. If his con- jecture that they end in a terminal sac as in the Crustacea proves correct, the latter would no doubt correspond to a part of the body-cavity. We refer the reader to our account of the coxal glands in the Scorpiones (pp. 24 and 87). I. The Genital Organs. According to Schimkewitsch, the genital organs arise in the anterior part of the abdomen, within the two longitudinal folds of the splanchnic layer, which have risen up into the yolk from the ventral side. In the median layer of each of these folds an ovoid thickening appears (Fig. 48 ^4). This consists of large central and flat peripheral cells, the latter representing an enveloping epithelial membrane (Fig. 48 B). The anterior end of the rudiment curves round towards the ventral side, and is said to correspond to the efferent ducts, while the rest represents the germ-glands. "When the young Araneid hatches, there is still no communication between the efferent ducts and the exterior ; this is established later by means of an ectodermal invagination (Schimkewitsch). [Pueceli, (App. Lit. on Araneae, No. VII.) traces the ducts to tubular growths of the abdominal mesodermal segments ; the openings of these ducts into, the coelom become connected with the genital cells which grow forward from the posterior end of the germ-band. Similar structures develop in all the abdominal appendages, but only those on the second segment persist. He regards them as modified nephridia.] VII. Acarina. Oviposition. The majority of the Acarina lay eggs, a few (e.g., Halarachne) are said to be viviparous. Some (e.g., Scutovertex) appear to be viviparous at certain times of the year and ovo- viviparous during the remainder of the season, others are habitually ovo-viviparous. The Acarid egg is surrounded by a more or less strong shell, sometimes covered with prominences ; in many species this protective shell is extremely thick and traversed by fine pore- canals.* The eggs are deposited in various places, according to the * [According to Trox'ssaet (App. to Lit. on Acarina, No. VII.), the female of Syringobiu chelopus among the plumicolous Sarcoptidae at times reproduces parthogenetically ; the eggs thus produced in the absence of males have no shells. — Ed.] 94 ARACHNIDA. habits of the parent. They are found in decaying wood, in damp earth, on the under surfaces of stones, in dung-heaps, on leaves, fruit, etc. Some, but by no means all, parasitic forms lay their eggs on or in the body of the host. The eggs are at times laid in a heap, at other times separately; in the latter case they are often stalked ; those of Myobia musculi have a process at the posterior pole by which they are attached to the fur of the mouse. According to Hallbr, many Oribatidae carry their eggs attached to their backs, others are said to lay them in a part of their cast-off chitinous Fig. 49.— Cleavage ami formation of the blastoderm in the egg of Tetranychus tdarius (after Clapar£de, from Balfour's Text-book). The yolk-granules are represented by clear circles (in .-1 and D). The nuclei, with the clear areas of protoplasm around them, are much larger than the granules. C, an egg in the stage of blastoderm-formation. integument.* The form of the eggs is most commonly elliptical (Fig. 50), sometimes oval, and more rarely globular (Fig. 49), or even discoidal. For their size, they are richly provided with food- yolk. 1. Embryonic Development. The embryonic development in these eggs is difficult to follow on account of their minute size, and is therefore not well known. [Neither of these assertions is quite correct. The carrying of the eggs is almost entirely confined to the genus Damaeus — it is most commonly the immature individuals, not the adults, which pile the eggs on their backs; it is manifest that at this period they cannot have any eggs of their own to carry. What happens is that the larvae are born with soft abdomens, and have the instinct of piling upon their backs dirt, rubbish, etc., as a protection ; they will pick up and carry the eggs and empty egg-shells, from which they may them- selves have emerged, but they will equally pick up and carry the eggs of other Acarina. The statement concerning the eggs being found in the cast integument has never been confirmed, and is very doubtful. — Ed.] EMBRYONIC DEVELOPMENT. 95 Claparede's account (No. 77) is still the most complete.* According to this writer, in Tetranychus telarius, the nucleus, surrounded by formative protoplasm, rises to the surface of the yolk (Fig. 49 A) and soon divides. Eepeated division (Fig. 49 B) gives rise to a large number of nuclei, each surrounded by an area of protoplasm. The nuclei remain lying at the surface of the egg, and by increasing- still further in number, they, with the protoplasm around them, give rise to the blastoderm (Fig. 49 G). pr Fig. 50.— Embryonic development and formation of the first larval integument in Myobia musculi (after Claparede, from Balfour's Text-hook). In D, the egg-integument has split, and the embryo, surrounded by the first larval integument, is in the act of leaving the egg. ch, chelicerae; pd, pedipalps; pHA the first three pairs of limbs; pr, proboscis (which has arisen through the fusion of the chelicerae and the pedipalps) ; sL-s*, four post-oral segments. The yolk is represented by the darker area. According to Robin and Megnin (No. 104), total cleavage occurs in the eggs of the Sarcoptidac. The egg, while still in the oviduct, was seen to hreak up into four cleavage-spheres. This, if correct, reminds us of the condition described in the Araneid egg, which, however, does not there lead to the complete division of the egg into cleavage-spheres. Total cleavage is said to he undergone also by the egg of Cliclifer (p. 2S), and the same has been main- tained, at least at a later stage, of the eggs of the Opiliones also (p. 32). The blastoderm was examined in a number of Acarina, and always consisted of a thin single layer of cells enclosing the yolk. As it develops further, thickenings take place at points corresponding to the future ventral surface, especially in the cephalic and caudal i-egions. The germ-band thus arises here (Fig. 50 B) in the same way as in other Arachnids. At first it is an ecpually thickened * [Henking's account is perhaps more correct.— Ed.] 96 ARACHNIDA. band, but later it breaks up into two symmetrical halves, a ridge of yolk pressing outwards in the median line. Here also there is agreement with the Araneae. The germ-band soon becomes segmented (Fig. 50 .4). The cephalic lobe, which, in Myobia as in the Araneae, curves over to the dorsal side (Fig. 50 B), and the caudal lobe become segmented off from the trunk. The part lying between theni, which corresponds to the cephalo-thorax, is divided up into a number of segments, the truncated rudiments of the mouth-parts and limbs soon appearing on these (Fig. 50 B). This segmentation is less distinct in other Acarina, and, as is well known, eventually disappears. The abdomen is still comparatively large in such an embryo ; in many Acarina it is much reduced, or is united to the cephalo-thorax. Before development has pro- gressed thus far, a delicate struc- tureless integument separates, in At ax, from the embryo, and surrounds it, like a second egg- integument, in the form of a closed envelope (Fig. 51, dm). In other Acarina, this process only takes place later, when the limbs are already present, so that these are found on the envelope in the form of sheaths surround- ing the actual limbs (Fig. 52, dm). This delicate envelope, though separated from the embryo, is thus seen to be a true larval integument. The embryo is now enclosed in a double envelope, and the dorsal surface which, up to this period, showed little signs of development, being covered only by a thin cell-layer, now commences to develop by the growth of the mesodermal elements towards this surface. The yolk for some time longer retains its former appearance (Figs. 50-53), but we must no doubt assume that the formation of entoderm has already begun. Nothing certain is as yet known of the develop- ment of the germ-layers and the rudiments of the organs in the Acarina. The limbs of the embryo lengthen (Figs. 51 and 53 A) and become segmented (Fig. 52). In the stage depicted in Fig. 51, and more especially in the following stages, the embryos of many Fio. 51.— Embryo of Atax Bonzi surrounded by the deutovum and the egg-shell (after Claparede). eh, chelicerae ; d, yolk ; dm, deutovum ; eh, egg-shell ; kl, cephalic lobe ; 2'!-j),,, the three pairs of limbs ; ped, pedipalps ; si, caudal lobe. THE FORMATION OF THE LARVAL INTEGUMENTS. 97 Acarina show great resemblance to those of the Araneae (Figs. 51 and 57 A). The chelicerae and pedipalps unite to form the proboscis (Fig. 53).* The abdomen (in Atax) now decidedly preponderates over the anterior part of the body (Fig. 53). There are only three pairs of limbs when the embryo breaks through its envelopes and begins free life (Figs. 51-53, p x -p^). We thus find, in the Acarina, a larval stage with only three pairs of limbs, as distinguished from the four pairs of the nymph and of the adult, which, in other points of both outer and inner organisation, the embryo greatly resembles.t 2. The Formation of the Larval Integuments and the Further Course of Development. It was mentioned that, in many Acarina, e.g., Atax, the embryo casts off a cuticular integument at an early stage when the limbs have not yet developed or are only indicated. Claparede's deut- ovum is thus produced, the embryo within the egg-shell thus becoming enclosed in a second envelope (Fig. 51). The resemblance of the "deutovum" with the embryo enclosed in it to an intact egg is increased by the fact that, after casting off the primary egg-shell (eh), the embryo undergoes further changes in its external form within the deutovum. In Trombidlum and Myobia this cuticular membrane is cast only after the rudiments of the limbs have appeared (Fig. 52). In Trombidium, this membrane is provided with appendages which surround the limbs like sheaths (Hen king), but this is not the case in Myobia. Here the limbs form in the usual way (Fig. 50 B), but when they have grown to a considerable length they become applied to the ventral surface of the body, and gradually become flattened to such a degree as hardly to project from the surface of the body. The whole embryo is once more oval and apparently devoid of appendages (Fig. 50 C). At this stage a cuticular membrane becomes detached from the embryo, bearing near its antero-dorsal extremity (in the nuchal region, according to Claparede) a tooth-like structure, composed of two thin chitinous processes closely applied to one another. This structure is not well depicted in Fig. 50 C and D, * [This is true of the forms described by Henking, but by no means holds good for all the Acarina, in the majority of which the chelicerae remain as perfectly distinct and movable organs. — Ed.] t [In the Phytoptidae the adult has only two pairs of legs. The larvae and nymphs do not always resemble the adults in other respects, for instance, in the Oribatidae, they differ essentially in external appearance, and the adult has a well-developed tracheal system which is entirely wanting in the larva. — Ed.] H 98 ARACHNIDA. /lect.cA where it appears more like a slit (at the left side of the inner envelope). Claparede thought that the tooth served for splitting the envelopes. It thus performs the same func- tion as the egg-tooth of the Araneae (p. 58), but it need hardly be pointed out that the difference in position of the two structures makes it im- possible to homologise them. We might rather compare the structure just described with the egg-tooth of theOpiliones (p. 33). The embryo, sur- rounded by the cuticular membrane, emerges from the egg-shell (Fig 50 D), which, however, con- tinues to surround the greater part of it. This recalls the cuticular membrane in the Araneae, which forms under the egg-shell and encloses the hatched and still motionless embryo. The limbs now grow out again, but are reduced as before, and a second cuticular integument is cast off, so that the greater part of the egg is enclosed by two integuments as well as by the egg-shell. The tritovum of Claparede is thus formed. Within it the embryo attains the six-limbed form in which it finally & ,-/' 4 , limbs of the nymph (partly broken off) ; r, proboscis (chelicerae and pedi- palps) of the nymph ; zh, intermediate integu- ment. DEVIATIONS IN THE METAMORPHOSIS. 107 number of statements (not indeed always reliable) made as to post-embryonic development, but also because of the number of variations occurring. "We must therefore refer to the literature already quoted for further particulars, and restrict ourselves to the description of a few ontogenetic peculiarities. The formation of the deutovum-membrane in the Acarid egg is apparently very common, and yet it seems to be indisputable that in some forms it does not take place. Claparede, who has given special attention to this point, states that the six-limbed larvae of Tetranychus hatch direct out of the egg-shell, without previously being surrounded by a special chitinous envelope. Tbe appearance of a six-limbed larva also is not universal, although it occurs in most families.* In Phytopta, for example, the larvae are four-limbed, i.e., provided with only two pairs of limbs, and some have been disposed to regard this as a primitive condition. But since, according to Nalepa (Nos. 100 and 101), the adult Phytopta also has only four limbs, this must be considered as a secondary condition both in the larva and in the imago. The great preponderance of the abdomen in the Phytopta and the consequent length of the body must also be regarded as a specialised condition. It is interesting, in this connection, to institute a comparison with the Demodicidae, which also have long abdomens. The six-limbed larva is found in its development and, according to Czokor (No. 78), passes through a course essentially agreeing with that described above. Taking into account the transformation of the six-limbed larva into the eight- limbed nymph, the occurrence of a four-limbed larva has been thought possibly to denote the more primitive character of the four-limbed form, from which the six-limbed form was to be derived. But we have already shown that such a conclusion is unwarrantable. Some light is thrown on the occurrence of the six-limbed larva by "Winkler's observations of Gamasus crassipes. Although the larva of this form has six limbs, four pairs were distinctly developed in the younger embryo (Fig. 57 A and B). Winkler's account is so clear, that all doubt appears to be excluded. t We must assume that one pair degenerates during a moult that takes place within the egg (formation of the deutovum). Shortly before the embryo hatches, when the limbs are already provided with the characteristic setae, there are only three pairs (Fig. 57 C). This statement, which we are hardly justified in doubting, is a strong argument in favour of the secondary origin of the six-limbed larva. The eight-limbed embryos of Gamasus crassipes observed by Winkler appear to be in a lower developmental stage than the six-limbed embryos (Fig. 57 A-C). We therefore assume that, in this form, a stage like that found in Pteroptus vcspcrtilionis is left out, this Acarid having an abbreviated course of development. The embryo of Pteroptus commences free life with eight limbs, i.e., at the nymph stage. It could, however, be shown that the embryo passes through a six-limbed stage in the egg while the latter is still within the mother (Nitzsch). X Limncsw pardina also leaves the egg as a nymph (Neumann). The young of the Phytopta, when they hatch, are very like the sexually mature adult, having * The six-limbed larvae have been observed in the Tetranychidae, Hydrach- nidae, Haiaearidae, Oribatidae, Trombidiidae, Gamasidae, Ixodidae, Tyro- glyphidae, Dermaleichidae, Sarcoptidae, Demodicidae, etc. t [This has since been confirmed by Wagner in Ixodes. — Ed.] J [This observation has not been confirmed, and appears very doubtful ; but cases probably exist in which the whole hexapod stage is passed through in the egg. — Ed.] 108 ARACHNIDA. only two pairs of limbs, and fully-developed mouth-parts. They differ from the adult chiefly in the absence of the external genitalia. These are developed in the course of two moults, and reproduction can now take place (Nalepa, No. 100). The development of Spluierogyna ventricosa appears still more abbreviated. This Acarid, the female of which is distinguished by the greatly swollen abdomen, is ovo-viviparous. The egg, after being laid, yields the sexually mature male and female, and copulation takes place soon after birth (Laboulbkne and Megnin). The course of development may be lengthened by the occurrence of a second nymph-stage following that which proceeds from the larva, and more or less resembling it in form. This is found in Halacarus spinifcr (Lohmann, No. 92), -/ie<£ Fig 57.— Embryos of Gamasus erassipes after the removal of the external egg-envelope, at various stages (after Winkler), abd, abdomen ; ch, chelicerae ; d, yolk ; eh, the cuticular embryonic integument ; kl, cephalic lobe ; ped, pedipalps ; p l -p i , limbs ; si, caudal lobe. and in various Gamasiclae (Reamer, No. 90, "Winkler, No. 106), but it ought to be more definitely ascertained whether these nymphs do not correspond to the pupal stage in other Acarina. It appears, further, that the nymph may be capable of reproduction before it attains the form of the sexually mature animal (Canestrini). This point was established for the Gamasiclae. Berlese distinguishes in this family several ontogenetic series which he describes as normal, and in which the larva, the nymph, and the imago succeed one another in the usual manner, and others which are abnormal, and in which earlier stages, i.e., nymphs, are already capable of reproducing themselves parthcnogcnetically . Such forms do not seem to attain to the complete form of the sexual animal. GENERAL CONSIDERATIONS. 109 It is said that several forms capable of reproduction may occur in this way in one and the same species ; Gamasus tardus, for instance, has no less than five such different forms, each of which might be taken for a different species (Berlese).* These are evidently very complicated conditions, which are far from being sufficiently understood. There is no doubt that early stages of development have repeatedly been regarded as different species, as is now definitely proved in the case of the well-known genus Hypopus (Megnin, Nos. 94 and 95, Michael, Nos. 98 and 99). The members of this genus are minute creatures with a smooth chitinous shell, convex on the dorsal side and flattened on the A - entral side, covering the whole of the body. Acarids with this characteristic appearance are often found on larval and on adult Insects, Myriopoda, etc., and were long regarded as adults. A closer study of the course of their development, however, proved that they merely represent early onto- genetic stages of Tyroglyphus and related genera, which, as a result of hitherto unknown circumstances, have deviated from the usual form of the nymph. These variations only affect isolated individuals, and it has been attempted to trace them back to unfavourable external conditions, which brought about such a modification of the inner organisation (Megnin). This explanation of the origin of the heteromorphic {Hypopus) forms has been disproved by Michael. General Considerations. Attempts have been made to separate the Acarina from the Arachnida, and to give this group the same value as the larger divisions of the Arthropoda (Arachnida, Myriopoda, Hexapoda, Haller, No. 83, A. C. Oudemans, No. 11). The grounds given for this classification appear to us too insufficient to deserve further discussion (p. 100). It rather appears to us that in the organisation and development of the Acarina there is sufficient resemblance to the Arachnida to justify their being classed among these latter, in accordance with the view until now commonly held. The Acarina represent a group of the Arachnida with highly specialised develop- ment, and are thus strongly differentiated in individual points of organisation from other Arachnids. Even the course of development has been influenced, and shows peculiarities which do not occur in other Arachnids. The chief of these are the different consecutive larval and pupal stages, and the free larval form provided with only six limbs. This latter must be considered as a secondary peculiarity. The best proof of this would be afforded by the appearance of a fourth pair of limbs in embryonic stages, which precede the six- limbed larva, if the statements made on this subject by "Winkler (No. 106, cf. p. 107) should be confirmed.! * [According to Michael both these observations are erroneous. — Ed.] t [This has been done for Ixodes by Wagner. — Ed.] 110 ARACHNIDA. VIII. General Considerations regarding the Arachnida. In studying the Arachnida, the point of greatest importance and interest consists in their relationship to those divisions of the Arthropoda classed with them as Tracheata, i.e., the Myriopoda and the Insecta. The Myriopoda, on account of their usually long form of body and the slight differentiation of the different parts of the body, demand less attention in this respect than the Hexapoda, in which the very marked division of the body into three regions calls for comparison with the segmentation of the Arachnida. In such a comparison, however, a serious difficulty at once arises in the different number of segments, and especially of limbs, found in the two groups.* The fusion of segments which often takes place among the Arachnida is of less consequence, since this may also occur to a greater or lesser degree among the Insecta. The fusing of the head and the thorax to form the cephalo-thorax must nevertheless be emphasised as an important Arachnidan character. The Insects, as is well known, carry on the head a pair of antennae, a pair of mandibles, and two pairs of maxillae, which, on account of their structure and ontogeny, are justly regarded as limbs ; further, there are three pairs of limbs on the thorax. The Arachnida have only two pairs of cephalic limbs (the chelicerae and the pedipalps), but four pairs of legs on the thorax. The attempts which have been made to harmonise these differences are too numerous to be treated here in detail. According to what may be described as the prevailing view, there is no homologue in the Arachnida for the antennae of the Insecta, but the chelicerae may be homologisecl with the mandibles, the pedipalps with the first maxillae, and the four ambulatory limbs with the second maxillae and the limbs that follow them. The chelicerae have, however, by some been considered to correspond to the antennae. "We are not disposed to accept either of these views, but, for reasons to be given later, compare the chelicerae to the second antennae of the Crustacea, for which a homologue is wanting in the Insecta. The first antennae of the Crustacea, which correspond to the antennae of the Insecta, are not present in the Arachnida. The pedipalps can at once be homologised with the mandibles of the Insecta (and Crustacea), the four pairs of ambulatory limbs with the two pairs of maxillae and the legs of the Insecta, but in this case one pair of thoracic extremities is wanting in the Arachnida. This, however, * [On this whole discussion compare Editorial note, p. 117.] GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. Ill does not appear to us important, since we attach no great value to this comparison of the Arachnida with the Insecta, and seek for the relationships of the former not so much in the domain of the "Tracheata" as among the branchiate forms, viz., the Xiphosura, as Ray Lankester and others have also done. We are, therefore, inclined to acrree with those zoologists who consider the Arachnida and the other air-breathing Arthropoda as two distinct series, and also assume a separate origin for the tracheae in these two divisions. The agreement existing between the organisation of the Arachnida and that of the Xiphosura compels us to adopt this view. We have already pointed out the agreement in outer structure between the Scorpiones and Limidus (Vol. ii., p. 357), especially in the numbers of the segments and limbs. In Limidus, as in the Arachnida, we find six pairs of limbs on the cephalo-thorax, so that a homology is suggested. We have just compared the first pair of limbs, the chelicerae, to the second antennae of the Crustacea, chiefly because the ganglia of these limbs, which arise post-orally, become united with the supra-oesophageal ganglion, as is the case also with the second antennae in the Crustacea (Vol ii., p. 164), and this process gains in significance when it is found repeated in the maxillary ganglia of Peripahis (p. 193). Xo such process is to be found in the Insecta, and we conclude that the limb in question is wanting in them. "We must not neglect to record the fact that, in the Opiliones and the Acarina, the chelicerae are said to be innervated from the thoracic ganglionic mass (Leydig, No. 40, b, and Winkler, No. 106). A final elucidation of this point is very desirable. The presence in the Araneae of another pair of cephalic limbs besides the two already mentioned has repeatedly been maintained. Two prominences are said to appear in front of the rudiments of the chelicerae and again to disappear (Croneberg, Jaworowski). It was assumed that these conjectural limbs became united with the rostrum (Croneberg, Lendl*), which, according to other observers, was found to have a paired rudiment (Schimkewitsch). There was a general tendency to seek in the rostrum the rudiment of one, or, indeed, perhaps of several pairs of limbs, and it was thought that this could even be proved in the adult animal (Scorpiones, Solifugae, * According to Lenbl, the vestigial limbs lie between the chelicerae and the pedipalps, and correspond to the mandibles of the Insecta, while the chelicerae, by their position and their manner of moving, show themselves to be true antennae. The shifting forward of the pedipalps pressed the conjectural mandibles against the rudiment of the upper lip, so as to fuse with it. 112 ARACHNIDA. Acarina — Croneberg). It should be noted that, according to Schimkewitsch, the so-called lower lip also arose from a similar paired rudiment, but in this case a pair of limbs seems out of the question. If such a vestigial pair of cephalic limbs is really present, it must be regarded (Croneberg, Jaworowski) as the missing antennae, and would be homologous with the first antennae of the Crustacea. This would necessitate no essential modification of our view. The first antennae, which were present in the ancestors, would still occur in the Araneae as vestiges, the chelicerae, however, corresponding to the second antennae. The pedipalps were compared by us with the mandibles of the Insecta. Each is composed of a masticatory ridge and a many- jointed palp. In the embryonic rudiment, however, both parts are said to consist of a number of joints ; if so, this limb would show a very primitive character, and a certain agreement with the biramose extremities of the Crustacea (Jaworowski). Indications of this biramose character are said to be found in the rudiments of other limbs also (Jaworowski). Further, whichever pair of limbs (chelicerae or pedipalps) is compared with the mandibles of the Insecta, the many-jointed character of the Arachnid limb affords a significant contrast to the Insectan mandible, which always consists of a single joint. Another primitive character is found in the presence of masticatory blades on the third and fourth limbs (in the Scorpiones and Opiliones), these extremities being thus partly utilised as mouth-parts, like the thoracic limbs of Limuhis which surround the mouth. The presence of pincers on the anterior limbs might also well be regarded as primitive, since such pincers are found in Limulus. We do not, however, lay any great stress upon this, as similar structures may arise independently of each other. The condition of the cephalo-thorax and its appendages in the various divisions of the Arachnida shows far more agreement with those of the Insecta than is found in the next section of the body — the abdomen — even if Ave overlook the reduced conditions which are exhibited here in the Acarina. We must here mention that the Solifugae, owing to the fact that the three posterior cephalo-thoracic segments are free, while the anterior region becomes swollen in a manner sucrsjestive of a head, have a certain resemblance to an insect. DO ' In addition to this, the abdomen shows the same number of segments as in the Insecta, and a pair of stigmata appears on the first GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. 113 "thoracic" or fourth cephalo-thoracic segment. These peculiarities have led to the Solifugae, which breathe through tracheae, being brought into relation with the Insecta ; but we have already shown (p. 36) that Ave cannot regard these characters of the Solifugae as primitive, nor consider the Solifugae themselves as intermediate forms between the Arachnida and the Insecta. In judging of the relationships of the Solifugae, it is important to note that in them also the chelicerae are innervated from the brain (Weissenborn, No. 16), and are thus proved to be homologous with the chelicerae of other Arachnids. An attempt to compare them with the antennae of the Insecta in order to explain their innervation will hardly be made, their whole development being opposed to this. In making a comparison with the Insecta, we should conclude rather that the antennae, which are to be regarded as a pair of cephalic limbs, are here wanting. The abdomen of the Arachnida is characterised chiefly by the great reduction of its segmentation, except in some divisions however, where the segments are very distinct. In the Scorpiones, the posterior part of the body is divided into a pre-abdomen and a post-abdomen, and is of great length. It might, indeed, be con- sidered as doubtful whether the lengthening of this part were not secondary, but for the fact that other Arachnida, during embryonic life, have sometimes this same number of segments, and also show indications of the division into pre- and post-abdomen (Araneae, pp. 50 and 57). In the fossil Xiphosura (Hemiaspis, Belinums), as well as in the Gigantostraca, the number of abdominal segments is larger than in Limulus, this makes it very probable that the abdomen of the latter has arisen through the fusion of a number of post-abdominal segments, and is thus homologous with the post-abdomen of the Scorpiones (Vol. ii., p. 358). The latter thus show, in the retention of the richly-segmented abdomen and in their segmentation generally, a very primitive character. It has been conjectured that the length and mobility of the abdomen are connected Avith the poison-sting which arms its extremity, and which is thus the more easily brought into use (Weissenborn). Great concentration of the organs is evident in the Arachnida, and the further forms are removed from those which we may rightly consider as the most primitive, the greater is the degeneration found in them, this degeneration reaching its highest degree in the Acarina. The derived forms of the Arachnida are thus simpler in their i 114 ARACHNIDA. organisation than the primitive forms, especially as certain systems of organs (circulatory and respiratory systems) may partly, or wholly, degenerate. The abdominal limb-rudiments are of peculiar importance in the comparison of the Arachnida with other Arthropods. Their number in the Scorpiones, as in Limulus, is six. [? cf. Brauer, Kishinodyb.] It is possible that in the Araneae, also, the same number of abdominal appendages was originally present (p. 51). The Arach- nida, like the Insecta, were derived from forms provided with a larger number of limbs. The first pair [second, pp. 10, 25, 57], is related to the genital aperture, while the following pairs show on their posterior surface the invaginations which give rise to the lungs. The lungs of the Arachnida may therefore be homologised with some probability with the gills of the Xiphosura (Vol. ii., p. 358, and Vol. iii., p. 77). This implies an origin for the Arachnidan tubular tracheae different from that in other "Tracheata" (Peripatus, Myrio- poda, Insecta), for there can be no doubt that the tracheae in the Arachnida are in the closest connection with the lungs.* Although the tracheae in a few Arachnids, e.g., the Solifugae, the Opiliones, and some Pseudoscorpiones and Acarina, seem to resemble each other greatly in structure, they must, in the one case, be derived from lungs or gills, and, in the other cases, from simple integumental depressions. Their later similarity of structure must be regarded as a phenomenon of convergence.! The presence of the stigmata in the abdomen only is in accord- ance with the view of the origin of the respiratory organs here adopted, but an exception occurs in the first pair of stigmata of the Solifugae which lies on the first "thoracic," or, rather, fourth cephalo-thoracic, segment. This must for the present be regarded as a secondary acquisition, and we may similarly try to explain the fact that, in the Acarina, stigmata occur in the cephalo-thorax at various points, often very far forward, in the cheliceral region. Similar displacements of the stigmata are also known to occur in Scolopendrella, where they also appear in the head in an unusual manner. * [See Simmons and Purcell (App. to Lit. on Araneae, Nos. VII., VIII.) and footnotes, p. 78.— En.] t [Tubular tracheae are not restricted to these four groups, but are also iouiirt in many Araneae associated with the lungs ; only the Scorpiones and the Pedi- palpi have lungs alone. This has led Bernard (App. to Lit. on Arachnida in gen No. III.) and Jaworowski (App. to Lit. on Araneae, No. II.) to the conclusion that the lung-books are not primitive structures giving rise to the trachea, but rather that both the lung-books and trachea are to be derived from simple sac-tracheae. — Ed.] GENERAL CONSIDERATIONS REGARDING THE ARACHNID A. 115 There are various other points of organisation in which the Arach- nida are removed from the Insecta, but approach the Xiphosura, and perhaps even the Crustacea. In dealing with the eyes, we tried to show that they cannot be classed together with those of the Insecta and the Myriopoda, but have had a different course of development (p. 68). They may, however, well be homologised with the median and lateral eyes of Limulus. In the origin of the Arachnid eyes, inversion plays an important part. Inversion has recently, also, been introduced by Claus as an explanation of the origin of the median eye of the -Crustacea (Xo. 57), and it appears not impossible that a closer connection may be found later between these processes. Further agreement between the Arachnida and the Xiphosura is found in the presence of an endoskeleton, which in the Scorpiones and Limulus is very similar in structure.* Another point which appears to us to be very characteristic, and which also fully applies to the Solifugae, in spite of their apparent deviation from the other Arachnids, is the presence of a large digestive gland (liver), such as does not occur in the Insecta, but is found in Limulus and the Crustacea. Another still more important point of agreement is yielded by the enteron and its appendages, if we grant that the testimony of ontogeny is reliable, viz., the origin of the so-called Malpighian vessels out of the entoderm. If this is the case, it would form an important reason for separating the Arachnida from the Insecta. Tubular appendages occur in the Crustacea at the posterior end of the metenteron ; the Malpighian vessels of the Myriopoda and the Insecta are, however, of ectodermal origin. Another point of resemblance between Limulus and the Arachnida is afforded by the presence of an artery running, in the Scorpiones, above the chain of ganglia, and forming a backward continuation of the oesophageal vascular ring (supra-neural vessel, supra-spinal -artery) ; a condition similar to this is met with in the ontogeny of Limulus. A sub-neural artery, indeed, occurs in the Crustacea, and a supra-neural vessel is also found in the Myriopoda (a fact * [There is considerable disagreement regarding the homology of the endo- sternite. Bernard (App. to Lit. on Scorpiones, Xo. I.), who has made a comparative study of the Arachnidan endosternite, comes to the conclusion that the endosternite of Limulus cannot be homologous with that of the Arachnids, the latter being part of an epidermal endophragmal system, while that of Limulus is mesodermal. On the other hand, Schimkewitsch (App. to Lit. on Scorpiones, No. YI.) maintains that the structure generally termed the endosternite in the Arachnida and Limulus is always mesodermal, and co-exists with, but is independent of, the series of ectodermal apodemes which -are so conspicuous in Galeodes. — Ed.] 116 ARACHNIDA. which makes this point of resemblance appear of less importance), so that this feature may perhaps be inherited from a common ancestral form. A less important agreement with the genital glands of Limulvs is afforded by the corresponding tubular network of genital glands in the Scorpiones. The coxal glands of the Arachnida, derived from the mesoderm, may, according to our present knowledge, be assumed with consider- able certainty to be nephridia, and are comparable with the organs which, in Limulus, occupy a corresponding position. These glands cannot be fully homologised with the antennal and shell-glands of the Crustacea, since these latter differ somewhat from them in position, i.e., belong to other segments. The nephridia that were present in every segment in the ancestral form have undergone great reduction, and the remnants are retained by their descendants in different segments, a feature probably connected with the varying form of the adult body in the different groups. We need hardly point out that the possession of coxal glands (especially strongly developed in youth) is a further distinction between the Arachnida and the Insecta, the latter not possessing any glands which in their development and position could be compared with the nephridia of the ancestral form. The Arachnid coxal glands arise from the mesoderm, the condition of which during embryonic development is a point of special im- portance. While, in the Insecta, the primitive segments are early subjected to change, in the Arachnida, they grow forward dorsally, and only undergo disintegration at a time when the dorsal heart is formed from them. The coelom, which disappears very early in the Insecta, is long retained in the Arachnida. This, which in itself is a primitive condition, further determines a greater simplicity in the rudiment of the heart, perhaps also in that of the coxal glands (nephridia), and probably also of the genital glands. The conditions thus produced recall those in the Annelida more than those in the remaining Arthropoda. It appears open to cuiestion whether much stress should be laid on the agree- ment existing between the cleavage, and the formation of the germinal layers and of the first rudiments of the organs in the Arachnida and the processes described for the Crustacea, or whether these should be explained by a certain similarity prevailing in these processes throughout the Arthropoda. This has already been pointed out in individual cases. It must remain equally doubtful whether the youngest stage of the germ-band in the Scorpiones, which has been compared with a certain ontogenetic stage in the Trilobites (p. 6), is of special importance in this connection. It can hardly be doubted, from all that has GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. 117 been stated above, that there is a close relationship between the Arachnida and Limulus, and, consequently, points of agreement with the Trilobites might be expected. It is, in this connection, a striking fact that the Scorpiones are of such great age, and that the forms now extant are not very unlike those found in the Silurian strata (PalacopJwnus uuncius, Xo. 15). In conclusion, we must again emphasise the fact that the apparent agreement of the Arachnida with the other Tracheata must he regarded as nothing more than a similarity determined hy their common Arthropodan nature and by a like development as the result of a similar manner of life. We must not assume a nearer connection between these divisions of the Arthropodan stock. We believe, rather, that the Arachnida, together with the Palaeostraca, proceeded from a common ancestral form, and subsequently diverged from one another, while the other Tracheata belong to a distinct stock, the two, however, being connected very far back. The Arachnida, according to our view of them, form a very uniform group. The most primitive forms are those in which the body is distinctly segmented, i.e., the Scorpiones and the Pedipalpi.* The Opiliones and the Pseudoscorpiones are affected by a reduction which goes still further in the Araneae, and reaches its highest degree in the Acarina, in which this far-reaching adaptation is accompanied by essential modifications in development.! Such modifications are also found in the Pseudoscorpiones, probably as the result of similar causes. [In addition to the editorial footnotes inserted here and there referring to Bernard's Arachnidan work, it is necessary to call separate attention to it in some detail, inasmuch as it has a profound bearing upon the question as to whether the Arachnids could be deduced from a Limuloid ancestral form. Arguing that the only scientific method of arriving at the ancestral form of the Arachnida is to compare all the known forms, and to sift out what are obviously the more primitive structural adaptations from the more specialised, this author arrives at the conclusion that the Solifugae come nearest the ancestral form in their segmentation, and in the simplicity of their endosternites. This endo- sternite has no resemblance whatever to the endosternite of Limulus, to which he would assign an entirely different origin (App. to Lit. on Arachnida in gen., No. I., and App. to Lit. on Scorpiones, Nos. I., VI.). He endeavours to show that the typical form of the Arachnidan body is an adaptation to the special manner of feeding. The Arachnids suck the blood of their victims, and, by a force-pump action of the oesophagus, distend the alimentary canal in a manner which would seriously interfere with the rest of the organisation. Their whole inner anatomy, he believes, can be shown to be simply so many adaptations to this serious * [According to Bernard, the Solifugae are in this respect the most primi- tive. — Ed.] t [This statement is a little misleading, for, in the adult Opiliones, only six segments are visible in the abdomen, while, in the Pseudoscorpiones, there are ten to eleven ; further, although the abdominal somites are fused in most adult Araneae (not in Liphistius), yet, in the young, eight to nine segments can be recognised ; these are not lost, but fused together, and, even in the Acarina, one form (Ixodes) exhibits marked segmentation ("Wagner). — Ed.] 118 ARACHNIDA. distention of the intestinal tract — adaptation, that is, of some much less specialised type than Limulus. All the chief organs are dealt with in detail, and, whether the author's conclusions are all of them ultimately confirmed or not, he has succeeded in placing on a new level, not only the controversy regarding the Arachuidan origin, hut also (by his association of physiology with morphology) the science of the whole group. So far his views have not met with much acceptance, and the Scorpiones are still generally regarded as the most primitive Arachnids finding their nearest allies in the Merostomata. — Ed.} LITEKATURE. Arachnida in General. 1. Croneberg, A. Ueber die Mundtheile der Arachniden. Archiv. f. Naturgesch. Jahrg. xlvi. 1880. 2. Eisig, H. Die Capitelliden des Golfs von Neapel. Mono- graphic der Fauna und Flora von Neapel. Berlin, 1887. (On the coxal and spinning glands of the Arachnida.) 3. Fernald, H. T. The Kelationships of Arthropods. Studies Biol. Lab. Johns Hopkins University. Baltimore. Vol. iv. 1890. 4a. Grenacher, H. Untersuchungen iiber das Sehorgan der Arthropoden. Gbttingen, 1879. 4b. Haase, E. Beitrage zur Kenntniss der fossilen Arachniden. Zeitschr. Deutscli. Geologisch. Gesellsch. Jahrg. 1890. 5. Jaworowski, A. Ueber die Extremitaten bei den Embryonen der Arachnoiden und Insecten. Zool. Anz. Bd. xiv. 1891. 6. Lankester, E. Eay. Limulus an Arachnid. Quart. Journ. Micro. Sci. Vol. xxi. 1881. 7. Lankester, E. Bay. On the sceleto-trophic tissues and the coxal glands of Limulus, Scorpio, and Mygale. Quart. Journ. Micro. Sci. Vol. xxiv. 1884. 8. Letjckart, E. Ueber den Bau und die Bedeutung der sog. Lungen bei den Arachniden. Zeitschr. f. Wiss. Zool. Bd. i. 1849. 9. Loman, J. C. C. Altes und Neues iiber das Nephridium (die Coxaldriise) der Arachniden. Bijdragen tot de Dierlcunde. Aflev. xiv. Amsterdam, 1887. 10. MacLeod, J. Becherehes sur la structure et la signification de l'appareil respiratoire des Arachnides. Archiv. Biol. Tom. v. 1884. 11. Oudemans, A. C. Die gegenseitige Venvandtschaft, Abstam- mung und Classification der sog. Arthropoden. Tijdschrift der Nederlandsche Dierlaindige Vereenigung. (2). Deel i. 1885-87. LITERATURE. 119 12a. Saint-Eemy, G. Contribution a l'etude du cerveau chez les Arthropodes tracheites. Archiv. Zool. Exper. (2). Tom. v. Suppl. 1887-90. 12b. Schimkewitsch, \V. Les Arachnoides et leurs amnites. Archiv. Slav. Biol. Tom. i. Paris, 1886. 13. Scudder, S. H. Bearbeitung der Arachniden. Zittel's Hand- bueh der Palaeontologie. Munchen and Leipzig, 1885. 14. Sturany, R. Die Coxaldriisen der Arachnoiden. Arb. Zool. Institut Univ. Wien. Bd. ix. 2. 1891. 15. Thorell, T., and Lindstrom, G. On a Silurian Scorpion from Gotland. K. Svensha Vetenslcaps Akad. Handlingar. Bd. xxi. 1885. Ann. Mag. Nat. History (5). Vol. xv. 1885. 16. "Weissenborn, B. Beitrage zur Phylogenie der Arachniden. Jen. Zeitschr. f. Naturw. Bd. xx. 1885. APPENDIX TO LITERATURE ON ARACHNIDA IN GENERAL. I. Bernard, H. M. Comparative Morphology of the Galeodidae. Trans. Linn. Soc. 1896. II. Bernard, H. M. The Apodidae. London, 1892. III. Bernard, H. M. An endeavour to show that the tracheae of the Arthropoda arose from setiparous sacs. Zool. Jahrb., Bd. v., and Ann. Mag. Nat. Hist. (6). Vol. xi. 1893. IV. Gaubert, P. Recherches sur les organes des senses et sur les systenies tegumentaire, glandulaire et musculaire des appendices des Arachnides. Ann. Sci. Nat. (7). Tom. xiii. V. Hansen, H. J. Orders and characters in different orders of Arachnids. Ent. Meddel. Kjobenhavn. Bd. iv. VI. Jaworowski, A. Homologie der Gliedmassen bei Arachniden und Insekten. Kosmos. Lemberg, 1891. VII. Pocock, Pi. I. On some points in the Morphology of the Arachnida (S. S.), with notes on the Classification of the Group. Ann. Mag. Nat. Hist. (6). Vol. xi. 1893. VIII. Wagner, J. Beitrage zur Phylogenie der Arachniden. Jen. Zeitschr. f. Naturw. Bd. xxix. 1895. IX. Are the Arthropoda a natural group 1 By ten authors. Nat. Sci. Vol. x. I. Scorpiones. 17. Blochmann, F. Ueber directe Kerntheilung in der Embryo- nalhiille der Skorpione. Morph. Jahrb. Bd. x 1885. 1 20 ARACHNIDA. 18. Ganin, M. S. On the Development of the Scorpion. (Kussian, without illustrations.) Supplement to the Protocol Univer. Kharkov. 1867. 19. Kowalevsky, A., and Schulgin, M. Zur Entwicklungsgeschichte des Skorpions (Androctonus ornatus). Biol. Centralbl. Bd. vi. 1886-87. 20. Lankester, E. Ray, and Bourne, A. G. The minute structure of the lateral and central eyes of Scorpio and Lirnulus. Quart. Journ. Micro. Set. Vol. xxiii. 1883. 21. Lankester, E. Eay. On the coxal glands of Scorpio, etc., and the brick-red glands of Lirnulus. Proc. Roy. Soc. London. Vol. xxxiv. 1882-83. p. 95. 22. Lankester, E. Eay. New hypothesis as to the relationship of the lung-book of Scorpio to the gill-book of Lirnulus. Quart. Journ. Micro. Sci. Vol. xxv. 1885. 23. Laurie, M. The Embryology of a Scorpion (Euscorpius italicus). Quart. Journ. Micro. Sci. Vol. xxxi. 1890. 24. Metschnikopf, E. Embryologie des Scorpions. Zeitschr. f. Wiss. Zool. Bd. xxi. 1871. 25. Muller, Joh. Beitriige zur Anatomie des Scorpions. Meckel's Arehiv. f. Anat. u. Phys. 1828. 26. Parker, G. H. The eyes in Scorpions. Bull. Mm. Comp. Zool. Harvard College. Vol. xiii. 1887. 27. Patten, W. On the origin of Vertebrates from Arachnids. Quart. Journ. Micro. Sci. Vol. xxxi. 1890. 28. Rathke, H. Zur Morphologie. Reisebemerkungen aus Taurien. Riga and Leipzig, 1837. APPENDIX TO LITERATURE ON SCORPIONES. I. Bernard, H. M. The Endosternite of Scorpio compared with that of other Arachnids. Ann. Mag. Nat. Hist. (6). Vol. xiii. 1894. II. Brauer, A. Beitriige zur Kenntniss der Entwicklungsgeschichte des Skorpions. Zeitschr. f. Wiss. Zool. Bd. lvii. and lix. III. Laurie, M. On the development of Scorpio fulvipes. Quart. Journ. Micro. Sci. Vol. xxxii. 1891 IV. Laurie, M. On the development of the Lung-books of Scorpio fulvipes. Zool. Am. 1892. V. Marchal, P. La glande coxale du Scorpion et ses rapports mor- phologiques avec les organes excreteures des Crustaces. Compt. rend. Tom. cxv. (Translated in Ann. Nat. Hist. (6). Vol. x.) LITERATURE. 121 "VT. Schimkewitsch, W. Ueber Bau und Entwicklung des Endo- sternites der Araclmiden. Zool. Jahrb. (Anat.). Bd. viii. 1894. II. Pedipalpi. 29. Bruce, A. T. Observations on the Nervous System of Insects and Spiders, and some preliminary observations on Phrynus. Johns Hopkins University Circulars. Baltimore. Vol. vi. 1886-87. No. 54, p. 47. 30. Grassi, B. Intorno ad un nuovo Aracnide Artrogastro etc. I. Progenitori dei Miriapodi e degli Insetti. Memoria V. Bull. Societa Entomol. Ital. Anno xviii. Firenze, 1886. 31. Tarnani, J. Die Genitalorgane der Telyphonus. Biol. Centralbl. Bd. ix. 1889-90. APPENDIX TO LITERATURE ON PEDIPALPI. I. Laurie, M. On the Morphology of the Pedipalpi. Journ. Linn. Soc. Vol. xxv. 1894. II. Pereyaslawzewa, S. (1) Les premiers stades du developpe- ment des Pedipalpi. (2) Les derniers stades du developpe- ment des Pedipalpi. Compt. rend. Tom. cxxv. 1897. III. Strubell, A. Zur Entwicklungsgeschichte der Pedipalpen. Zool. Anz. xv. (Translated in Ann. Nat. Hist. (6). Vol. x. 1892.) III. Palpigrada. 31a. Hansen, H. J., and Sorensen, W. The order Palpigrada Thor. and its relationship to the other Arachnida. Entomologisk Tidskrift. Aug. 18th, 1897. And Grassi, No. 30. IV. Pseudoscorpiones. 32. Barrois, J. Sur le developpement des Chelifers. Compt. rend. Acad. Sci. Paris. Tom. xcix. 1884. p. 1082. 33. Croneberg, A. Beitrag zur Kenntniss des Baues der Pseudo- scorpione. Bull. Soc. imp. Nat. de Moscou. Tom. ii. 1888. 34. Metschnikoff, E. Entwicklungsgeschichte des Chelifer. Zeitschr. f. Wiss. Zool. Bd. xxi. 1871. 35. Stecker, A. The development of the ova of Chthonius in the body of the mother, and the formation of the blastoderm. Ann. Mag. Nat. Hist. (4). Vol. xviii. 1876. p. 197. (Translated from Sitzungsber. bohm. Gesellsch. Wiss. Prag, 1876.) APPENDIX TO LITERATURE ON PSEUDOSCORPIONES. I. Barrois, J. Memoire sur le developpement de Chelifer. Rev. Suisse Zool. 1896. 122 ARACHNIDA. II. Bernard, H. M. Notes on the Chernetidae, with special reference to the vestigial stigmata, and to a new fcrm of Trachea. Journ. Linn. Soc. Zuol. Vol. xxiv. III. Bertkau, Ph. Zur Entwicklungsgeschichte der Pseudoscorpione. C. B. Ver. Rheinl, 1891. IV. Bouvier, E. L. Sur la ponte et le developpement d'un Pseudo- scorpionide, le Garypus saxicola. Bull. Soc. ent. France. 1896. V Vejdowsky, J. F. Sur la question de la segmentation de l'oeuf et la formation du blastoderme des Pseudoscorpiones. Congr. interned. Zool. ii. 1892. V. Opiliones. 36. Balbiani, E. G. Memoire sur le developpement des Phalangides. Ann. Sci. Nat. (5) Zool. Tom. xvi. 1872. 37. Faussek, V. Ueber die embryonale Entwicklung der Gesch- lechtsorgane bei der Afterspinne (Phalangium). Biol. Cen- tralis. Bd. viii. 1888-89. 38. Faussek, V. Zur Embryologie von Phalangium. Zool. Anz. Jahrg. xiv. 1891. 39. Faussek, V. On some ontogenetic stages of the Opiliones. Trud. ross. estestv. Obshchetsvo. St. Petersburg. Zool. Tom. xx., pp. 46-53. 40a. Henking, A. Untersuchungen iiber die Entwicklung der Phalangiden. Zeitschr. f. Wiss. Zool. Bd. xlv. 1887. 40b. Leydig, F. Ueber das Nervensystem der Afterspinnen (Phalan- gium). Arcliiv. f. Anat. u. Phys. 1862. 41. MacLeod, J. Sur l'existence d'une glande coxale chez les Phalangides. Bull. Acad. Roy. Sci. Belgique. (3). Tom. viii. 1884. APPENDIX TO LITERATURE ON OPILIONES. I. Bertkau, P. Die Entwicklung der Coxaldriise bei Phalangium. Zool. Anz. Bd. xv. 1892. II. Faussek, V. Zur Embryologie von Phalangium. Zool. Anz. Bd. xiv. III. Faussek, V. Zur Anatomie und Embryologie der Phalangiden. Biol. Centralblatt. Bd. xii. (Translated in Ann. Mag. Nat. Hist. (6). Vol. ix. 1892.) IV. Faussek, V. On the ontogeny and anatomy of the Phalangidae (Russian). Trud. ross. Obshchestvo estestv. St. Petersburg Zool. Tom. xxii. (No. III. is a resume of this larger work). LITERATURE. 123 V. Lebedinsky, J. Die Entwicklung der Coxal-driise bei Phalan- gium. Zool. Anz. Bd. xv. 1892. VI. Purcell, F. Uber den Bau der Phalangidenaugen. Zeitschr. f. Wiss. Zool. Bd. lv. (1). 1894. VI. Solifugae. 42. Birula, A. Einiges iiber den Mitteldarm der Galeodiden. Biol. Centralbl. Bd. xi. 1891-92. 43. Croneberg, A. Ueber ein Entwicklungsstadium von Galeodes. Zool. Anz. Jahrg. x. 1887. 44. MacLeod, J. Sur la presence d'une glande coxale chez les Galeodes. Bull. Acad. Roy. des Sci. Belgique (3). Tom. viii. 1884. 45. Lankester, E. Bat. Limulus an Arachnid. Quart. Journ. Micro. Sci. Vol. xxi. 1881. p. 644. APPENDIX TO LITERATURE ON SOLIFUGA. I. Bernard, H. M. Comparative Morphology of the Galeodidae. Trans. Linn. Soc. Zool. 1896. II. Birula, A. Beitrage zur Kenntniss der Anatomischen Baues Geschlechtorgane bei dem Galeodiden. Biol. Centralbl. Bd. xii. 1892. VII. Araneae. 46. Balbiani, E. G. Memoire snr le developpement des Araneides. Ann. Sci. Nat. Zool. (5). Tom. xviii. 1873. 47. Balfour, F. M. Notes on the development of the Araneina. Quart. Journ. Micro. Sci. Vol. xx. 1880. 48. Barrois, J. Recherches sur le .developpement des Araignees. Journ. Anat. et Phys. Paris, 1877. 49. Bertkau, Ph. Ueber die Respirationsorgane der Araneen. Archiv. f. Naturgescli. Jahrg. xxxviii. 1872. 50. Bertkau, Ph. Ueber den bau der Augen etc. bei den Spinnen. Verhandl. Naturhist. Ver. Rheinlande und Westfalen. Jahrg. xlii. 1885. p. 218. 51. Bertkau, Ph. Ueber den Verdauungsapparat der Spinnen. Archiv. f. mikr. Anat. Bd. xxiv. 1885. 52. Bertkau, Ph. Beitrage zur Kenntniss der Sinnesorgane der Spinnen. I. Die Augen. Archiv. f. mikr. Anat. Bd. xxvii. 1886. 53. Bruce, A. T. Observations on the Embryology of Spiders. Amer. Nat. Vol. xx. 1886. 124 ARACHNIDA. 54. Bruce, A. T. Observations on the Embryology of Insects and Arachnids. Baltimore, 1887. 55. Carriere, J. von. Kritische Besprechung der neueren Arbeiten iiber Bau und Entwicklung des Auges der zehnfiissigen Crustaceen und Arachnoiden. Biol. Centralbl. Bd. ix. 1889-90. 56. Claparede, E. Recherches sur revolution des Araignees. Naturkundige Verhandl. Provinciaal Utrecht sch Genootshap Kunst. Wiss. Deel i. Stuk i. Utrecht, 1862. 57 Claus, C. Ueber den feineren Bau des Medianauges der Crustaceen. Anz. d. Akad. Wiss. Wien. No. xii. 1891. 58. Emerton, H. Observations on the development of Pholcus. Proc. Boston Soc. Nat. Hist. Vol. xiv. 1870-71. 59. Herold, M. Untersuchungen iiber die Bildungsgeschichte der wirbellosen Thiere im Eie. I. Theil. Yon der Erzeuguns der Spinnen. Marburg, 1824. 60. Kennel, J. von. Die Ableitung der sog. einfachen Augen der Arthropoden, namlich der Stemmata der Insectenlarven, Spinnen, Scorpioniden etc. von den Augen der Anneliden. Sitzungsber, Naturf. Gesellsch. Dorpat. Bd. viii. 1888. 61. Kingsley, J. S. The Embryology of Spiders. Critical notice of ]STo. 72, Schimkewitsch. Amer. Nat. Vol. xxi. 1887. 62. Kishinouye, K. On the development of Araneina. Journ. Coll. Sci. Imp. University, Tokio. Japan. Vol. iv., Part i. 1891. (This journal was only accessible after this work had gone to press. The chief results recorded have, however, been referred to by us.) 63. Lendl, A. Ueber die morphologische Bedeutung der Glied- maassen bei den Spinnen. Mathem. naturw. Berichte aus Ungam. Budapest and Berlin. Bd. iv. 1886. 64. Locy, W. A. Observations on the development of Agalena naevia. Bull. Mies. Comp. Zool. Harvard College. Vol. xii. 1886. 65. Loman, J. C. Ueber die morpholog. Bedeutung der sog. Mal- pighi'schen Gefiisse der echten Spinnen. Tijdschrift der Nederlandsche Dierkundige Vereenigung. Ser. 2., Deel i. 1885-87. 66. Ludwig, H. Ueber die Bildung des Blastoderms bei den Spinnen. Zeitschr. f. Wiss. Zool. Bd. xxvi. 1876. 67. Mark, E. L. Simple eyes in Arthropods. Bull. Mus. Comp. Zool. Harvard Coll. Vol. xiii. 1887. LITERATURE. 125 68. Morin, J. Zur Entwicklungsgesch. der Spinnen. Biol. Centralbl. Bd. vi. 1886-87. 69. Morin, J. On the Development of the Araneae. Zapisk. novoross. Obsh. estestv. Odessa. Tom. xiii. 1888 (Russian). 70. Sabatier, A. Formation du blastoderme chez les Araneides. Compt. rend. Acad. Sri. Paris. Tom. xcii. 1881. {Ann. Mag. Nat. Hist. (5). Vol. vii. 1881.) 71. Salensky, W. On the Development of the Araneae. Zapisk. Kievsk. Obshch. estestv. Tom. ii., Pt. i. 1871. (Russian. Abstracted in Hofmann's and Schwalbe's Jahresb. Anat. Phi/s. Bd. ii. 1875. p. 323.) 72. Schimkewitsch, W. Etude sur le developpement des Araignees. Arcliiv. Biol. Tom. vi. 1887. 73. Watase, S. On the Morphology of Compound Eyes of Arthropods. Studies Biol. Lab. Johns Hopkins University. Baltimore. Vol. iv. 1890. APPENDIX TO LITERATURE ON ARANEAE. I. Damix, X. Ueber Parthenogenesis bei Spinnen. Verh. Ges. Wien. xliii. (Translated Ann. Mag. Nat. Hist. 1894.) II. Jaworowski, A. Die Entwicklung der sogenannten Lungen bei der Arachnoiden und speciell bei Trochosa singoriensis, nebst Anhang iiber die Crustaceenkiemen. Zeitschr. f. Wiss. Zool. Bd. lviii. III. Jaworowski, A. Ueber die Extremitaaten, deren Driisen und Kopfsegmentierung bei Trochosa singoriensis. Zool. Anz. Bd. xv. 1892. IV. Jaworowski, A. Die Entwicklung der Geschlechtsdriisen bei Trochosa singoriensis. Verh. Ges. deutsch. Naturf. 1895. V. Jaworowski, A. Die Entwicklung des Spinnenapparates bei Trochosa singoriensis, mit Beriicksichtigung der Abdominal- anhange und der Fliigel bei der Insekten. Jen. Zeitschr. f. Naturio. Bd. xxx. 1896. VI. Kishinouye, K. Note on the Coelomic cavity of Spiders. Joum. Sci. Coll. Japan. Vol. vi. 1894. VII. Purcell, F. Note on the Development of the Lungs, Enta- pophyses, Tracheae, and genital ducts in Spiders. Zool. Anz. Bd. xviii. VIII. Simmons, 0. L. Development of the Lungs of Spiders. Amer. Joum. Sci. (3). Vol. xlviii. Also in Tuft's Coll. Stud. No. 2. 12G ARACHXIDA. VIII. Acarina. The literature on the ontogeny of the Acarina is so extensive that only a limited number of the works can here be quoted. Fuller references to literature will be found in the works of Furstexberg (No. 80), Henkixg (No. 85), and Lohmanx (No. 92). 74. Bexeden, P. J. vax. Eecherches sur l'histoire naturelle et le developpement de l'Atax ypsilophora. Mem. Acad. Roy. Belgique. Tom. xxiv. 1850. 75. Berlese, M. A. Polymorphisme et Parthenogenese de quelques Acariens (Gamasides) Archiv. Ital. Biol. Tom. ii. Turin, 1882. 76. Caxestrixi, G. Osservazioni intorno al genere Gamasus. Atti del Real. Institute* Veneto d. Sc, Lett. etc. (5). Tom. vii. 1880-81. 77. Claparede, E. Studien an Acariden. Zeitschr. f. Wiss. Zool. Bd. xviii. 1868. 78. Czokor, J. Ueber Haarsackmilben und eine neue Varietat derselben bei Schweinen. Verli. k. k. zool. hot. Gesellsch. Wien. Bd. xxix. 1880. 79. Frauexfeld, G. vox. Zool. Miscellen. Khyncholophus oedi- podarum. Verh. k. k. zool. hot. Gesellsch. Wien. Bd. xviii. 1868. 80. Furstexberg, M. H. F. Die Kratzmilben des Menschen u. d. Thiere. Leipzig, 1861. 81. Gudden, R. Beitrage zu den durch Parasiten bedingten Haut- krankheiten. Archiv. f. physiol. Heilkunde. Stuttgart, 1855. 82. Haller, G. Zur Kenntniss der Tyroglyphen und Verwandten. Zeitschr. f. Wiss. Zool. Bd. xxxiv. 1880. 83. Haller, G. Die Mundtheile und systematische Stellung der Milbeu. Zool. Anz. Jahrg. iv. 1881. 84. Haller, G. Ueber den Bau der vogelbewohnenden Sarcoptiden (Dermaleichiden). Zeitschr. f. Wiss. Zool. Bd. xxxvi. 1882. 85. Hexking, H. Beitrage zur Anatomie, Entwicklungsgeschichte und Biologie von Trombidium fuliginosum. Zeitschr. f. Wiss. Zool. Bd. xxxvii. 1882. 86. Koexike, F. Zur Entwicklung der Hydrachniden. Zool. Anz. Jahrg. xii. 1889. 87. Kramer, P. Zur Naturgeschichte der Milben. Archiv. f. Naturgesch. Jahrg. xlii. 1876. LITERATURE. 127 88. Kramer, P. Ueber Dendroptus. Archie, f. Naturgesch. Jahrg. xlii. 1876. 89. Kramer, P. Ueber die Segmentirung b. d. Milben. Arckiv. f. Naturgesch. Jahrg. xlviii. 1882. 90. Kramer, P. Ueber Gamasiden. Archiv. f. Naturgesch. Jahrg. xlviii. 1882. 91. Laboulbene, A., et Megnin, P. Memoire sur la Sphaerogyna ventricosa. Journ. Anat. et Phys. Annee xxi. Paris, 1885. 92. Lohmann, H. Die Unterfamilie der Halacaridae Murr. u. die Meeresmilben der Ostsee. Zool. Jahrh. Abth. f. Syst. Bd. iv. 1889. 93. MacLeod, J. Communication preliminaire relative a l'anatomie des Acariens. Bull. Acad. Roy. Sci. Belgique. Ser. iii. Tom. vii. 1884. 94. Megnin, P. Memoire sur un nouvel Acarien de la famille des Sarcoptides, le Tyroglyphus rostro-serratus et sur son Hypopus. Journ. Anat. et Phys. Paris, 1873. 95. Megnin, P. Memoire sur le Hypopus. Journ. Anat. et Phys. Paris, 1874. 96. Megnin, P. Sur les Metamorphoses des Acariens de la famille des Sarcoptides et de celles de Gamasides. Compt, rend. Acad. Sci. Paris. Tom. lxxviii. 1874. 97. Michael, A. D. British Oribatidae. Vols. i. and ii. London. Ray Society, 1883. 98. Michael, A. D. The Hypopus question, or the life history of certain Aearina. Journ. Linn. Soc. Zool. Vol. xvii. 1884. 99. Michael, A. D. ^Researches into the life histories of Glyci- phagus domesticus and G. spinipes. Journ. Linn. Soc. Zool. Vol. xx. 1890. 100. Xalepa, A. Anatomie der Phytopten. Sitzungsber. k. Akad. voiss. Wien. Bd. xcv. 1887. 101. Nalepa, A. Beitrage zur Systematik der Phytopten. Sitz- ungsber. k. Akad. tciss. Wien. Bd. xcviii. 1889. 102. Xeuman, C. J. Sur le developpement des Hydrachnides. Entonx. Tijdsskrift. Stockholm. Tom. i. 1880. 103. Xitzsch, C. J. Ueber die Fortpflanzung des Pteroptus vesper- tilionis. Archiv. f. Naturgesch. Jahrg. iii. 1837. 104. Eobin, C, et Megnin, P. Memoire sur les Sarcoptides plumicoles. Journ. Anat. et Phys. Paris, 1S77. 128 ARACHNIDA. 105. Winkler, W. Das Herz der Acarinen nebst vergleichenden Bemerkungen uber das Herz der Phalangiden und Cherne- tiden. Arb. Zool. Inst. Wien. Bd. vii. 1888. 106. Winkler, W. Anatomie der Gamasiden. Arb. Zool. Inst. Wien. Bd. vii. 1888. APPENDIX TO LITERATURE ON ACARINA. I. Jourdain, S. Sur le developpement du Trombidium holoseri- ceum. Compt. rend. Acad. Sci. Paris. Tom. cxxv. 1897. II. Kramer, P. Ueber die Typen der postembryonalen Entwick- lung bei der Acariden. Archiv. f. Naturgesch. Bd. lvii. 1891. III. Kramer, P. Zur Entwicklungsgeschichte und Systematik der Susswassermilben. Zool. Anz. Bd. xv. 1892. IV. Lohmann, H. Die Halacarinen der Plankton-Expedition. Leipzig, 1893. V. Schaub, H. von. Ueber die Anatomie von Hydrodroma. Sitzungsber. k. Akad. loiss. Wien. Bd. xcvii. 1888. VI. Supino, F. Embriologia degli Acari. Atti. Soc. Veneto-Trent (2). Tom. ii. VII. Troussart, E. L. Sur l'existence de la parthenogenese chez les Sarcoptides plumicoles. A?i?i. Soc. ent. France. Tom. lxiii. 1894. And Bull. Soc. ent. Paris, 1894. VIII. Wagner, J. Zur Entwicklungsgeschichte der Milben, Fur- chung des Eies, Entstehung der Keimbliitter und Entwick- lung der Extremitiiten bei Ixodes. Zool. Anz. Bd. xv. 1892. IX. Wagner, J. Die Embryonalentwicklung von Ixodes cal- caratus. Trud. St. Peterb. Obshch. Tom. xxiv. CHAPTEE XXII. PENTASTOMIDAE. Our knowledge of the Pentastomidae still rests principally on Leuckart's observations, supplemented by a few smaller treatises, and recently confirmed and amplified by Stiles. 1. Embryonic Development. The eggs of Pentastomum are surrounded by two envelopes (Fig. 58 A and B, h). The early embryonic development takes place gradually as the ovum passes down the uterus. Cleavage is total (Leuckart, Macalister). The egg breaks up into a number of cells of about equal size, the further fate of which could not be ascer- tained. Macalister describes the formation of a blastoderm and a germ-band, but his statements are not conclusive. According to Leuckart, a germ-band is not formed. The embryo secretes a surface cuticle at an early stage, a disc-like thickening appearing on the dorsal surface of this cuticle. When the cuticle detaches itself from the embryo and forms a third envelope to the latter (Fig. 58 A and B, eh), it remains connected at this thickened disc (dorsal cone, rz). The chitinous integument also, which is now secreted as a covering for the embryo, is correspondingly thickened at this spot, and takes the form of a pit-like depression. The "dorsal cone," which at first connects these two chitinous thickenings, becomes constricted and broken through, but a trace of it is left attached to the embryo ; this, in P. taenioides, is shaped like a raised cross situated in a cup-shaped groove (Fig. 58 B and C, rk). The remainder of the" dorsal cone" is retained on the detached integu- ment as a circular thickening (the so-called facet, Fig. 58 B, /). This structure recalls the micropyle or the dorsal organ of the Crustacea, with which Leuckart has compared it. A certain external similarity in structure is found between the so-called primitive tracheae of the Acarina and the dorsal cone of the Pentastomum ; but these "tracheae" are paired and lie ventrally, so that there is no real agreement between them. K 130 PENTASTOMIDAE. The early shedding of a cuticular integument within the egg, which must be regarded as a moult, recalls the formation of the deutovum-membrane in the Acarina (p. 96) ; similar processes occur also in the Crustacea (Vol. ii., p. 118). Before the dorsal cone is broken through — i.e., before the cuticular envelope is completely detached from the embryo — two pairs of truncated appendages have developed on the ventral side. These are limbs on which claws soon appear. A narrower posterior portion — the so-called tail — has, previous to this, become marked off from the compact trunk (Fig. 58 A and B), to the ventral surface of which it is applied. This caudal appendage is characteristic of the embryos of a few species of Pentastomum. In P. taenioides it is c. Sw Pi p. Fig. 58. Embryos in the egg-integuments and free larva of Pentastomum taenioides (after Leuckart). (1st, stigma of gland ; eh, embryonic integument; /, "facet"; h, egg-integu- ments ; m, oral plate ; p x and p 2 , truncated limbs ; rh, dorsal cross (dorsal organ) ; rz, dorsal cone ; s, caudal appendage. The boring apparatus of the embryo is not shown. somewhat large (Fig. 58 B and C), while in P. proboscideum it is merely a small bifid appendage (Fig. 59, s). The embryo of P. oxycephalum has no caudal appendage, but presents a round posterior extremity. In this form the embryo leaves the egg (Van Beneden, Schubart) ; it is therefore very unlike the parent in shape, and has to pass through radical transformations before attaining the adult form (Leuckart). The Larval Development. The further course of development is marked by the transference of the eggs into the intermediate host and the development of a THE LARVAL DEVELOPMENT. 131 four-limbed larva. The form whose ontogeny was examined by Leuckart, P. taenioides, inhabits, in its sexual condition, the nasal cavity of the dog. The eggs are laid in the nasal mucus, and with this they reach the exterior. For the further development of the embryo an intermediate host is necessary. This, in the case of P. taenioides, is a rabbit, which, by swallowing the eggs, introduces them into its stomach, where the egg-integuments become detached and the larva set free. In P. proboscideum also (Stiles), the early stages are similar to the above. The eggs of this form are found in the lungs of the boa constrictor ; from the lungs they pass into the intestine, where they are found in quantities in the fa?ces, with which they leave the body. They, too, must be swallowed by an intermediate host in order to develop further. Stiles was successful in introducing them into mice. The larva, which has a blunt anterior, and a pointed posterior end, i.e., which is sup- plied with a tail, has two pairs of truncated limbs, provided wuth chitinous claws fur- nished with a sup- porting apparatus (Fig. 58 C, and Fig. 59, st). The two claws are attached to a chitinous ring, and seem to be quite in- dependent of the supporting apparatus. This structure sug- gests that the limb consists of a terminal, and a basal seg- ment, the limb being thus regarded as two- jointed. Stiles, who adopts this view, thought the limb more distinctly marked off from the body than did Leuckart, who regarded it as consisting of one joint only. At the anterior end of the body lies a boring apparatus composed ■of several chitinous spines (Fig. 59, ba), which has been compared Fig. 59. — Quadrupedal larva of Pcntastomum proboscideum, from the ventral side (after Stiles), ba, boring apparatus ; dst, stigma of gland ; dz, gland - cells ; kr, claws ; to, mouth ; ma, stomach ; n, rudiment of the nervous system ; oes, oesophagus ; ^',-j'o, truncated limbs; ro, dorsal organ, seen through the transparent body ; s, caudal appendage ; st, apparatus for supporting the claws ; tp, sensory papillae. 132 PENTASTOMIDAE. with the mouth-parts of the Arthropoda, especially with those of the Acarina, hut such a comparison is hardly permissible on account of the position of this apparatus and its origin in front of the mouth ; it must probably be regarded as a larval organ (Stiles). Near the boring apparatus are two small papillae, which have been regarded as tactile organs (tp). The mouth, in P. proboscideum, lies somewhat far back, about on a level with tbe anterior truncated limb (Fig. 58, m). It is surrounded by a chitinous horsesboe-shaped band, and leads into a narrow oesophagus, which passes into a wider stomach. According to Stiles, there is no anus, although one is to be seen in Jacquart's not very accurate drawings. An accumulation of cells surrounding the oesophagus represents the rudiment of the nervous system (m). Stiles also found within the larva a large accumulation of richly granulated cells distributed in a definite manner, some of which are no doubt glandular cells. Two circular structures lying at the bases of the anterior extremities are regarded as the external apertures of glands (so-called stigmata of the glands, Figs. 58 and 59, dst). The Encysted Larva. The larva which has become free in the intestine of the intermediate host, by the help of the boring apparatus at the anterior pole of tbe body and the limbs, traverses the wall of the intestine and passes into the other organs, e.g. y the liver, where it becomes at- tached and en- closed in a fibrous cyst derived from the tissues of the host. It here passes through a number of moults, during which it throws off the limbs and the boring apparatus. The caudal appendage also disappears, and the larva assumes a compact cylindrical form. Leuckart found, seven weeks after ^ ,ma ces. U-ed. Fig. 60. — Encysted larva of Pentmt^mvm tacnioidcs from the viscera of a rabbit, nine weeks after infection (after Leuckart). a, anus ; ag, efferent duct of the genital gland; dst, glandular stigmata; ed, proctodaeum ; gd, genital gland ; m, mouth ; ma, stomach ; n, rudiment of the nervous system ; oe, genital aperture ; oes, oesophagus. THE LARVAL DEVELOPMENT. 133 infection, in the cysts of P. taenioides, besides the worm-, or, rather, maggot-shaped larva, two cast integuments, on which could be distinguished remains of the embryonic chitinous structures, viz., the dorsal cross and the chitinous oral horseshoe-shaped band, and probably also the remains of the truncated limbs. Several further moults then take place, a long time being occupied by this develop- ment ; five to six months, according to Leuckart, pass before the larva of P. taenioides attains its full development in the intermediate host. The development of P. 2^'oboscideum is somewhat more rapid, but also occupies several months (Stiles). While the larva remains in the cyst, and during the course of several moults, the most important change which takes place is the development of the internal organs; the external form, however, ov. s?iu^ ma- Fig. 61. — Encysted female larva of Pentastomum taenioides from the viscera of a rabbit, about four months after infection (after Leuckart). a, anus ; ed, proctodaeum ; Ih, larval integu- ment (detached cuticle) ; m, mouth ; ma,, stomach ; mu, retractor muscles of the pharynx ; n, nervous system ; od, oviduct : oe, genital aperture ; oes, oesophagus ; or, ovary ; tn, nerve running from the oesophageal ganglion to the tactile papillae ; vwj, vagina. also undergoes a few changes, to be described below. The internal organs of the free larva, as far as could be ascertained, seem to pass direct into those of the encysted larva and of the sexually mature animal. The intestinal canal, which was not extensive in the free larva, widens and becomes differentiated into its separate regions, pharynx, oesophagus, and stomach. The latter soon becomes very large (Fig. 60, ma). It ends blindly posteriorly, and only becomes connected later with the proctodaeum (ed). The accumulation of cells round the oesophagus present in the 134 PENTASTOMIDAE. free larva (Fig. 59, n) during later larval life develops into the sub-oesophageal mass and the oesophageal ring, which represent the central nervous system of the adult. The sub-oesophageal mass, in early larval life, is much larger than in the adult animal, and occupies a considerable part of the ventral surface (Leuckart, Figs. 60 and 61, n). The rudiments of the genital organs can be recognised early, but, according to Leuckart, it is at first impossible to distinguish the two sexes. A long, unpaired strand lying dorsally to the stomach, the germ-gland (Fig. 60, gd), forks anteriorly to form two strands (the rudiments of the efferent ducts, ag). These two strands oir. /7UI. <% ed Fig. 62. — Encysted female larva of Pentastomiim prohoscideum (the so-called P. subcylindrimm) from the viscera of a mouse, six and a half weeks after infection (after Stiles), a, anus ; ed, proctodaeum ; lit, hook-sac ; Ih, larval integument (detached cuticle) ; m, mouth ; ma, stomach ; n, rudiment of the nervous system ; od, oviduct ; oe, genital aperture ; ol, upper lip ; ov, ovary ; rs, receptaculum seminis ; vag, vagina. embrace the anterior part of the stomach, and, after reuniting ventrally, open externally in the region of the ganglionic mass (a). There is very little difference in this respect in the male ; the genital aperture in the adult male retains its primitive position in the anterior part of the body, not far behind the mouth. The genital aperture in the adult female is, however, found at the posterior end of the body, quite near the anus (Fig. 62, oe) ; and Leuckart assumes that it has been thus displaced on account of greater growth of the part between it and the mouth taking place simultaneously Avith arrest of growth in the posterior region. Fig. 61 represents THE LARVAL DEVELOPMENT. 135 a transitionary stage, in which the genital aperture is already shifted further back than in Fi" 60. The differentiation of a vagina from the primitive genital duct has here already taken place. In Fig. 62, the genital aperture has already assumed its final position near the anus. Stiles speaks of a differentiation of the sexes at an early stage ; but the stages described by him in P. proboscideum seem to us to be somewhat more advanced than those observed by Leuckaet in P. tacnioides. According to Hoyle, it appears that the genital glands may originally have been paired. If this were the case, we should have, in the fusion of the germ- glands to form a single organ, a process similar to that in the Acarina (p. 101). The position of the (female) genital aperture at the posterior end of the body, which is in opposition to what is usual in the Arachnida, might, according to Leuckart's explanation, be regarded as secondary. The body of the encysted larva after the first moults looks quite smooth, but later a series of rings make their appearance (Fig. 62). These first arise in the middle of the body, and spread anteriorly and posteriorly. These superficial markings cannot be regarded as equivalent to actual segmentation on account of their late appearance and their development. In some Pentastomidae, e.g., P. protelis (Hoyle), they are somewhat broad, and constrictions form between them, thus increasing the resemblance to a true segmentation. In P. proboscideum also such an appearance can be remarked, and is still more conspicuous in P. taenioides. In other Pentastomidae, raised rings are found like broad hoops round a barrel, separated by inter- spaces (Van Beneden, Jacquart). Small circular apertures in the chitinous integument are found distributed all over the surface of the body, and later, in consequence of the formation of rings, arranged in transverse rows upon it. These resemble the two glandular stigmata of the four-limbed larva (Fig. 62), and were regarded by Leuckart as the apertures of integumental glands. A differentiation of the chitinous covering of the body which arises in later larval stages is found in the so-called circles of spines which appear at the posterior edge of each ring, and are characteristic of the fully-formed larva (Fig. 63, st). The larva of P. taenioides, which was formerly taken for a sexually mature form and called P. denticulatum, has the circles of spines specially well developed. They are probably of advantage to the animal in locomotion. Still more important aids in locomotion and attachment are the hooks — two pairs of claw-like chitinous structures (Fig. 63, h), which develop in two sac-like depressions of the integument in front of the mouth (Fig. 62, Jit). The hooks have no connection 136 PENTASTOMIDAE. with the truncated limbs of the larva, nor can they be regarded as limbs, as might appear from their origin as depressions and in front of the mouth. At a later stage they shift further back towards, or even behind, the mouth (Fig. 63). A further differentiation of the surface is found in the appearance of a large number of papillae arranged in pairs at the anterior end of the body (Fig. 63, tp), which have been considered tactile organs (Leuckart, Stiles). The last larval form and its transference to the final host. While these external and internal ontogenetic processes have been taking place the body has lengthened, and has thus been forced to curl up in the cyst, within which the general form of the adult animal is reached. The larva (Fig. 63) now breaks through the cyst, and wanders away from the part hitherto inhabited by it, the circles of spines assisting it in this process. Should the intermediate host in which it lives at this time fall a victim to a beast of prey, the larva possibly passes direct out of the mouth of the latter into its nasal cavity, there, by renewed ecdysis, throwing off its spiny covering, and finally attaining the complete organisation of the sexually-mature Pentastomum. But if no such favourable opportunity is afforded the larva of reaching its final host, it becomes re-encysted within the body of the intermediate host. Encysted larvae which are swallowed by a beast of prey with the flesh of the host, and thus reach the intestine of the former, if sufficiently mature, break through the intestinal wall, and by active locomotion reach the respiratory tissues and the nasal cavity (Gerlach, Stiles). a- Fig. 63.— Free larva of Pentastomum taenioides (the so-called P. denti- cvlatum), from the liver of the rabbit or the nasal cavity of the dog (after Leuckart). a, anus ; d, intestine ; ft, hooks ; m, mouth ; st, circles of spines ; tp, tactile, papillae. 3. General Considerations. The most important point in the ontogeny of Pentastomum is the appearance of a larva furnished with two pairs of limbs. This larval form distinctly indicates that in Pentastomum we have an Arthropod, a fact which is not so evident from the organisation of the adult. It was this larval form above all that led to the classing of the Pentastomidae with the Acarina. The similarity LITERATURE. 137 would be still greater if a six-limbed larva also appeared in Pentastomum, as was maintained by De Filippi. Unfortunately, but little reliance can be placed upon this otherwise important statement, as may be seen from a glance at his figures. A direct comparison of the Pentastomum larva with an Acarid larva is inadmissible on account of the absence of mouth -parts in the former. In this case degeneration may, indeed, have gone even further than in the Acarina, and it is possible that Pentastomum may be derived from forms resembling these latter animals. Certain Acarids, e.g., the Pliytopta, in which two pairs of limbs disappear, and in which the body is lengthened (pp. 107 and 108) might be regarded as indicating the possible line of origin of a form like Pentastomum (Leuckart). But it must be expressly pointed out that there is no definite ground for this view, and that Pentastomum might, with almost equal justification, be derived from some other group of Arthropoda. Unfortunately, the organi- sation of the adult also fails to afford any definite clue, but only makes it clear that Pentastomum is a form much reduced by parasitism. Important systems of organs, such as the respiratory and excretory systems, which elsewhere, by their characteristic development, help to determine systematic position, are wanting. A distinct blood vascular system also is not developed. In the transversely striated musculature, on the other hand, we have an Arthropodan character, and it has already been pointed out that the genital organs can, perhaps, be interpreted in this sense. The ovary in its structure recalls that of the Arachnida, the eggs bulging out like follicles on its surface, and giving the organ an aciniform appearance. LITERATURE. 1. Bexedekt, P. J. van. Recherches sur l'organisation et le developpement des Linguatules (Pentastoma). Ann. Sci. Nat. (3). Zool. Tom. xi. 1849. 2. Filippi, F. de. Nuova linguatula con embrioni di particolar forma. Archiv. Zool., Anat. e Fisiol. Fasc. i. Tom. i. Genova, 18G1. 3. Gerlach, A. C. Pentastomum denticulatum bei zwei Ziegen. Jahresber: d. 1c. Thierarzneischule zu Hannover, ii. 1869. 4. Hoyle, W. E. On a new species of Pentastomum (P. protelis), from the Mesentery of Proteles cristatus, etc. Trans. Roy. Soc. Edinburgh. Vol. xxxii. 1887. 138 PBNTASTOMIDAE. 5. Jaquart, H. Mecanisnie de la retraction des ongles des Felis et des crochets des Linguatules trouvees dans les poumons des serpents. Journ. Anat. et Phys. Paris. Annee iii. 1866. 6. Leuckart, R. Bau unci Entwicklungsgeschichte der Pentas- tomen. Leipzig and Heidelberg. 1860. 7. Lohrmann, E. Untersuchungen liber den anatomischen Bau der Pentastomen. Archiv. f. Naturgesch. Jahrg.lv. 1889. 8. Macalister, A. On two new species of Pentastoma. Proc. Roy. Irish Acad. Ser. 2. Vol. ii. Dublin, 1875-77. 9. Schubart, T. D. Ueber die Entwicklung des Pentastomum taenioides. Zeitschr./. Wiss. Zool. Bd. iv. 1853. 10. Stiles, Ch. W. Bau und Entwicklungsgeschichte von Pentas- tomum proboscideum u. P. subcylindricum. Zeitschr. f. Wiss. Zool. Bd. Iii. 1891. (This work contains a very complete bibliography of the Pentastomidae.) APPENDIX TO LITERATURE ON PENTASTOMIDAE. I. Spencer, W. B. The Anatomy of Pentastomum teretiusculum. Quart. Journ. Micro. Sci. (2). Vol. xxxiv. CHAPTER XXIII. PANTOPODA. Oviposition and Care of the Brood. The Pantopodan female does not deposit her eggs in the usual manner, but transfers them to the male, who attaches them to his third pair of limbs, the so-called ovigerous limbs (Figs. 74, 3, p. 157), and carries them about until the embryo is mature. The eggs are usually collected into large clumps, containing as many as 100. Several such clumps are found on one male, so that, if Avell laden, he may be found to carry 1000 eggs (Dohrn). Although in such cases, and generally among the Pantopoda, the eggs are very small, they are comparatively large in Pallene (0*25 mm. in diameter), being, for example, 125 times the size of an egg of PhoxichiUdium or Tanystylum (Morgan). Pallene carries only a few glutinous egg-clumps, each containing only two eggs (Dohrn). Nymphon, according to Hobk, has specially large eggs (in N. brevicaudatum, 0*5 to 0*7 mm. in diameter), but yet carries a great number. The large eggs are very rich in yolk, the smaller ones naturally have less yolk. The eggs are spherical, and each is surrounded by a delicate membrane (Fig. 64, D). 1. Cleavage and Formation of the Germ -Layers. The cleavage of these eggs is total (Dohrn, Hoek, Morgan) ; but those genera which have small eggs (e.g. PhoxichiUdium and Tanystylum) show equal cleavage, those with larger eggs (Pallene, Nymphon) unequal cleavage (Morgan). Up to the present time but little has been known concerning the early ontogenetic processes in the Pantopodan egg. Many years ago (1843) Kolliker gave an account of the total cleavage of the egg, and Dohrn has more recently described a few stages in the cleavage of the eggs of Pycnogonum that confirm the above conclusion. Hoek, in examining the Challenger material, found a few ontogenetic stages, which, however, could naturally only give a very incomplete idea of 1 40 PANTOPODA. the development of the embryo. Hoek afterwards tried to complete his account by means of observations made on living specimens (Pdllene, No. 7). Morgan next investigated the cleavage and formation of the germ-layers in these animals (Nos. 10 and 11), and in a more recent work (No. 12) he gives a detailed description of these processes in several Pantopodans. We shall thus have to rely chiefly on his account of these processes. In Pallene, the first line of cleavage divides the egg up into two blastomeres, one of these being large, and the other only about a quarter of its size (Morgan). Each of the two spheres is again divided into two by a cleavage taking place at right angles to the first, so that two micromeres and two macromeres are now formed. The third line of cleavage is perpendicular to the two former lines, and gives rise to four micromeres and four macromeres. This stage is followed by one of eight small and eight large cleavage-spheres. From this point onward the micromeres and the macromeres do not divide at the same rate. At a later stage, sections present an appearance like that given in Fig. 65 A, except that the pole of the micromeres consists of smaller cells .than are there figured. The cells are pyramidal, but their boundaries do not in all cases extend to the centre. We here find an indication of transition to the next im- portant stage. Unequal cleavage seems also to occur in the eggs of Nymphon brevicaudatum, which are rich in yolk, for, according to Hoek's figure (Fig. 2, PI. xix., No. 6), one half of the egg at a late stage is composed of smaller cells than the other half. The nuclei of the pyramidal cells, together with the surrounding protoplasm, shift to the periphery (Fig. 64 A and B), the boundaries of the blastomeres being retained to a certain extent (dp). To some degree, however, they disappear, this being specially noticeable towards the centre of the egg (A and B). The nuclei are surrounded by areas of protoplasm, which send out processes into the yolk. Since these complexes of protoplasm, increasing in number by division and shifting closer together, yield the blastoderm (Fig. 64 C), a stage like that seen in the Araneae is passed through, i.e., the yolk- mass appears first divided up into pyramids which disintegrate later. According to Hoek's description, this breaking up of the yolk can still be distinguished in later stages after the blastoderm has formed (cf. figure of Nymphon brevicaudatum, No. 6, PI. xix., Fig. 5). In the centre of the egg a cavity appears (Fig. 64 A, fh), which must be regarded as the cleavage-cavity. Its occurrence, however, does CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 141 not seem to be constant (Morgan), and in any case it soon disappears again. This also, if proved to be correct, would constitute a certain analogy with a condition described for the Araneae (p. 39). In 35. Fig. 64. — Sections through eggs of PaUene in various stages of blastoderm-formation (after Morgan). In D, an imagination (e) appears in the blastoderm, round which cells (probably the first mesoderm-cells) are given off. bl, blastoderm ; d, yolk ; dp, yolk-pyramids ; e, aperture of the invagination ; eh, external and internal integument ; fh, cleavage-cavity (?) them also there is a transition from total to superficial cleavage. There is also a concentration of the blastoderm towards that pole at which later the first indications of the embryo appear (Fig. 64 C). 142 PANTOPODA. The peripheral cells, which were also formerly present at the opposite pole (A and B), disappear. At a time when the blastoderm only partly surrounds the egg, a few cells of amoeboid form are seen lying below it (Fig. 64 C). According to Morgan, cells are given off first at the pole of the micromeres, and then at other parts of the periphery. These cells arise by division of the pyramidal blastoderm-cells in a tangential direction, a process which Morgan, comparing it with the result of observations on other Pantopoda, considers to be one of delamination ; a lower cell-layer forms, which is no doubt to be regarded as the entoderm. This view does not appear sufficiently supported by the facts as yet known, and Morgan's observations have made possible another assumption with regard to the formation of the germ-layers. At the pole of the egg that is richer in cells a thickening appears, which has been compared by Morgan to the primitive cumulus of the Araneid egg (p. 42). A depression then appears at this point (Fig. 64 D, e), and from this an active proliferation of cells takes place. Morgan himself regards this as the formation of the meso- derm, and believes that some of the amoeboid cells which grow into the yolk are also of entodermal nature. The two germ-layers are not yet distinct from one another. In any case, the whole process shows great similarity to the formation of the germ-layers in the Araneae. Amoeboid cells are formed which grow into the yolk, and give rise later to the enteron. That some of the cells which originate near the invagination represent the rudiment of the mesoderm cannot be doubted. These cells soon increase greatly in number, and become arranged into two bands, the mesoderm-bands. The invagination which, on account of its relation to the formation of the germ-layers, might be regarded as the blastopore, is held by Morgan to be the stomodaeum. The two genera Tanystylum and Phoxichilidium, possess smaller eggs less richly provided with yolk, and these differ in their develop- ment from the larger eggs just described, inasmuch as they undergo equal cleavage, by means of which the egg breaks up into two, four, eight, and sixteen blastomeres of equal size. In consequence of this, the pyramidal cells of a later stage are also approximately equal in size (Fig. 65 A). The fact that the yolk contained in such an egg is smaller in quantity than in the other egg leads to a difference in the further development. An actual blastoderm is not at first formed, as in Pallene, but forms later by a process of delamination (Fig. 65 B). A cleavage-cavity also seems to arise, as may be seen in Fig. 65 B. CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 143 Each of the pyramidal cells divides tangentially into an inner and an outer cell, both of these cells then continuing to divide. The outer cells form at the periphery a regular layer, the blastoderm (Fig. 65 C, bl), while many of the inner cells lose their regular boundaries. A yolk-mass thus arises in which isolated cells lie (C, d and z). The inner cells, which were evidently the richer in yolk, have now fused to form a common mass. The embryo thus shows a condition similar to that of other Arthropoda, e.g., the Araneae, there being a peripheral layer of cells (the blastoderm) and an inner yolk-mass with cells distributed in it. The latter, indeed, arise in a different way in the Pantopoda, as is shown in Fig. 65 B. The formation of the germ-layers could not be more exactly made out in eggs with equal cleavage, but Morgan assumes that the enteron is formed from the inner cells (the entoderm). In these forms also, Morgan early a. D6. C. Fin. 65.— Sections through eggs of Tanystylum (A and B) and Phoxichilidium (C) in the final stage of cleavage (A) and in the stage of delamination and blastoderm-formation (B and C) (after Morgan), bl. blastoderm ; d, yolk-mass ; r, the cells which become detached from the peripheral cells (blastoderm) and shift inward. noticed a depression of the peripheral cell-layer, which, like the depression already described in Pallene, he regarded as the rudiment of the stomodaeum. This depression is triangular, a fact which has led to its being compared with the triangular stomodaeum of the Araneae. In view of the comparatively slight knowledge which we possess of the first ontogenetic processes in the Pantopoda, it would be too presumptuous to try to form further conclusions. It has already been mentioned that a certain agree- ment with the conditions in the Araneae exists. The splitting of the blastoderm into two layers maintained by Morgan recalls the processes in the Pseudo- scorpiones (p. 28) ; but these also are too little understood to allow of further comparison. The commencement of development and the further differentiation at the one pole might be compared to the formation of the germ-layers in the 144 PANTOPODA. Araclmida over a limited area of the blastoderm. Morgan states very definitely that this budding-off of an inner layer of cells or multipolar delamination takes place in Pallene slowly, while the outer layer of cells is growing round the yolk ; this we might perhaps refer to an ingrowth of cells with a circumcrescence of the yolk, and compare this process with the corresponding one in the Scorpion. It is advisable to direct the attention of future observers to this point. When the depression of the blastoderm described above appears, the entoderm, according to Morgan, is already formed ; the depression could not, therefore, be compared with the blastopore, although in other respects such a comparison is suggested, all the more so that Morgan thinks that the mesoderm arises round this depression. The fact that other processes in the Pantopoda resemble those in the Araclmida is proved by the formation of a germ-band which, however, is much degenerated, but at the same time shows a certain resemblance to that of the Araclmida. Eggs rich in yolk no doubt represent the more primitive condition in the Pantopoda, and the formation of a blastoderm (of the usual Arthropodan consti- tution) and of a germ-band must also be regarded as primitive. The reduction of the yolk probably had a great influence on the ontogenetic processes, which thus attained the condition in which they are now found (p. 154). 2. The Further Development of the Embryo. Our knowledge of the development of the embryo and the origin of its organs is still very incomplete. The following accounts refer chiefly to Pallene, which was made the subject of careful investigation by Morgan. We must, however, point out that Pallene, unlike other Pantopoda, remains within the egg almost up to the time when the adult form is reached (pp. 148 and 153). When the invagination already mentioned has appeared on the thickened side of the blastoderm, other thickenings of the surface take place. Two of these are oval in form (Fig. 66, g), and lie in front of the triangular depression (to). These represent the rudiments of the supra-oesophageal ganglion. Extending posteriorly from the invagination are two rows of thickenings ; these are the rudiment of the ventral chain of ganglia (gu-gjy) ; laterally to these the first indications of the limbs appear as distinct thickenings (Fig. 67, A). These rudiments, taken as a whole, form a band on the ventral surface of the egg, narrow anteriorly, but broader posteriorly, which may with safety be compared with the germ-band of other Arthro- pods. As the yolk-mass is not very large, the germ-band covers a great part of the egg. As the embryo develops further, it extends laterally, covering a still larger part, so that it can no longer be designated as a distinct germ-band, but rather as the embryonic rudiment surrounding the egg. During this process the embryo has also grown somewhat longer (Fig. 67 A). THE FURTHER DEVELOPMENT OF THE EMBRYO. 145 The order of appearance of the limbs varies in the different forms. In Pallene, according to Morgan, the first to develop is the most anterior pair; these limbs lie near the mouth and are chelate, but their first appearance has not been observed with certainty. The next pair to arise is the fourth, and, in the gap which naturally occurs between these limbs, two pairs of ganglia are visible, those of the second and third pairs of limbs (Figs. 66 and 67 A). The fourth pair of limbs is followed by the fifth and sixth. The third pair develops later, but the second pair is alto- gether wanting in Pallene, and the seventh pair, like the third, appears a short time before the embryo leaves the egg-envelope. Pallene is thus seen to possess, as an embryo, all the limbs of the adult. In most other Pantopoda, however, this is not the case, only three pairs of limbs being usually developed within the egg-envelope. Nymphon brevicaudatum resembles Pallene in possess- ing all the limbs of the adult at the time of hatching (Hoek). While the limbs are appearing and gradually developing, the rudi- ment of the nervous system also undergoes further differentiation. Five pairs of large ganglia can be distinctly made out (Fig. 67 A). They belong to the segments carrying the second to the sixth limbs. It would be interesting to discover the relations of the ganglia which innervate the first pair of limbs to the supra-oesophageal ganglion, i.e., whether they represent a post-oral pair of ganglia fused with the supra-oesophageal ganglion. The two anterior of these five pairs of ganglia approximate closely to each other later (Fig. 72 B), and in the adult these two ganglia, belonging to the second and third limbs, are united. The ganglia of the first three pairs of ambulatory limbs (Fig. 67 A), which appear early, are followed at a much later stage by those of the fourth pair (the seventh pair of limbs) and the abdominal ganglia. In each of the ectodermal thickenings which represent the rudi- Fig. 6C— Superficial aspect of an egg of Pallene, showing the anterior part of the embryonic rudiment (after Morgan), g, rudiments of the supra-oesophageal ganglia ; gji-gjy, ventral ganglia belonging to the segment carrying the second, third, and fourth pairs of limbs ; m, mouth ; I, first limb ; IV, rudiment of the fourth limb. 146 PANTOPODA. ments of the ganglia a pit-like depression appears (Fig. 67 A and B, e), round which the cells of the thickening show a regular epitheloid arrangement (Morgan). An ectodermal depression thus takes part in the formation of the ganglion. The invagination closes later, but its cavity can still be recognised after the ganglion has shifted inward, and has lost its connection with the ectoderm (Fig. 68, e). "When the two anterior pairs of ganglia unite they appear as a single pair, in which, however, there are four pits, which proves that this one pair is composed of two. Morgan's statement as to the participation of ectodermal invaginations in the formation of the ventral ganglia is so definite, that we do not seem justified in doubting this fact (cf. Figs. 67 a. and 68). He himself compares these structures with the ventral organs of Pcripatus (p. 189), and there is no doubt a certain similarity between the two ; but it must be pointed out that the ventral organs are by no means in such direct connection with the ganglia as are the depressions in the Pautopoda. A participation of an ectodermal invagination similar to the above in the formation of the brain cannot be established, although it is just here that we should expect it, when we take into account the cerebral pits in the Arachnida. The development of the ex- ternal shape of the body is com- pleted by the addition of the missing appendages, the length- ening of the embryo, and the commencement of segmentation. The first pair of limbs shifts anteriorly and dorsally. At its base, the proboscis or beak appears to arise as an unpaired anterior outgrowth of the body, carrying the mouth at its extremity. At the posterior end of the body, the vestigial abdomen appears as a small pointed appendage, at the end of which the anus forms. The first of the internal organs to claim attention is the enteron. The entoderm has become arranged into an epithelium surrounding the yolk-mass (Fig. 68, ent), and from this, diverticula, also filled Fio. 67.-^4, embryo of Pallene empusa, seen from the ventral side. B, part of a trans- verse section through the same, to show the paired depressions (c) on the ventral surface (after Moroan). I- VI, limbs; bg t ventral chain of ganglia, the depressions (e) being visible in the ganglia ; ect, ecto- derm ; ent, entoderm ; mes, mesoderm. THE FURTHER DEVELOPMENT OF THE EMBRYO. 147 with yolk, grow out at an early stage into the limbs (di). These are the intestinal caeca, which, in the larva (Fig. 72 A), as well as in the adult, run far into the limbs. This arrangement recalls that in Chelifer, where the yolk also extends far into the limbs (p. 29, and Fig. 16). This is also the case in the Acarina, and in the embryos of some Araneae, e.g., Agalena (Locy). This peculiar feature is known to be retained throughout life in the Pantopoda, in which the trunk is much reduced as compared with the limbs. These latter also contain the genital organs in the adult, and this explains the fact that a process of the mesoderm at an early stage runs between the ectoderm and the entoderm into the rudiment of the m«s* Fig. 68. — Transverse section through an embryo of Palknc empusa at a somewhat older stage than in Fig. 07 A. The ventral depressions (c) have closed (after Morgan). lg, ventral nerve-strand showing fibrous structure on the dorsal side ; coe, mesodermal cavity in the limb ; d, yolk ; di, intestinal caeca of the limbs ; e, the closed ectodermal invagination ; ect, ectoderm ; cut, entoderm ; roes, mesoderm ; p, pair of ambulatory limbs. limb. According to Morgan, a cavity bordered by a mesodermal •epithelium lies at the base of each limb, the mesodermal process extending from this point into the limb (Fig. 68, mes). Morgan does not hesitate to speak of the body-cavity of the limbs. In any case we thus have here the primitive segments which, taken together, represent the two already segmented mesoderm-bands. These latter, together with the rudiments of the ganglionic chain and the limbs on each side, form the germ-band (Fig. 66), although this is con- siderably reduced in accordance with the small size of the egg. As these mesoderm-bands develop at the thickened part of the blastoderm, 148 PANTOPODA. the region beneath which the mesoderm extends may be regarded as the germ-band, the Pantopoda, as has already been pointed out, agreeing in this respect with other Arthropods. Should the appearance of primitive segments and their extension into the limbs be confirmed, a strong resemblance to the Arachnida would be established. Pcripatus, indeed, and many of the Insecta, show the same arrangement, but we do not feel confident in laying so much stress either on this or on the similarity to the ventral organ which Morgan specially points out. Trans- verse sections of embryos of Pallene (Morgan) and of Nijmplwn (Hoek) show unmistakable similarity to sections of a spider. The further development of the mesoderm, its relation to the adult body-cavity, and the formation of the heart, have not yet been ascertained with sufficient certainty. The heart appears in the dorsal middle line after the mesoderm has already given rise to a number of schizocoele-like cavities. More accurate accounts of the participa- tion of the primitive segments in these processes (the further differentiation of the mesoderm and the formation of the heart) would be of great interest. The mesodermal tissue with its cavities increases in extent as the yolk-mass degenerates. The latter is absorbed by the surrounding entodermal epithelium. Yolk-cells do not appear to play any special part in this process, and may, indeed, be wanting. The enteron becomes connected with the stomodaeum, which is derived by Morgan from the invagination already mentioned as appearing very early. The proctodaeum does not appear until very late, when the seventh pair of limbs and the abdomen form. The Form of the Larva and its Transformation into the Adult. The Larva. Most of the Pantopoda develop through metamor- phosis. The larvae usually have three pairs of limbs, but some leave the egg in a more advanced condition ; the young Pallene y for instance, when hatched is provided with all the limbs of the adult, and this higher stage of development is also attained in the egg by a few species of the genus Nymphon. The various species of this genus differ from each other in this point; in some of them the larva, at hatching, has only four or five pairs of limbs (Hoek). The many Pantopodan larvae that have been described differ only slightly from each other, and are easily derived from a larval form provided with three pairs of limbs. This form, which was first THE FORM OF THE LARVA. 149 carefully examined by Dohrn, has a compact body (Fig. G9), some- times almost square, or else rounded, seldom long or oval. The body is not externally segmented, although it carries three pairs of limbs ; in this respect this larva bears a certain resemblance to the Crustacean Nauplius. It has been compared with the latter, but the resemblance is merely superficial. Fig. 69. — Larva of Achclia lacvis immediately after hatching (after Dohrn). 7-777, limbs; bg, strands of connective tissue ; d\ spine on limb 7 with gland (dr) ; da, enteron ; /, filament of the glandular secretion ; g, brain (with the eyes above it) ; m, muscles ; s, proboscis ; v, vacuoles in the gland. The larva, as already stated, is supplied with three pairs of limbs. The most anterior limb has three joints, and is chelate. At its base it has a movable spine (Fig. 69, d), which, in other genera, is considerably longer than in the larva of Achelia depicted in Fig. 69. This appendage brings about a certain similarity to the biramose limbs of the Crustacea, but we would not lay 1 50 PANTOPODA. any great stress upon this point. A tolerably large spine, comparable with that on the first limb, also occurs on the two following limbs (Fig. 69). That on the first extremity, however, is distinguished from the others by having at its point the aperture of a gland (dr). The fine filaments which can be produced through this aperture serve for attaching those larvae which, after quitting the egg-envelope and undergoing the first moult, fix themselves on the ovigerous limbs of the male. The second and third pairs of limbs possess hooks only (Fig. 69, 77. and 777.). The muscles of all the limbs, especially the first, are well developed. Whereas the first are used for fixation, and especially for prehension, the two posterior pairs are used for crawling and climbing. These larvae live among algae, Hydroids, etc. Another feature of the external organisation of these larvae is the proboscis, or beak, which arises as a ventral conical outgrowth between the bases of the anterior limbs (Fig. 69, s). At its tip lies the oral aperture. It appears as if the proboscis arose near the stomodaeum as an ectodermal outgrowth, although some have been inclined to attribute its origin to fusion of the upper lip with a pair of limbs (Adlehz). It is impossible to decide whether we are justified in comparing it to the provisional proboscis of Clielifer y which it cannot fail to recall, on account of the slightness of our knowledge of this latter organ. The intestine is already provided with outgrowths, the anterior pair of which are beginning to extend into the first pair of limbs (Fig. 69). From the intestine, fibres of connective tissue extend to the body-wall. The anus does not yet seem to be present (Dohrn), and no doubt does not appear until later with the rudiment of the abdomen (Fig. 71 B). The nervous system of the larva consists of the supra-oesophageal ganglion and only two pairs of ganglia on the ventral side. Im- mediately above the supra-oesophageal ganglion lie the two eyes in close contact (Fig. 69). The manner in which these arise is of special interest, as it appears to offer a further point of agreement with the Arachnida. The eyes, like the nervous system, attain full development during metamorphosis. The two eyes of the former stage are now joined by another pair. So as to understand how these develop we shall have to explain briefly the structure of the Pantopodan eye, which is as yet very insufficiently understood. These eyes, like those of the Araneae, consist of a corneal lens, a subjacent hypodermis DEVELOPMENT OF THE EYE. 151 (vitreous body), a thick layer of retinal cells, and a layer of pigment behind the whole. In the retina, the cell nuclei lie in front of the rods ; these latter therefore belong to the posterior part of the cells, and thus come into direct contact with the pignient-layer (Fig. 70, st). The nerve-fibres become connected, however, with the outer ends of the visual cells, so that here also the same conditions prevail as are found in the posterior middle eyes and the lateral eyes of the Araneae (Fig. 34, p. 64). This last point, which seems to be implied in the description given by Hoek, has recently been established by Morgan (No. 12). The ontogenetic stages, as well as the adult structure, closely resemble those of the Arachnida, as may be seen by comparing Fig. 70 with the ontogenetic stages of the eyes of the Scorpiones and the Araneae illustrated in Fig. 10, p* 14, and Fig. 35, p. 65. In Fig. 70, an invagination extending from one side below the hypo- dermis is suggested. The retina and the pigment-layer thus arise, and out of the superjacent hypodermis the layer forming the vitreous body, which secretes the lens on its outer side. An inversion thus takes place in the formation of the eye, and its innervation would be from the first explicable in the same way as that of the eye of the Araneae. In earlier stages in the development of the eyes, an invagination is not so distinctly recognisable as in the eyes of the Araneae. The different layers of cells lie somewhat close to one another, and Morgan assumes that no actual (complete) invagination takes place, but rather that new cells are continually being added to the eye from the point of ingrowth, and that thus finally the layers, like those in the Arachnid eye, are formed (Fig. 70). A thicken- ing of the hypodermis, which appears laterally to the eye, perhaps yields the new cell-material. This hypodermal thickening recalls the one found near the Crus- tacean eyes and those of Limulus (Vol. II., pp. 280 and 359). The development and the structure of the Pantopodan eyes suggests through- out a comparison with those of the Arachnida. Morgan's statement that the ettr r Fig. 70. — Longitudinal section through one of the posterior eyes of the larva of Tanystylum (after Morgan), c, cuticle ; cct, hy, ectoderm (hypodermis) ; gl; vitreous body ; pi, pigment- layer ; r, retina ; st, rods. 152 PANTOPODA. rods arise through fusion of the rods of two neighbouring cells, makes the similarity appear still more striking, and leads to the same conclusion in both cases ; viz. , to a derivation of these apparently simple eyes from compound eyes. Our knowledge of the eyes of the Pantopoda is, however, still too slight to allow of any definite conclusions ; Morgan even adopts an altogether opposite view, and explains the inversion which in all cases is present in these eyes, by the degeneration of the posterior part of an optic invagination and the better development of the anterior part. In this way he derives the inverted Pantopodan eyes from such simple eyes (ocelli) as those of the Insecta, being guided in this decision chiefly by a certain bilateral symmetry in the Panto- podan eye. But that method of development as it appears in the ontogeny of the eye, i.e., the growth of the invagination towards one side, is merely a caenogenetic process, and serves for the quicker attainment of the structure now possessed by the adult eye. It has the significance of an abbreviated develop- ment. As a logical consequence of this view, a corresponding assumption must be made for the Arachnid eyes. We cannot here examine Morgan's conclusions Fig. 71. — Larvae of Tanystylum in two different stages, seen from the ventral side (after Morgan), a, anus ; abd, abdomen ; ig, ventral chain of ganglia ; m, mouth ; s, proboscis ; I-IV, first four limbs. more closely, but refer to the original treatise and to our own view of the eyes of the Arachnida given above (p. 68). On the other hand, it must be mentioned that the description recently given by Claus (No. 2) of the origin of the median eye in the Crustacea, involuntarily recalls the condition of the eyes in the Pantopoda. The median eyes of the Crustacea are said by Claus to arise by inversion, and seem to have their elements arranged like those of the Pantopodan eyes. The rods lie on the inner side, directed towards the pigment- cup of the eye. while the nerve-fibres join them from the opposite side, where also lie the nuclei of the retinal cells. The principal change which brings about the transformation of the larva into the adult is the formation of new segments at the posterior part of the body. The limbs already present either pass over directly to the adult, merely growing and developing further, or TRANSFORMATION OF THE LARVA INTO THE ADULT. 153 some of them, usually the second or third, and in many cases both of these or even all the three anterior pairs temporarily degenerate, the corresponding adult limbs growing out at the same points (Dohrn, Hoek). In Pallene the second pair is wanting, and does not even occur as a vestige, while in Tanystylum the first pair is wanting as a functional appendage, but appears ontogenetically as a Avell-developed pineer-carrying limb (Fig. 71 A and B), and only gradually degenerates in the later larval stages ; it is still present in the adult as a small, vestigial two-jointed bud (Morgan). The position of the second and third pairs of degenerated limbs is marked by the appearance of the apertures of what are presumably excretory glands (coxal glands). The tubular spine of the first limb, through which the above gland opens, is thrown off in one of the moults and gives place to an ordinary short spine. It has therefore the significance of a larval organ. The first indication of the formation of new body-segments is, according to Dohrn, found in a paired swelling of the intestine behind the last of the larval limbs, accompanied by a bulging of the body-wall. At the same time, in the posterior part of the ventral surface, a thickening of the ectoderm appears which is the rudiment of a new pair of ganglia. The ectoderm begins to become wrinkled in the posterior part of the body and rises up above the newly- formed lower layer. The larva now moults, after which it is evident that a limb has appeared on the bulging of the body-wall just mentioned ; into this limb an intestinal caecum is continued. It is thus clear that this is a new limb, which soon develops and becomes jointed (Fig. 71 A and B). The other limbs form in the same way. Only when the body thus lengthens do the three anterior pairs of limbs also take part in the transformation (Dohrn). The short abdomen arises as a posterior sac-like swelling, and the anus appears upon it (Fig. 71 B). The transformation of the six-limbed larva just described takes place in some forms, as has already been mentioned, within the egg-envelope ; Nymphon breuicollum, for example, leaves the egg when provided with five well-developed pairs of limbs (Fig. 72 A and B), and the first rudiments of a sixth pair. Other points of its organisation, especially the shape of the limbs with the intestinal caeca extending far into them, can be made out without further assistance from Figs. 72 A and B. The young of Nymphon brevicaudatum possess all the limbs at hatching (Hoek), and the same condition is found in the genus Pallene (Dohrn, Morgan). 154 PANTOPODA. During metamorphosis, the rudiments of the genital organs which were not observed in the six-limbed larva become recognisable. In the larva with four pairs of limbs (Fig. 71 B), a compact mass of cells, the first rudiment of the genital gland, lies in the median line dorsally to the enteron, somewhat near the fourth pair of limbs (or first ambulatory limbs). The anterior end of this mass splits later into two parts, which grow out towards the bases of the limbs just mentioned. The posterior end of the germ-gland then splits in the same way, the genital tubes which run into the limbs thus arising. The wide tubular rudiment of the heart has formed at the anterior part of the body, also from mesoderm cells, dorsally to the genital rudiment, and thus directly beneath the integument. Fig. 72.— Larvae of Nymphon brevicollum soon after hatching. A, dorsal, li, ventral aspect (after IIoek). I-V, the five anterior limbs; bg, ventral chain of ganglia; d, yolk-mass; dl, diverticulum of the yolk-filled enteron in the limb; g, brain ; s, proboscis. The differences observed in Pallene and Nymjihon give rise to the question as to which method of development is to be considered the more primitive among the Pantopoda ; in this respect the appearance of larval organs and the casting of a larval integument, observed by Dohrx in Pallene, suggest that the direct development of this form must be regarded merely as an abbreviation of the indirect method of development, and that the latter is the more primitive. In consequence of the more complete development of the embryos in the egg the latter must have a richer supply of nutritive material. The large amount of yolk in the eggs of Pallene and Nymplwn would, under these circumstances, appear as a later acquisition, and it then seems doubtful whether we ought to ascribe to the first ontogenetic processes of these eggs a truly primitive character, although we feel inclined to do so on account of the greater resemblance of their development to that of other Arthropoda. The course of development in Phoxichilidium differs from that of other Pantopoda in that the form of the larva undergoes THE LARVA OF FHOXICHILIDIUM. 155 considerable degeneration before passing into that of the adult. This is connected with its parasitic manner of life. On leaving the egg, the larva of Phoxichilidium possesses on the whole the organisation of the usual sixdimbed Pantopodan larva, but is distinguished from the latter by the fact that the usually hookdike terminal joints of the two posterior pairs of limbs are Fig. 73. — Various larval stages of Phoxichilidium, (after Dohrn, Semper, and Adlerz). A, free larva with the tendril-like flagellae on the two posterior pairs of limbs (77 and 777). B-D, larval stages found in Hydroid polyps. (A is more highly magnified than the other figures.) 7-777, limbs ; d, intestine with its caeca ; dr, glands of the first limb ; h, larval integument in the act of becoming detached ; n, ventral chain of ganglia ; s, proboscis. much lengthened, and form long flagellae, which can coil up like tendrils (Fig. 73 .4). These flagellae, which may be much longer than those represented in Fig. 73 (e.g., in Phoxichilidium femoratum, (Hoek)), are probably used for attachment, the larvae winding them round the Hydroids (e.g., Hydractinia, Podocoryne, Tuhularia, 156 PANTOPODA. Plumularia), which are chosen by them as hosts. Dohrn assumes that the larvae, after attaching themselves to the Hydroids by the help of the flagellae, throw off during a moult the two posterior pairs of limbs that carry the latter, and pass through the oral aperture of the polyp into its gastral cavity. They are certainly found later in such a position, and here pass through the further stages of their development. The tendril-like flagellae seem not to occur in all Phoxichilidia, for R. von Lendenfeld lias described a larva of Ph. plumulariac not distinguished in any way worth mentioning from the usual Pantopodan larva. This larva further differs from other Phoxichilidia in its manner of life ; it does not penetrate into the polyps, but only attaches itself to them by the help of its pincers and by burying its beak in the host's body at the base of the head. The larva remains in this position until it has almost attained the form of the adult animal. "VVe may gather from v. Lendenfeld's description that the further development of the forms discovered by him takes place as in other Phoxichilidia, for he also mentions a two-limbed stage. It has already been stated that the larvae cast off the flagellae and limbs at ecdysis (Semper, Dohrn). The larva moults several times (Fig. 73 B), the second and third pairs of limbs degenerating completely (Semper) ; but, according to Adlerz, some vestiges of the posterior pairs are retained (Fig. 73 C and D), and it is in place of these that the second and third limbs of the adult arise. The larvae, several of which often occur in one polyp, with their large anterior limbs, have a very peculiar appearance (Fig. 73) in this stage. In the following stages the limbs are found to degenerate still further (this is also evident from the figure given by Adlerz), but, with the bulgings of the intestines, the rudiments of the posterior segments begin to appear. The ganglia of these develop and the outgrowths of the body-wall which yield the limbs soon appear (Semper, Adlerz). These processes seem, on the whole, of the same nature as those before described. When three pairs of ambulatory limbs have formed, and the fourth is present as a rudiment, the young Phoxichilidium leaves the polyp and leads a free life. 4. General Considerations. Although much has been written as to the relationships of the Pantopoda, these are still far from clear. The ontogeny of these animals, as far as it is now known, unfortunately throws little light upon the subject. In comparing the Pantopoda with other divisions of the animal kingdom, attention is turned chiefly to the Crustacea and the Arachnida. The form of the larva is of the greatest GENERAL CONSIDERATIONS. 157 importance in a comparison with the former, while in comparing the Pantopoda with the Arachnida the shape of the adult receives more attention. It cannot be denied that the whole appearance of the Pantopoda suggests a certain similarity to the Araneae. But a nearer comparison at once reveals a difficulty, inasmuch as the Pantopoda possess one pair of limbs more than the adult Arachnid. An attempt has been made to overcome this difficulty by considering the first two pairs of limbs of the Pantopoda (Fig. 74, 1 and 2) as ecpuivalent to the chelicerae and the pedipalps of the Arachnida, and the third to the sixth limb of the former as equal to the ambulatory limbs of the latter (Fig. 74, 3-6). The ovigerous limbs (Fig. 74, 3) would thus represent the first pair of ambulatory limbs of the Arachnida, and the seventh limb would be the homologue of Fig. 74. — Male of Nymphon hispidum seen from the ventral side. The setae are omitted (after Hoek, from Lang's Text-book). 1-7, limbs ; aft, abdomen ; s, proboscis. the first pair of abdominal limbs. In view of the fact that in the Insecta an abdominal segment is separated from the posterior part of the body, and may enter into the closest relation to the thorax, such a view might be defended. Those Avho adopt it consider that the addition of another pair of limbs to those already specialised for locomotion was determined by the withdrawal of the third limb from the ambulatory series for use in the care of the brood. According to this view, the four pairs of ambulatory limbs of the Pantopoda would not be homologous with those of the Arachnida. The last homology must, however, be regarded as possible, and in that case the loss of an anterior limb in the Arachnids would have to be assumed. It has already been pointed out (p. Ill) that the rostrum has been conjectured to represent a pair of limbs. 158 PANTOPODA. Iii carrying further these attempts to homologise the limhs, this last assumption leads to certain difficulties as to the position of those now under consideration. A careful examination of the various views held on this subject, which are all more or less speculative, would lead us too far, but we must draw attention to the fact that the ovigerous limbs have by some been regarded not as independent limbs, but as belonging to the second limb. Schimkewitsch, who adopted this view (Nos. 14 and 15), in defending it laid weight on the fact that the rudiments of the pedipalps in the embryos of the Araneae are biramose (pp. 52 and 112). Each of the branches is said to give rise to a limb. This view is not supported by ontogeny, since, in the Pantopodan larva, the second and third limbs arise quite separately. Just as little does ontogeny support the view that the tripartite proboscis of the Pantopoda arises through the fusion of a pair of limbs with an (unpaired) upper lip. A third pair of limbs would then be added, for it cannot be assumed that the paired pieces are merely parts of one limb. The loss of two pairs of limbs by the Arachnida has even been suggested (Croneberg, p. 111). The ontogeny of the Pantopoda seems to show that the beak is, as Dohrn assumes, only an outgrowth of the lips of the stomodaeum. The number of the ganglia corresponds to that of the limbs, Adlerz, indeed, finds (in the adult), besides the ganglia of the second and third limbs, another pair which innervates the proboscis. A final decision on this point will only be possible when the ontogenetic conditions are clearly established. The first limb is innervated from the brain, while the second and third limbs receive their nerves from the first and second ventral ganglia. It would be of the greatest importance to make certain whether an originally post-oral ganglion unites with the brain, as in the Crustacea and the Arachnida. If this is not the case, the limbs lost in the Arachnida must be considered to be the first limbs of the Pantopoda, and their homologues must be sought in the conjectural rostral limbs of the Arachnida. It does not, however, seem probable that the first chelate limbs should be true antennae, and consequently not comparable to the chelicerae of the Arachnida. We have already several times pointed out various resemblances between the development of the Pantopoda and that of the Arach- nida, but these do not appear to us sufficient to lead to further conclusions as to the relationship of the two groups. Morgan, chiefly on account of his ontogenetic researches, has recently spoken in favour of such a relationship. It appears to us that, in taking up this position, he was largely influenced by the structure of the Pantopodan eyes. But Claus has recently shown (Vol. ii., p. 167, and Vol. iii., p. 115) that the median eyes of the Crustacea also arise by invagination, and that their component parts apparently have the same position as those of the Pantopodan eyes (No. 2), so that in this character there is similarity to the Crustacea just as much as to the Arachnida. In assuming the loss of an anterior limb, we are obliged to shift back any connection between the Pantopoda and the Arachnida to very early times in the history of the Arthropoda, before the THE FORM OF THE LARVA. 159 Arachnid a arose from forms nearly related to the Xiphosura, for the Arachnida agree with the Xiphosura in many more points than with the Pantopoda. If we must remove the union of the two to such a remote period, the few points of comparison again lose their significance, seeing that they refer chiefly to the more highly developed forms and not to the lower forms. To derive the Pantopoda directly from the Arachnida, however, seems im- possible, the latter having attained far too high a grade of organisation to allow of such a derivation. Even if the Pantopoda were originally related to the Arachnida or some other segmented form, they have in their whole organisation become far removed from it, and have become markedly specialised. The decided preponderance of the limbs over the trunk, and the almost complete degeneration of the latter (Fig. 74) determined the displacement of the internal organs (intestinal caeca and genital glands) into the limbs. The opening of the genital organ on the second joint of the limbs is probably a consequence of this change, and thus has a secondary character. In those cases in which the genital apertures are found, not on several limbs, but only on the seventh pair, as in Pycnogonum, we might be inclined to derive this condition from that in Limulus and the Arachnida, in which the genital apertures lie in the first [second] abdominal segment, and to regard it as primitive, but such an assumption is not supported by any convincing evidence. The reduction of the trunk as compared with the limbs becomes still more marked through the degeneration of the abdomen. The latter is merely a short, truncated appendage of the body (Fig. 74), but the presence of two pairs of ganglia (Dohrn) shows that it originally consisted of more than one segment. In Ammothea and Zetes, the abdomen shows externally a division into two parts, and in some other Pantopoda evidence of a larger number of segments (three to seven) is said to be forthcoming (Hoek, Xo. 7, pp. 453 and 454). Should the Pantopoda prove to be connected at the roots with the Arachnid stock, they would thus in a certain way be related to the Crustacea. The latter, however, appear to us to be too far removed in structure to admit of any relation between the Panto- podan larva and the Nauplius. Those recent observers who have most thoroughly studied the ontogeny of the Pantopoda cannot find any close relation between the two. Hoek regards the larva as representing the primitive racial form, just as the Nauplius was 160 PANTOPODA. formerly regarded. Dohrn considers it to be, like the Nauplius, a modified Annelid larva, and derives the Pantopoda from forms resembling the Annelida. Morgan, however, is unable to accept this conclusion, but regards it as a secondary larval form which can no longer be referred to the Annelidan larva. It seems to us that this last view might easily be reconciled with that of Dohrn. Dohrn derives the Pantopoda from the Annelida, without relating them to the Crustacea and the Arachnida. He thus regards them as a distinct, independent group, and this is also Hoek's view (No. 7). Morgan, on the contrary, favours the relationship to the Arachnida, a view towards which Schimkewitsch also inclines (No. 15). He attributes the Pantopoda to the same racial form as the Arachnida, but believes that they branched off early and developed in a different direction. The most recent investigator of the Pantopoda, G. 0. Sars (No. 13), does not connect them with either the Crustacea or the Arachnida, but wishes to make them into a separate class. In consequence of all these varying opinions we are unable to define with any degree of certainty the systematic position of the Pantopoda. On the whole, according to the present state of our knowledge, we shall do best to follow the conclusions of Dohrn (No. 4). If, notwithstanding this last decision, we have appeared to place the Pantopoda as next in order to the Arachnida, and to dwell on the possibility of the relationship of these two forms, this was done for practical purposes, since we should otherwise have been obliged to classify them in a less satis- factory manner, because they seem to show some slight similarity in their development, and a convergence in some anatomical points, to the Arachnida.* LITERATURE. Only a few of the many treatises describing the ontogenetic stages of the Pantopoda are here enumerated. The following literature, however, will afford further references. 1. Adlerz, G. Bidrag till Pantopodernas Morfologi och Utveck- lings historia. Bihang till k. Svenska Vetenskap. Akad. Handlingar. Ed. xiii. Afd. iv. No. 11. Stockholm, 1888. 2. Claus, C. Ueber den feineren Bau des Medianauges der Crustaceen. Anz. k. k. Akad. Wiss, Wien, Mai, 1891. No. 12. * [Ihle (App. to Lit. on Pantopoda, No. I.) holds that it is impossible to ally the Pantopoda with the Arachnida or with the Crustacea, but thinks that the Mvriopoda may he regarded as the ancestral stock. He lays special stress on the presence of a pair of abdominal appendages. — Ed.] LITERATURE. 161 3. Dohrn, A. Untersuchungen iiber Bau unci Entwicklung der Arthropoden. 2. Pycnogoniden. Jen. Zeitsclir. f. Natunv. Bd. v. 1870. 4. Dohrn, A. Die Pantopoden des Golfes von Xeapel. Fauna und Flora des Golfes von Neapel. Monographie iii. Leipzig, 1881. 5. Faxon, "W. Bibliography to accompany "Selections from Embryological Monographs." Pycnogonida. Bull. Mus. Comp. Zool. Harvard College. Vol. ix. 1882. p. 247. (Contains the older bibliography.) 6. Hoek, P. P. C. Keport on the Pycnogonida. Voyage of H.M.S. Challenger. Zoology. Vol. iii. 1881. 7. Hoek, P. P. C Nouvelles Etudes sur les Pycnogonides. Archiv. Zool. exper. Tom. ix. Paris, 1881. 8. Hodge, G. Observations on a species of Pycnogon (Phoxichili- dium coccineum) with an attempt to explain the order of its development. Ann. Mag. Nat. Hist. (3). Vol. ix. 1862. 9. Lendenfeld, Pv. vox. Die Larvenentwicklung von Phoxichili- dium plumulariae. Zeitsclir. f. Wiss. Zool. Bd. xxxviii. 1883. 10. Morgan, T. H. Preliminary Note on the Embryology of the Pycnogonids. Johns Hopkins Univ. Circidars. Baltimore. Vol. ix. No. 80. 1890. 1 1 . Morgan, T. H. The relationships of the Sea-spiders. Biological lectures delivered at the Marine Biologiccd Laboratory of Woods Holl. Boston, 1891. 12. Morgan, T. H. A contribution to the Embryology and Phylogeny of the Pycnogonids. Studies Biol. Lab. Johns Hopkins Uvniersity. Baltimore, Vol. v. 1891. 13. Sars, G. 0. Pycnogonidea. Den Norske Nordhavs-Expedition 1876-7S. Bd. xx. Christiania, 1891. 14. Schimkewitsch, W. Etude sur l'anatomie de l'Epeire. Ann. Sci. Nat. (6). Zool. Tom. xvii. 1884. 15. Schimkewitsch, W. Les Arachnides et leurs affinites. Archiv. Slaves Biol. Tom. i. Paris, 1886. 16. Semper, C. XJeber Pycnogoniden und ihre in Hydroiden schmarotzenden Larvenformen. Arb. Zool. Inst. Univ. Wiirzburg. Bd. i. 1874. APPENDIX TO LITERATURE ON PANTOPODA. I. Ihle, J. E. W. Ueber die Phylogenie und Systematische Stellung der Pantopoden. Biol. Centralbl. Bd. xviii. 1898. M CHAPTEE XXIV. TARDIGRADA: The eggs of the Tarcligrada are laid either singly {Macrobiotus Hufelandi) or several together, and are left in the cast-off skin of the mother. In the case of eggs laid singly, the egg-integument is thickly studded with small prominences which render its examination very difficult. When several eggs are laid together the egg-envelope is smooth and transparent. The species investigated by Kaufmann seems to have been comparatively easy to study, and he was able to establish the fact that its cleavage is total and equal, as v. Siebold had already stated. Kaufmann followed the process of cleavage up to the formation of a morula stage composed of cells of about equal size. He then observed the separation of a peripheral cell-layer from the central mass, and the flexion of the embryo which supervenes. The concave side of the embryo seems to correspond to the ventral surface, for the limb-rudiments here arise on the two sides. Two pairs of prominences appear first ; these are apparently the two anterior pairs of limbs, which are followed by the third and fourth pair. When the young leave the egg, they possess the full number of limbs and have the general form of the mother. v. Siebold's statement (No. 4, p. 553), that the Emydiac have only three pairs of limbs when they leave the egg, may be traced to a misunderstanding of Doyere's account (No. 1). This author states that the limbs are here not fully developed, not that one pair is wanting. It does not appear from v. Siebold's account that he himself investigated this point, which is of interest in con- nection with the comparison that has repeatedly been made between the Tardigrada and the Acarina. The accounts of the ontogeny of the Tardigrada are unfortunately so scanty that we can hardly gain anything from them applicable to the whole group. We cannot even tell for certain if a blastoderm and germ-band develop, although this is probable. The armature of the mouth is evidently a product of the stomodaeum, as may be gathered from the study of the adult anatomy ; mouth-parts (in the sense in which the term is used of the Arthropoda) apparently do not appear TARDIGRADA. 163 even as rudiments.* For this reason alone, the association of the Tardigrada with the Arachnida and especially -with the Acarina, which has repeatedly heen attempted, chiefly on account of the number of limbs, cannot be maintained. "With regard to the number of their limbs, the Tardigrada cannot be compared with any other division of the Arthropoda, and the form of their limbs is so simple as to distinguish them in this respect also from all other Arthropoda. The segmentation of the body in the Tardigrada is peculiar, inasmuch as the abdomen is wanting and the anus lies in front of the last pair of limbs. In other points also the organisation of the Tardigrada shows peculiarities which distinguish it from that of other Arthropods; we may mention, by way of example, the unstriped muscle-fibres and the absence of special respiratory organs, and of a vascular system. For these and other reasons (cf. Plate, !No. 3) we are led to consider the Tardigrada as a lateral branch of the Arthropod stock which separated from it near its root, and developed in a special and unique direction. LITERATUEE. 1. Doyere, M, Memoire sur les Tardigrades. Ann. Sci. Nat. (2). Tom. xiv. Zool. 1840. 2. Kaufmann, J. Ueber die Entwicklung und systematische Stel- lung der Tardigraden. Zeitsclir. f. Wiss. Zool. Bd. iii. 1851. 3. Plate, L. Beitrage zur Naturgeschichte der Tardigraden. Zool. Jahrb. Abth. f. Anat. Bd. iii. 1888. 4. Siebold, C. Th. vox. Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere. Berlin, 1848. pp. 552 and 553. APPENDIX TO LITERATURE ON TARDIGRADA. I. Erlanger, R. von. Beitrage zur Morphologie der Tardigraden. Zur Embryologie eines Tardigraden, Macrobiotus macronyx. Morph. Jahrb. 1895. And Biol. Centralbl. Bd. xiv. and xv. II. Kennel, J. von. Die Verwandschaftsbeziehungen und die Abstammung der Tardigraden. Sitzmigsb. Ges. Dorp. 1891. * [Erlanger (App. to Lit. on Tardigrada, Xo. I.) has made an exhaustive investigation into the development of Macrobiotus macronyx. He finds that cleavage is total and equal and results in the formation of a long, oval blastula with a eleavage-eavity ; an invagination-gastrula arises whose blastopore, which occupies the position of the future anus, soon closes ; a very short proctodaeum is formed, and the archenteron divides into a pharynx, gullet, and stomach ; four pairs of enterocoeles form, and the embryo becomes divided into a head and four thoracic segments. The head-coelom enters into connection with the first pair of appendages ; the coelomic pouches of the second and fourth thoracic segments enter into relation with the remaining appendages, while that of the third segment gives rise to the gonads and (?) to a pair of enteric glands. The oral papillae, the salivary glands, the nerve-ganglia, and the eyes all arise from the ectoderm. Erlanger regards the head and first two thoracic segments as the cephalo-thorax ; the third and fourth segments as the abdomen, behind which is a transitory post-abdomen. — Ed.] CHAPTER XXV. ONYCHOPHORA (Peripatus). Structure of the Eggs and Nourishment of the Young by the Mother. The eggs of Peripatus pass through their development in the uterus, but there is considerable variation in this respect in the different geographical species. This point has been carefully investi- gated up to the present time in P. novae-zealandiae (Australia), P. capensis and P. Balfouri (Africa), and P. Edwardsii, torquatus, and Imthurni (South America). These species differ even in the size of the egg and of the mature embryo. The oval eggs of P. novae- zealandiae are 1/5 mm. long and 1 mm. broad, and the young which hatch from them are not much larger than the eggs themselves ; the eggs of P. cajyensis and P. Balfouri are 0"4-0'6 mm. long, but the newly-hatched young of the former has a length of 10-15 mm., and that of the latter is about half as long. In P. Edioardsii the mature embryo attains the length of 22 mm., i.e., a third of that of the mother, while the egg is here only O04 in diameter. The species in which the young are largest at birth have thus the smallest eggs,. and vice versa. The explanation of this striking fact is to be found in the circumstance that in the South American species the egg or embryo remains in close connection with the mother, and is nourished by means of a "placenta and an umbilical cord" (Fig. 88, p. 179).* This accounts for the extraordinarily small size of the eggs in this case, and for their being devoid of nutritive material. In the African species the eggs are larger, but the newly-hatched young are smaller than in P. Edwardsii, there being no correlation between the size of the egg and that of the embryo, the latter, although not * We are here following the definite statements of v. Kennel (No. 4), which rest upon his own observations, although we are aware of the statements made by Hutton (No. 3) as to the size of the newly-hatched young of P. novae- zealandiae. These, according to this latter author, measure from 8 to 10 mm. Since v. Kexnel's statements were not contradicted by the more recent observers of the Peripatus of New Zealand, we must assume that the difference is only apparent, and that the large size of the embryo as compared with that of the egg must be traced not to its greater mass, hut rather to its increase in length and to its extension after leaving the egg. CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 165 attached to the wall of the uterus, is nevertheless nourished by fluid yielded by that organ. In the New Zealand species, such nourishment from the mother is not needed, since the embryo is not essentially larger than the egg. In this case, therefore, the material for the development of the embryo must be contained in the egg itself. It is actually found that the egg of P. novae- zeal andiae is very rich in yolk, as are those of most Arthropods. The general course of its cleavage also agrees with what is found, for instance, in the Insecta. Considering the close relationship of Peripatus to the Arthropoda, which can hardly be disputed, it seems likely that the condition of the egg of P. novae-zealandiae is the primitive condition. It is probable that Peripatus, like the terrestrial Arthropoda generally, origin- ally produced eggs rich in yolk which it laid. This state of things is recalled by the presence of a firmer egg-envelope in P. novae-zealandiae, already pointed out by Sedgwick (No. 11) ; the laying of eggs not fully developed also in this same species points in this same direction, even though we find that eggs laid thus early do not attain full development (Hutton, No. 3). The capacity for developing the eggs within the body must have been secondarily acquired. The ■egg of the New Zealand species, which is rich in yolk and develops within the uterus, represents the first step in this newly-acquired course of development. An accumulation of nutritive material in an egg which develops within the uterus is unnecessary, and this is opposed to the assumption that in P. novae- zealandiae we have a specialised form in which the egg has been secondarily supplied with yolk. A further step in adaptation would be represented by P. capensis. The eggs here show a spongy structure as if penetrated by fluid yolk, and this, as well as the method of their development, seems to indicate that they to a certain extent represent a degenerate condition of eggs originally rich in yolk. Isolated granules of yolk also appear in these eggs, and in P. Balfouri the egg is still somewhat rich in definite yolk-masses. In the species found in the West Indies, the nourishment of the embryo by the mother has become so complete that no trace of the former rich supply of yolk remains in the eggs which have become extraordinarily small. These biological conditions naturally find expression also in the method of development of the different species. * 1. Cleavage and Formation of the Germ-Layers. Although the early development, the cleavage and the formation of the germ- layers, has repeatedly been investigated in different species, our knowledge of these processes still remains very incomplete. This fact is accounted for by the * [Willey (App. to Lit. on Onychophora, No. II.), from the study of the «gg of P. novae-britanniae, has come to conclusions which are exactly the reverse of those given above ; and here he is in agreement with v. Kenxel, who believes that the ancestral Peripatus discharged a small yolkless egg into the water, and that the intra-uterine development was concomitant with the adaptation of the parent to a terrestrial existence. These authors then conclude that the development of yolk in the eggs of P. novae-zealandiae is quite a secondary condition, which, Willey believes, culminates in the return to the oviparous condition observed by Dexdy (App. to Lit. on Onychophora, No. I.) in P. oviparus, which Willey regards as a secondarily acquired habit. — Ed.] 166 OXYCHOPHORA. difficulty of obtaining material, for the eggs, taken from living animals brought to Europe, were sometimes in such a bad condition (Sedgwick, No. 10, Pt. I., Figs. 7-13), that the researches made on them could not be of any great value. Some of the observations also are incomplete, or, as in the case of the South American species examined by v. Kennel, important differences of opinion arise between observers (v. Kennel and Sclatek) which can only be finally settled by further research. A connected description of the first ontogenetic processes and their inter-relationships in the various species is as yet impossible. We shall first consider the development of P. novae-zealandiae which, for the reasons given above, we regard as showing the most primitive condition, and then deal with the African species. The South American species, from what we as yet know of them, seem to claim a position distinct from the others. Peripatus novae-zealandiae. Cleavage is here superficial. The eggs are rich in yolk, and the cleavage-nucleus appears to have a peripheral position. Its division gives rise to nuclei surrounded by islands of protoplasm ; these for the most part also lie peripherally, but single nuclei shift towards the centre of the egg, as may be seen in the figures given by Lilian Sheldon (Fig. 75, Xo. 12) It is no doubt due to the distribution of the nuclei in the yolk that this latter breaks up to some extent into distinct rounded areas (Fig. 76 A), although Lilian Sheldon was not always able to prove that these were regular yolk-pyramids either in origin or form. This break- ing up of the yolk led former observers (Hutton, No. 3, v. Kennel, No. 4, Pt. I.), who could only make observations on insufficient material, to the conclusion that the egg of P. novae-zealandiae underwent total cleavage. According to the descrip- tion given by L. Sheldon, the cleavage - nucleus and the nuclei which first arise seem to lie on the later dorsal side and opposite to the point at which the blastoderm forms. These nuclei increase in number and form a peripheral accumulation (protoplasmic or polar area, Fig. 76 A, a), starting from which, circumcrescence of the yolk takes place (formation of the blastoderm). The active increase in number 75.— Section through the egg of P. novae- zealandiae (after L. Sheldon). In the yolk are I lie nuclei surrounded by areas of protoplasm. TERIPATUS NOVAE-ZEALANDIAE. 167 of the cells and their constant shifting towards the periphery, leads to the almost complete circumcrescence of the yolk as far as a point lying almost opposite the original accumulation of nuclei, where the yolk remains uncovered. Here an ingrowth of cells then takes place, the appearance of an invagination being thus produced (Fig. 77 A and B). The point of invagination is the blastopore (bl), and the base of the depression is formed of yolk in which nuclei can be recognised (Fig. 77 B). The germ-layers do not yet appear to be differentiated from the cell-mass surrounding the blastopore, which represents the rudiment of the germ-band. Miss Sheldon seems to assume that the part of the cell-mass underlying the superficial cell- layer (or ectoderm) yields chiefly mesoderm, while the entoderm arises from the cells lying in the yolk, and which, according to Miss Fig. 76. — Portions of sections through the egg of P. novae-zealandiae, showing the blastoderm- formation (after L. Sheldon). A shows the " polar area " and the cleavage of the yolk. B, the commencement of the circumcrescence of the egg. a, " polar area " ; ds, "yolk segments." Sheldon, arise and multiply by a process of free nuclear forma- tion (I), as she was unable to observe any karyokinesis.* The blastopore lengthens later and then resembles a small groove, the base of which is formed by the nucleated yolk. We here have a resemblance to the condition in P. capentis illustrated in Fig. 84 A. As far as we can gather from the description of L. Sheldon, the process of circumcrescence is regarded by her as an epibolic gastrulation. The yolk-mass, with the nuclei contained in it, would correspond to the entoderm. A study of the figures, however, has compelled us to form another conclusion, which gains in probability from the fact that we are here dealing, as in the case of many * [It is extremely doubtful if there is such a process as free nuclear formation. All recent research on nuclei tends to prove that every nucleus originates from a pre-existing nucleus either by mitotic or amitotic division. — Ed.] 168 ONYCHOPHOIU. Arthropods, with an egg very rich in yolk. Whether the blastoderm is really formed by circumcrescence of the egg starting from one pole, or whether the nuclei contained in the yolk, by shifting to the surface, help to form it, the peripheral accumulation of cells which recurs in the same way in various stages claims identification with the cell-accumulation in the neighbourhood of the blastopore {cf. Figs. 76 and 77). We should then not be obliged to assume gastrulation through epibole, which is unusual in eggs so rich in yolk, but should rather assume that at the point where this cell-accumulation is found a depression (invagination) occurs (Fig. 77 B). Whether the base of this depression is formed of yolk (containing nuclei), or whether a closed archenteron is present, would in this case still have to be decided. If the blastopore lengthened later [cf. also P. capensis) there would be a resemblance to the gastru- lation of the Insects. In these latter, as in Peri- patus, the mouth and anus show a connection with the two terminal points of the long blastopore. In this conception of the cleavage and formation of the germ -layers, it may be noticed that the invagina- tion apparently takes place at the animal pole of the egg. But if it is remem- bered that in P. capensis the brain arises in the im- B mediate neighbourhood of . ^-^^ M- the blastopore, it will be seen that we must rather regard this as a shifting of the vegetative pole, or the region of entoderm-forma- tion, towards the animal pole, than as a gastrulation at the animal pole. The same is the case in the Insecta and in many Crus- tacea (Vol. ii. , pp. 141 and 142). The view here adopted receives a general support from the conditions in the Crustacea, in which the cir- cumcrescence of the yolk (or the formation of the blasto- derm) takes place from one point, gastrulation after- wards occurring in that region (Vol. ii., p. 115). We are unable in the present state of knowledge to obtain any light upon these processes from the ontogeny of P. capensis. &«°?&^>&& ■^mm?o®m? , 'o •sOO ^mmMmmmm L ~-CC i 0°°isr O c n o % qo oQnO oOOOOo r ° n Cfpn Qo ??J \S.o " Fig. 77.— Sections through the egg of P. novae-zealandiae showing the formation of the blastoderm and invagina tion (after L. Sheldon), bl, blastopore. PERIPATUS CAI>ENSIS. 169 Peripatus capensis. In consequence of the eggs of Peripatus capensis being poor in yolk, their cleavage is apparently total. According to Sedgwick, an animal pole (corresponding to the later dorsal side) can be dis- tinguished from a vegetative pole. Two meridional furrows divide the egg into four blastomeres of equal size, each of which contains a portion of the animal and a portion of the vegetative protoplasm. These cleavage-planes are said not to cut through the whole egg, the blastomeres being united centrally. At a later stage, an equatorial furrow separates the smaller ectoderm-cells (animal pole) from the larger entodermal blastomeres. At the close of segmentation, the cells are very loosely connected, the smaller ectoderm-cells are closely applied together, while the larger entoderm-cells are amoeboid and scattered irregularly within the egg-membrane. The stage which follows may roughly be compared with the blastula. The entoderm- cells draw together and lie directly beneath the smaller ectodermal cells, which then grow round the entodermal .--" elements, a solid (and vJ''(' > ' therefore epibolic) gastrula being thus formed in the course of further develop- ment. The archenteric cavity is said to arise in — E. the entoderm through the formation of "vacuoles"! It opens externally at the point which has remained unaffected by the cir- cumcrescence, and thus corresponds to the blasto- pore. Behind this, an increase in number of the cells of the superficial layer takes place, which leads to a thickening of this layer and then to a separation of the lower layers, that have thus arisen, as the mesoderm. During the lengthening of the blastopore, which soon occurs, and the simultaneous increase in length of the whole embryo (Fig. 84 A), the mesoderm grows forward on both sides of the blastopore and thus yields the mesoderm-bands. The rudiment of the germ-band is thus produced (Sedgwick). Fio. TS.— Section through a 16-celled embryo of ]'. Edwardsii lying in the uterus (after J. v. Kennel). E, embryo; i.Uv\ inner wall of the uterus; Ue, uterine epithelium. 170 ONYCHOPHORA. In the better preserved eggs figured by Sedgwick, large cavities can be seen in the protoplasm, and this leads us to conjecture that, in the normal condition, the eggs might be filled with a more or less fluid mass of yolk. These spaces in the body of the egg are very large, occupying a large part of its interior, so that, when the very unsatisfactory condition of the material investigated is taken into account, we are led to the conclusion that the cleavage may in this case also be superficial. The cavities in the blastula-stage just described would then be filled with yolk, and the gastrula would perhaps be formed by invagination, as was conjectured in the case of P. novac-zealandiac. As we have not personally examined these eggs, wc do not feel justified in giving definite expression to this view, but we cannot refrain from making a conjecture which appears to us so- probable. There would in this case be a certain similarity between the African, and the New Zealand species, especially as it may with probability be assumed that eggs poor in yolk are to be derived from eggs rich in yolk. This last view is held by Sedgwick himself, and in a later treatise (No. 10, Pt. iii.) he calls- the egg of P. capensis meroblastic, because of the central connection mentioned above as existing between the blastomeres. The American species. On account of the small size of their t»W«»flBM c oo° Us. f ~. E E. ire..._L' ' " r^ -_-> f M "+~- ■"'&•'■&*• w t.Uro. - ' 3P o - Pig. 70.— Sections through embryos of P. Edwardsii together with the uterine wall (after J. v. Kennel). E, embryo; am, amnion; a.Uw, outer wall of the uterus; i.Uvj, inner wall of the uterus; Ue, uterine epithelium [embryonic derivative, Sclater and Willey]. and the connection between these and the wall of the uterus, the American species differ entirely in their develop- ment from the forms we have so far considered. The small eggs, poor in yolk, undergo a total and apparently fairly regular (equal) course of cleavage. The embryo, even at this early stage, appears to obtain nourishment from the uterus, for it increases in size in a marked manner (v. Kennel). When it has reached the 32-cell stage, it forms, according to v. Kennel, a solid cell- mass, completely filling the narrow lumen of the uterus, and thus in close contact with the inner surface of the uterine epithelium (Fig. 78). This THE AMERICAN SPECIES. 171 latter at first consists of very deep cells which, however, under the influence of the growing embryo, seem to flatten. The embryo then, according to v. Kennel, enters into direct connection with this epithelium, this change being accompanied by a peculiar alteration in the shape of the former. The embryo, which is said to give off fluid and to decrease in size, becomes applied to the epithelium as a lenticular cell-mass (Fig. 79 A). The figures show the close nature of the connection between the embryo and the epithelium, the latter may, indeed, occasionally become detached from the wall of the uterus, and may surround the embryo as a special layer (Fig. 79 B, Ue). The central part of the embryo now rises from the surface of the uterus, while the edges, which still remain in contact with the latter, become somewhat approximated through these changes; the em- bryo thus assumes the form of a cap open towards the surface of the uterus (Fig. 79 B). From the surface of the embryo a few amoeboid cells become detached ; some of these be- come applied to the uterine epithelium and, finally, these amoeboid cells unite and give rise to an envelope which sur- rounds the whole embryo, and which has been termed the Fig. SO. — Median section of a pear-shaped embryo of P. Ed- wardsii, with amnion and uterine wall (after J. v. Kennel). om, amnion ; n, umbilical cord ; j).c, embryonic, p.u, uterine placenta ; Ue, uterine epithelium ; I'w, wall of the uterus ; w, point of ingrowth. amnion or serosa (v. Kennel, Fm\ 80, am). The margins of the cap-shaped embryo now become approximated and fuse together, so that the embryo becomes a closed vesicle. The embryo then grows out from the wall into the cavity of the uterus ; its point of attachment narrows and thus forms a stalk (Fig. 80, ?i). A proliferation of cells then takes place at the base of the stalk, this growth being called by v. Kennel the "embryonic placenta." Corresponding to this is a 172 ONYCHOPHORA. circular thickening of the uterine epithelium, which, as the " uterine placenta," enters into close connection with the former (Fig. 80, p.e and p.u). The stalk connecting the embryo with the placenta continues to narrow, and is described by v. Kennel as the "umbilical cord." According to this account the embryo becomes closely con- nected with the wall of the uterus, and a thickening of the connective tissue layer of the latter takes place, causing a constriction of the uterine lumen in front of and behind the region which contains the embryo, thus forming a closed brood-cavity (Fig. 88, p. 179). The amnion and the uterine epithelium are now separated from the embryo by a considerable cavity (Fig. 80). The germ-layers begin to form by an active increase and a con- sequent ingrowth of the cells which commences opposite the point of attachment of the embryo (Fig. 80, w). In comparing the development of P. Edicardsii with that of other species of Peripatus, the point at which this ingrowth takes place will recall the accumula- tion of cells in the blastoderm in P. novae-zealandiae, in which invagination eventually occurs, and which at the same time represents the first indications of the germ-band. In the South American species this point of ingrowth, which in position corresponds to the ventral side of the embryo (the latter is attached by its dorsal surface), must be regarded as the blastopore. From this point the ingrowth proceeds continuously, and fills the whole inner space of the embryo down to the " umbilical cord " (Fig. 80). The cells of the latter have shifted apart, leaving a central lumen, and have become arranged into an epithelium such as is also found all round the embryo, except at the point of ingrowth (Fig. 81). This outer epithelium corresponds to the ectoderm. The further differentiation of the germ-layers is said by v. Kennel to take place through the appearance of a cavity in the more dorsal part of the central cell- mass and the regular arrangement of the cells in its neighbourhood (Fig. 81, ent). The cell-layer thus differentiated, the entoderm, is in this way distinguished from the ventral cell-mass lying at the blastopore, which represents the mesoderm. This latter remains connected with the ectoderm for a long time, even during the later changes of shape of the embryo, and at this point (w) new cell- material is continually produced (v. Kennel, Sclater). In the above description of the first ontogenetic processes in P. Edicardsii, we have followed the account given by v. Kennel liecause the material at his disposal, with regard to quantity and state of preservation, seems to guarantee the reliability of his statements, but it should be mentioned that these processes THE AMERICAN SPECIES. 173 have received another interpretation. Although this latter has been opposed by v. Kennel for very important reasons (No. 5), it has been adopted by Sclater, and seems to have a certain value in so far as it affords some explanation of the peculiar early developmental stages. According to Sclater (No. 9), cleavage gives rise to a blastula formed of large cells, and containing a small cavity (Fig. 82 A). An invagination then takes place in this (pseudogastrula, Fig. 82 B). The invaginated part alone yields the embryo (Sclater)) while the outer layer, by the peculiar growth of its cells, sejiarates from the embryo and becomes very thin, thus forming a membrane which envelops the embryo (Fig. 82 C, a). From the embryo itself another envelope arises, by the splitting off of single cells, this latter corresponding to the amnion described by v. Kexnel. The figures given by Sclater agree on the whole with those of v. Kennel, but they are interpreted by the two authors in an entirely different way. What v. Kennel regards as uterine epithelium is considered by Sclater as an embryonic envelope, for this no doubt is the meaning of his pseudogastrula. Fig. 79 B (v. Kennel) must therefore be regarded as the stage of invagination corresponding to Fig. 82 B (Sclater' s pseudogastrula), and Fig. SO must be interpreted in a similar way. Fig. 79 A, according to this view, should be regarded as an older stage, similar to that represented in Fig. 82 C. Further, the two stages in which the conjectural vesicle has either thin or thicker walls ought not to be unhesita- tingly derived one from the other, as is done by Sclater. Indeed, far stronger proofs must be brought forward for the view adopted by Sclater before it can be finally accepted ; it nevertheless appears to us worth mentioning because it seems best to account for the origin of the embryonic envelopes which are attributed to Pcripatus.* In any case, the two en- velopes which are said to surround the embryo suggest the double embryonic envelopes (amnion and serosa) of the Insecta, all the more that this double embryonic integument may have arisen here as there by the formation of folds in the blastoderm. The position of the embryo in relation to the folds might even correspond to that of the Insectan germ in relation to the embryonic integuments, but we know too little of the * [Willey's observations on the development of P. novac-britanniac (App. to Lit. on Onychophora, No. II. ), in which he finds that the egg gives rise to a large, thin-walled vesicle (trophoblast) with a thickened invaginated embryonic area, tend to support Sclater's views regarding the relations of embryonic envelopes in P. Edwardsii, and are opposed to those of v. Kennel.^To us they appear conclusive on this point. — Ed.] Fig. SI.— Median section through a pear- shaped embryo of P. Edwardsii (after v. Kennel), ent, entoderm ; n, um- bilical cord ; w, point of growth. 174 ONYCHOPHORA. ontogeny of Peripatus to be able to make further comparisons. We must, however, add the description given by L. Sheldon (No. 12) of the earlier stages of P. novae-zealandiae, according to which the embryo proper still within the egg-shell is surrounded by a layer of yolk (the ectodermal yolk of Miss Sheldon). Unfortunately no details as to the significance and origin of this " external yolk " are known, but we might in this also see an embryonic envelope, especially as structures resembling nuclei are found in this outer layer. We are led to adopt this assumption all the more on account of the condition of those Insecta (or Myriopoda) in which the germ-band sinks into the yolk, a condition which finally leads to formation of the embryonic envelopes. In consequence of the Insectan embryonic envelopes arising in this way the embryo here also may be apparently surrounded by an outer layer of yolk, which in reality lies between the embryonic envelopes. The presence of such envelopes derived from folds is not confirmed by what is found iu P. capensis. In this form nothing of the kind has been observed, nor can we assume that such a feature has been overlooked. The ectoderm of P. capensis is only so far peculiar in that it is in the younger stage extraordinarily rich in vacuoles and of a spongy texture, and, in consecpience of its structure, is aide to take in nourishment endosmotically (Sedgwick, No. 10). L. Sheldon connects this structure with the so-called ectodermal yolk of the New Zealand species, but we can- not consider this a happy com- parison. On the other hand, this condition of the ectoderm helps to explain the formation of the external organs of nutrition in the embryos of the American species, whether these are formed direct from the ectoderm of the embryo itself or represent a specially differ- entiated portion of the embryonic envelopes. If, in dealing with the early ontogeny of the different species of Peripatus, we have appeared to dwell almost entirely on the rela- tive probabilities of the processes described, we can only again point out how very little is known with certainty of the earliest develop- ment of this animal. The great importance of this form forces us to take into account statements that are not sufficiently confirmed. We have therefore tried to gather together the facts as yet known into a connected whole, but do not for one moment assume that the conclusions arrived at are final. Fig. S2.— Sections through [embryos of P. 2m- thnrni at various stages (after Sclater). E, embryo ; a, outer, i, inner cell-layer of the embryo ; m, (cuticular) membrane bounding the uterus internally ; v, placenta-like growth of cells. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 175 2. The Development of the External Form of the Body. In spite of the variations found in the first ontogenetic stages of the several species of Peripatus, the latter differ very little from one another in the development of the external form of the body. In the following descriptions we shall first deal chiefly with P. capensis, which was very carefully examined first by Moseley (No. 6), then by Balfour (No. 1), and later by Sedgwick (No. 10, Pt. i.). P. capensis. It has already been shown, in describing the forma- tion of the germ-bands, that a thickening of the blastoderm arises behind the lengthening blastopore by the proliferation of cells, this spot being recognisable externally as an oval area (Fig. 83). "We saw that at this point the mesoderm originates, and extends forwards in the form of two bands, to the right and left of the blastopore. In each mesoderm-band segmentation takes place, a segmentation which may in all respects be compared to that of the Annelida. For instance, at the anterior ends of the two bands, cell-complexes are cut off and commence, by the formation of cavities, to form the primitive segments (Fig. 84 A and Z>), fresh rudiments being continually added posteriorly. At the posterior end of the blasto- pore the mesoderm-bands pass over into an undifferentiated cell-mass. During the differentiation of the mesoderm-bands another im- portant change takes place in the embryo. The lips in the middle region of the elongated blastopore approach one another and fuse, so that the only remains of the blasto- pore are an anterior and a posterior --'"" ^^v. aperture (Fig. S4 A and B). These /' "\ two apertures are henceforward retained / ' •- , .©, '-.'■'• '■% (C and D), giving origin (in connection j: B w~~. with ectodermal invaginations) to the mouth and the anus. \ ■ / ; .'*7 The next changes in the embryo \ • j &•. take place as follows : as the differ- \ entiation of the primitive segments Continues, the first Of these Shift Fig. S3.— Embryo of P. capensis (after further forward, and, in addition to ^IZZ W ' blastopore; w ' zone this internal segmentation of the em- bryo, an outer segmentation now appears (Fig. 84). At the anterior end the cephalic lobes begin to appear, and it is to be specially noted that, as rudiments, they show great resemblance to the body-segments. The posterior end of the hitherto straight embryo curves round 176 ONYCHOPHORA. K. B. M. ventrally, thus covering the posterior aperture derived from the constriction of the blastopore (Figs. 84 and 85). Before describing the further development of the embryo, we must glance at the corresponding processes in other species of Peripatus. The observations recorded above on the development of the external form have dealt chiefly with the shaping of the ventral surface, this being first developed as two symmetrical halves. We are here reminded of the development of the eggs of the Oligochaeta and Hiru- dinea that are rich in yolk, and still more of that of the eggs of the Myriopoda, Insecta, and Arachnida. In Peripatus, as in these, a germ-band forms. Its composition out of two halves is still more distinct in P. novae-zea- landiae. In this form, in consequence of the large size of the egg, caused by the abundance of its yolk, the two halves of the germ-band lie somewhat far apart, separated by a ventral protrusion of the yolk-mass covered with ectoderm and entoderm (Fig. 86 A and B), so that a kind of ventral yolk-sac arises resembling the one met with in the Araneae (p. 54). While already well developed at the anterior end, the two halves of the germ-band become less and less differentiated posteriorly, and end near the blastopore in the as yet undifferentiated cell-mass (primitive streak of English authors). At first sight there appears to he a fundamental difference between P. capcnsis with P. Edwardsii and P. novcte-zealandiae regarding the position, relative to us. - ra- il. m.- Fio -Embryos of P. capensis to illustrate the closing of the blastopore, the segmentation of the mesoderm, and the flexure of the embryo (after Balfour and Schimkewitsch). a, anus ; U, blasto- pore ; m, mouth ; us, primitive segments ; w, zone of growth. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 177 the anus (? = blastopore), of this undifferentiated cell-mass (primitive streak). In the two first species this zone of growth is undoubtedly situated behind the anus (i.e., behind the blastopore = anus in P. capensis, Fig. 84 A, w, and in the region of the blastopore and behind the anus in P. Edwardsii, Fig. 89 A), whereas in P. novae-zealandiae this zone of growth appears, from a superficial examination, to be situated in front of the anus (Fig. 86 A), at least the two halves of the germ-band bend forward and unite in front of that aperture. According to L. Sheldon this is only an apparent difference, for in sections of P. novae-zealandiae through this region the zone of growth of the mesoderm (primitive streak) is found to be situated, as in P. capensis and P. Edwardsii, behind, not in front of, the anus. The possibility of the zone of growth being situated in front of the anus is of interest when we make a comparison between the embryos of Peripatus at this stage and those of the Annelida. Such a comparison made between the embryo of P. novae-zcalandiae (Fig. 86 A) and that of Clepsine among the Hirudinea (Vol. i., Fig. 152, p. 322) reveals a striking similarity between the two, especially in the configuration of the mesoderm-bands which, in the Hirudinea, however, unite in front of the anus, a condition which, it is true, is suggested from a superficial examination of the embryo of P. novae-zealandiae, but which is not substantiated by the investiga- tion of sections, and one which would, moreover, seem improbable from a comparison with the two other species of Peripatus mentioned above. Further, in the Hirudinea, there appears to be no connection between the blastopore and the anus, which makes a comparison with Peripatus more difficult than would otherwise be the case. In other Annelida, however, the primitive mesoderm- cells are met with at the posterior edge of the blastopore (Vol. i., pp. 264 and 283), but the condition is different from that in Peripatus in so far as the blastopore does not pass direct into the anus. In view of these possible differ- ences between the species of Peripatus inter se and of the possible resemblances to some Hirudinea, a renewed investigation of the relations of the growing zone in P. novae-zealandiae, which must be regarded as the most primitive form [one of the most specialised according to Willey and v. Kennel], is much to be desired. The South American species, in conse- quence of the small size of their eggs and of the connection of the latter with the wall of the uterus, have a different form in the early stages. Our description of the embryo ceased at a stage in which it was somewhat pear-shaped (Figs. 80 and 81). From this stage it passes into the mushroom stage (Fig. 87), the embryo proper gaining in size as compared with the umbilical cord, through extending in both directions at right angles to the axis of the cord (Fig. 87). These two directions correspond to the length and the breadth of the embryo. Growth takes place at first chiefly in the first of these two directions, Fig. 85. — Embryo of P. capensis (after Balfour and Sedgwick), a, anus; m, mouth. 178 ONYCHOPHORA. af„ with the result that the embryo becomes elongate (Fig. 88). Dorsally, it is attached by the umbilical cord, while the ventral surface is free. The blunt end becomes the head, and the pointed end the posterior extremity. The blastopore, which is probably secondarily displaced, lies quite near the latter (Fig. 89 A, bl). The space between the blastopore and the umbilical cord is much longer than that between the latter and the anterior end, since, starting from the blastopore, new cell-material is continually being produced posteriorly. Two mesoderm-bands, divided up into the primitive segments, are present as in P. capensis, but, in conse- quence of the smaller size of the egg, the paired nature of the germ-band is not so distinct, although it can be recognised here also (Fig. 89). The mouth arises in a position corresponding to that in P. capensis, but quite independent of the blastopore, the latter, as a small and shallow depression, n J9f„ having come to lie ^ai. at an early stage at the posterior end (Fig. 89 A, bl). The anus also is said by v. Kennel to arise independently of the blastopore. It arises in front of the latter as a slit-like depres- j sion (Fig. 89 A, a). J If the accounts given of P. capensis by Sedg- wick prove correct, we shall have to conclude that, in P. Edwardsii also, the oral and anal apertures were originally connected with the blastopore, since the position of the two apertures is similar to that in P. capensis. With regard to the position of the growing zone, P. Ediuardsii, according to v. Kennel, agrees entirely with P. capensis and P. novae-zealandiae ; for since this zone proceeds from the blastopore, and the latter lies behind the amis (Fig. S9 A), the undifferentiated cell-mass is also found behind it. The connection of the embryo with the mother must here again be referred to. According to v. Kennel, the embryo is connected with the mother by means of the umbilical cord, as well as by the Fio. S6. — Embryos of P. novae-zealandiae. A, ventral, and B, lateral aspect (after L. Sheldon), a, anus ; at, antennae ; ex, limbs ; m, mouth. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 179 Fig. 87. — Mushroom-shaped embryo of P. Edwardsii in the brood-cavity (after v. Kennel), b, brood-cavity ; e, embryo ; n, umbilical cord ; p, placenta ; it, lumen of the uterus. embryonic and uterine placenta (Fig. 80, p. 171, and Figs. 87-89) The great development of these organs shows that, in the younger stages, they contribute to its nourishment. They degenerate later, and the embryo is then, like that of P. capensis, nourished by the uterine secretion. In conse- quence of the close organic connection of the embryo with the uterus, the former is unable to shift from its position. The embryo, firmly enclosed in its brood-sac (Fig. 88), can only move on into the vagina by the growth of the parts lying between the ovary and the brood-cavity itself, and by the gradual absorption of the posterior parts. When the embryo which lies nearest the vagina passes over into the latter, its brood-cavity must be com- pletely absorbed before the next embryo can reach the vagina. The extrusion of the embryos in the South American species of Penpatus closely resembles the passage of Insectan eggs from the oviduct into the efferent apparatus. There also the empty follicle left after the expulsion of the -egg is completely absorbed before the next ■egg is able to pass out. The further development of the external form of the body consists ■essentially in the lengthening of the body, the marking off of the head and trunk, and the appearance of the limbs •and sensory organs. It agrees on the whole in the different species, so that separate accounts are here unnecessary. An important change in the form of the young embryo is brought about by the great development and marking off FlG . ss.— Embryo of p. Edwardsii in Of the cephalic segment from the trunk the brood-cavity (after J .v. Kennel, r ° _ from Langs Text-book of Comp. (Figs. 86 and 89). This change, which <.). e, embryo; ep, placenta. 180 ONYCHOPHORA. occurs early, is introduced by the shifting forward of the first pair of primitive segments to the extreme anterior end of the body, where they become considerably enlarged. A pair of large swellings (cephalic lobes) thus arise at the anterior end ; these soon become marked off from the body by a transverse furrow, and thus constitute the cephalic segment. On the ventral side of these lobes is the oral aperture ; on the dorsal side a pair of prominences appear (Fig. 86 A and B) which soon increase in size and become recognisable as the rudiments of the antennae. In P. capensis these are said to appear before the limbs (Sedgwick), but this distinction seems to be of no great significance ; in P. Edwardsii the antennae are said to appear simultaneously with the rudiments of the truncated legs, which they closely resemble. They are, however, distinguished from the latter by their more dorsal and pre-oral position (Figs. 86, 90, and 91). In front of the rudiments of the antennae, and lying more medianly, there are, at an earlier stage, two small prominences (Fig. 90, x), which shift later towards the anterior margin of the head (Fig. 94 A and B). These prominences, which were observed by v. Kennel in P. Edivardsii, and the nature of which is as yet unknown, can still be recognised at a later stage than that depicted in Fig. 94 B, and disappear R. B. --m. from view only when folds begin to form in the cephalic integument. Wc shall refer to them again at the end of this section (p. 187). The limbs arise as latero- ventral outgrowths of the segments consecutively from before backward (Figs. 86, 90, and 91). The segmentation of the body is brought about chiefly by the outgrowth later- ally of the primitive segments. The embryo, especially in its lateral parts, thus appears notched (Figs. 86 and 90). The paired nature of the germ-band is still indicated by the presence of a median ventral furrow (Fig. 89). This especially applies to P. Edivardsii, in which also the limbs appear later than in the African and Australian species. This retardation is no doubt due — U. Fig. 89.— Embryo of P. Edwardsii. A, ventral, and B, lateral aspect (after v. Kennel), a, anus; hi, blastopore; m, mouth; n, umbilical cord. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 181 to the modified method of development within the uterus, the close connection of the embryo with the wall of the uterus leading to the later development of its external form. In P. novae-zeal andiae the limbs are to be found while the two halves of the germ-band are still far apart (Fig. 86), and in P. capensis also they appear early. The embryo, at an early stage, becomes curved, and, as the body ■elongates, its posterior end becomes rolled up ventrally (Fig. 85, p. 177), this being determined either by its position within the egg- shell or (secondarily as in P. Edwardsii) within the brood-cavity. In P. Edwardsii the posterior end forms several coils. The posterior extremity of the embryo of P. capensis is also at first bent in towards the ventral surface of the body (Fig. 85), but subsequently this pos- terior region grows parallel with that surface, the bend being retained at the middle of the body, and the embryo lies in the egg-envelope in such a way that the anterior and posterior halves of the body are almost parallel to one another, the head touching the posterior end. In P. novae-zcalandiac, in a stage earlier than that illustrated in Fig. 86 A and B, a ventral flexion apparently occurs in the embryo, the latter consequently assuming a curved form, but it soon straightens again to some extent, and retains the form shown in Fig. 86 A and B (L. Sheldon, No. 12, Pt. i. ). Here also the two halves of the germ-band are at first very far apart, as may be seen from Fig. 86 A and B. In keeping with the unspecialised external form of the adult Peripatus, the further development of the embryo is very simple, and, apart from the anterior region of the body, presents no specially noteworthy features. The formation of the limbs continues in the manner above described (Fig. 91), until the final number is reached. Where the two halves of the germ-band lie far apart, as in P. novae-zeal andiae, they eventually shift together to form the ventral surface, a process which is assisted by the gradual absorption of the yolk. The dorsal surface at the Fig. 90. — Anterior part of an embryo of P. Edicardsii, dorsal aspect (after v. Kf.nxel). at, antenna ; k, max- illary segment ; op, segment of the oral papillae ; p, first adult trunk segment ; x, prominence in front of the antennal rudiment (c/. pp. ISO and 187). same time assumes its final shape. The annulations of the body, and the papillae which are seen on its surface in the adult condition, appear in the form of folds and slight elevations of the epidermis. 182 ONYCHOPHORA. The terminal region of the body, up to the time when the adult form is assumed, is almost button-shaped. At its lower side, either in a depression (as in P. Edicardsii) or on a papilla, as in P. capensis, lies the anus. Two slight outgrowths, the anal papillae, which apparently belong to the terminal section, must be regarded as rudi- ments of limbs, and thus indicate a true segment. The limbs themselves have assumed their adult form, being better marked off' from the body, and exhibiting a ringed appearance not unlike segmentation At their free ends the two cuticular chitinous claws arise. The limbs have shifted from their former more ventral position to their final position between the dorsal and the ventral surface. With regard to the position of the anus it must be mentioned further that, in consequence of its being found in front of the growing zone, it must be related to a true segment. In various drawings made by v. Kennel and Sedgwick of sections cut through the anal aperture, well developed primitive segments are seen round the terminal region of the intestine. We must then in any case- assume a shifting forward of the anus which originally belonged to the terminal region of the body. The relation of the anus to the segmentation of the body in the adult does not seem satisfactorily settled, nor is it clear whether it subsequently shifts out of the segmented region to the extreme end. The development of the anterior region of the body is less simple than that of the trunk. Complications arise in the former through two other segments besides the actual cephalic segment being drawn into the formation of the adult head, and through the corresponding modification of the append- ages of these segments. We thus find in Peripetias a state of things already met with in the Crustacea, and still more closely re- sembling conditions found in the Arachnida, Myrio- poda, and Insecta. In the cephalic segment, the rudiments of the antennae have undergone alteration; they have lengthened considerably, and rings like those on the limbs have appeared on them (Fig. 91, at). The Fio. 91.— Embryos of P. capensis of different ages (after Sedgwick), at, antenna; aw, eye; /, fold, contributing to the formation of the buccal cavity; k, jaw ; op, oral papilla ; p,-p t i,, first three pairs of limbs. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 183 eyes (azi), as slight ectodermal depressions, are already present as rudiments (P. capensis), situated somewhat ventrally to the antennae. In P. Edwardsii they arise immediately behind the bases of the antennae. The further development of the mouth and of the two pairs of limbs connected with it is of special importance. The anterior aperture remaining after the partial closing of the blastopore does not directly give rise to the mouth, but becomes carried inwards by an invagination of the ectoderm, the stomodaeum, and thus forms the aperture between the latter and the enteron.* Neither does this second aperture represent the mouth of the adult, for it becomes covered by various outgrowths of the ectoderm, which form above it a secondary buccal cavity. This process commences by the appearance of a fold on the outer side of the limb next in order to the antenna (Figs. 91 and 92, 7c); this fold is closely applied to the limb, and is continued posteriorly along the ventral surface of the embryo (Figs. 91 and 93, /, and Fig. 92, p). It appears notched, and, in P. Edwardsii, is repre- sented by a series of papillae lying one close to the other (Figs. 92 and 94). In later stages these two folds shift closer towards the oral aperture, and thus press the limb-rudiments that lie on the inner side of them towards the mouth. As the folds grow still higher, these limbs, to- gether with the stomodaeal aperture, come to lie in a cavity, the adult buccal cavity (Fig. 94), the limbs themselves becoming the jaws of the adult. The distal part of each of the limbs, at the time when the formation of the buccal cavity just described begins, appears deeply notched, and the two strong chitinous teeth arise at this part (Fig. 94 A and B). These terminal teeth, which are to be compared with the double claws on each of the legs, prove, even in the adult, that these jaws are true limbs. Several other structures contribute to the complete development of the buccal cavity. Between the cephalic lobes, and ventrally to them, a somewhat long prominence arises (Fig. 94, ol), which * Cf. below, p. 196. Fig. 92.— Anterior portion of an. embryo of P. Edwardsii, seen from the ventral side (after v. Kennel, from Lang's Text- book of Comp. Anat.). Tc, jaw ; no, aperture of the nephridium belonging to the segment of the oral papillae (op) ; p, pa pillae of the folds which sur- round the jaws laterally. 184 ONYCHOPHORA. lies directly in front of the sharp edge of the stomodaeal aperture, and thus, when that aperture is walled in by the lateral folds, shifts with it into the cavity thus formed (Fig. 94 B). The folds then unite in front of this unpaired papilla, the upper lip of v. Kennel (Fig. 95). The posterior unnotched continuations of the lateral folds form the posterior boundary of the buccal cavity, on the floor of which the primitive aperture of the stomodaeum now lies surrounded by the jaws and the upper lip. From the above it will be seen that, in Peripatus, there are three distinct apertures, each of which in turn must be regarded as the oral aperture: (1) the primitive blastopore mouth (Fig. 84 D, m), which persists in the adult as the opening between the oesophagus and the stomach-intes- tine; (2) the stomodaeal mouth (Fig. 93, m), which in the adult puts the buccal cavity into communication with the pharynx; and (3) the external opening of the buccal cavity, which functions as the mouth in the adult, and is formed by the concrescence of two ectodermal folds. The shifting forward of the lateral folds towards the oral aperture has also caused the ventral organs of the first two segments to shift into the buccal cavity (Fig. 94 A, vo l and vo 2 ). We shall refer to these again later. Another pair of folds exactly like those which have walled in the oral aperture are sometimes present, according to v. Kennel, on the outer side of the lateral folds, but these do not seem to be of constant occurrence. They, however, seem further to support the view, which appears very probable, that the folds found near the mouth of Peripatus do not represent limb-rudiments, as has been conjectured by Moseley. The third pair of limbs are less closely connected with the mouth than are the jaws, for while they also shift towards the oral aperture, they remain outside the lips of the buccal cavity (Fig. 95, op). Apart from the fact that no chitinous hooks develop on them, they retain to a greater degree the character of limbs. They are early distin- guished from the other limbs by their greater development (Fig. 91 B, op). These limbs are known as the oral papillae, at the tips of which the slime-glands open. In the adult, these papillae lie as far forward as the jaws (Fig. 95), and the segment to which they belong must therefore be reckoned as a cephalic segment. Three Fig. 93.— Cephalic part of an embryo of P. capensis (after Sedgwick). at, antenna ; /, oral fold ; k, jaw ; in, stomodaeal aperture ; op, oral papillae ; sp, aperture of the sali- vary gland ; vo, aperture of the ventral organ. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 185 segments beside the primary cephalic segment, viz., those of the antennae, of the jaws, and of the oral papillae thus take part in the formation of the head. The head of the embryo, in consecpience of the great development of the cephalic lobes, at first appears very large in proportion to the rest of the body (Figs. 86 and 89). In the course of development, however, it decreases in size, the mouth shifts more to its anterior end, and the form of the adult is thus practically attained. R. B. g+no Fio. 94. — Anterior parts of embryos of P. Edwardsii, ventral aspect (after v. Kennel). at, antenna ; /,, the folds broken up into papillae, surrounding the mouth; /,, the folds lying outside of/ ; g+vo, ganglion and ventral organ of the cephalic segment with the slit-like depression of the ventral organ ; k, jaws ; ol, upper lip ; op, oral papillae ; j\ , p,/, first and second pairs of legs ; vo x ~vo 4 , ventral organs of the jaws, oral papillae, and of the first two trunk-segments; vo., is divided into an anterior and a posterior part; x, prominences in front of the antennal rudiments (pp. ISO and 1S7). The young are born provided with the complete number of limbs. Their development, according to authors, lasts unusually long (Sedgwick, No. 11). P. novae-zecdandiae is said to require eight to nine months for its development, and P. capensis thirteen months (?). The umbilical cord, which in the South American species connects the embryo with the placenta, at the time when the embryo lengthens and coils up its posterior end, changes and finally degenerates ; its lumen closes first near the embryo. The embryo is now nourished by swallowing the surrounding albumen, 186 ONYCHOPHORA. a method of feeding which also occurs in the embryos of P. capensis ; in addition to this there is a kind of endosmotic inception of nutritive fluid. Interpretation of the cephalic appendages of Peripatus. The nature of the two posterior pairs of cephalic appendages of Peripatus cannot be doubted. They correspond to the limbs of the trunk, and might Avithout further question be assumed to be limbs which, when two (primary) trunk -segments were fused with the head, were transformed into jaws and oral papillae. This is not the case with the antennae, which are distinguished from the limbs of the trunk by their dorsal and pre-oral position. In this respect they entirely agree with the antennae of the Myriopoda and the Insecta, with which we a consider them homo- logous. The antennae of Peripatus seem un- doubtedly homologous with those of all the other Tracheata, but not with those of the Annelida. The an- tennae, not only of Peripatus, but of the Myriopoda and In- secta, have been com- pared with regard to their position to the cephalic tentacles of the Annelida, which are found (pre-orally) in the cephalic seg- ment, occupying the same position with relation to the neural plate as do the antennae of the higher forms with relation to the brain. The manner in which the antennae of Peripatus originate, however, seems to us to tell against such a comparison. The antennae, both as rudiments and when developing, show great agreement with the trunk-limbs (Figs. 91-94), a fact which is strikingly evident in the figures given by Sedgwick and v. Kennel. Like the limbs, they are externally ringed, and a process of the primitive segment runs into each of them, so that they too are hollow cones. Indeed, a canal is said to run from the primitive segment of the antenna Fig. 05.— Head of P. Edwardsii, ventral aspect (after Sedgwick, from Lang's Text-book of Comp. A net.), a, antenna (the greater part of which is removed) ; op, oral papilla. In the buccal cavity are the double jaws. The cavity itself is surrounded by the folds cut up into papillae. THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 187 to the exterior, and this would correspond to the nephridial canal of the trunk -segments. In fact, there is such close agreement between the antennae and limbs as rudiments, that it is difficult to believe that they are essentially different structures. We should feel inclined rather to attribute to them the same origin, and to assume merely that the antennae had shifted further forward. This conjecture is supported by a comparison of the antennae of the Insecta with those of Peripatus. The former also as rudiments show not only in form, which would there be less remarkable, but also in position the closest agreement with the (primary) trunk- appendages, indeed, to begin with they even lie post-orally (Fig. 147). We might conclude from this that the antennae of Peripatus and those of the Insecta were homologous structures, but that they could not be compared with the cephalic tentacles of the Annelida, in other words, that they were originally appendages of the primary trunk and not of the primary cephalic region. If we accept this view we shall have to assume that the primary cephalic region has greatly degenerated, and that a primary trunk-segment (the first) has to a certain extent taken its })lace. An indication as to how and why this happened is to be looked for in the fusion of the other and undoubted trunk- segments in the adult head. The utilisation of the anterior limbs as mouth- parts was accompanied by their partial transformation into sensory organs (palps of the Insecta), and the final preponderance of one pair as feelers. Again the brain would in this case have to be reckoned as belonging to the first (primary) trunk-segment, and could not be derived from the neural plate alone. This view, however, presents no difficulty when we see how, in Peripatus, the ganglia of the maxillary segment passes from a post-oral to a pre-oral position and is absorbed into the brain (p. 193). In the Crustacea the ganglia of the second antennae undergo corresponding changes. The entire filling up of the so-called cephalic segment in Peripatus by a regular pair of primitive segments with unbroken epithelial walls agrees with what is found in a trunk-segment, but not with the condition of the cephalic region in the Annelida. If the primary cephalic segment which, in the Annelidan Trqchophore, carries the cephalic tentacles, has really undergone degeneration, we might expect to find traces of this fact. The two small prominences which appear in front of the antennal rudiments, the significance of which is still obscure, might be regarded as vestiges of this kind (Figs. 90 and 94, x). We might conjecture that they are possibly vestiges of the primary Annelidan tentacles. This interpretation of them, which appears to us very plausible, also leads to a striking agreement with the Crustacea. In homologising the cephalic appendages of the latter (Vol. ii., p. 166), a similar view was adopted, the same significance being ascribed to the frontal sensory organs as is now given to the small prominences (x) in front of the antennal rudiments in Peripatus. It cannot be regarded as altogether improbable that the adult Peripatus should still retain vestiges of this organ, the agreement of which with the frontal sensory organs still functioning in many Crustacean larvae would be stil 188 ONYCHOPHORA. more striking. The prominences now under consideration are, according to v. Kennel, retained for a long time, and have perhaps escaped observation in later stages owing to the development of papilla-like prominences of the integu- ment, such as occur in great numbers in the adult. St. Remy (No. 8) describes and illustrates a paired ganglionic swelling on the brain of the adult Peripatus ("formation de nature inconnue"), which in position corresponds to the two prominences on the head of the embryo, and which might well be regarded as the lobe of a primary tentacle-nerve. In the posthumous works of Balfour also, similar structures are described as pairs of nerves (running to various points of the dorsal surface), and of these one might belong to such a sensory organ. We cannot refrain, in this connection, from calling attention to the sensory organs found in the cephalic region in many Myriopoda (e.g. , Lithobius, Polyxenus, and Glomeris), the innervation of which is said to take place from the "optic thalamus" (Tomosvaky, No. 22, p. 760). We must however impress upon the reader that the actual material required for a successful comparison of this peculiar sensory organ with the frontal organ of the Crustacea, or with the still insufficiently investigated prominences of Peripatus has not yet been obtained. In the younger embryos of Peripatus (such as that illustrated in Fig. 91 A), the change of position of the antennae, if these are considered as proceeding from limbs, is not very marked, especially as compared with the corresponding change that takes place in the Insecta. The position of the eyes in Peripatus is less easily reconciled with this view. The eyes lie further back than the antennae, close to that part of the brain which must be derived from the first (primary) trunk-segment. The eyes, however, may well be ascribed to the primary cephalic segment, especially as, in Peripatus, they agree with the eyes of the Annelida rather than with those of the Arthropoda. This can only be explained as having been brought about by the shifting of the various parts which participate in the formation of the head. 3. The Formation of the Organs. The Ectodermal Structures. The Integument. The ectoderm forms a single layer of cubical cells over the greater part of the body of the embryo. In P. capensis these cells, especially on the dorsal surface, are said by Sedgwick not to be sharply demarcated externally, and to exhibit a spongy structure. Sedgwick on this account ascribes to them the capacity for absorbing fluid nourishment, and believes that the placenta described by v. Kennel might arise as a more specialised ectodermal organ for taking in nourishment. The changes undergone by the ectoderm when trans- formed into the adult integument are not very important. The delicate cuticle which occurs in the adult is secreted externally. At some points, e.g., on the ventral side of the limbs, the ectoderm becomes multilaminar and here secretes a thicker layer of chitin, and this is also the case at the distal extremities of the limbs where the claws are formed. THE NERVOUS SYSTEM AND THE VENTRAL ORGANS. 189 The Nervous System and the Ventral Organs. The nervous system and the ventral organs arise from two massive thickenings of the ectoderm formed by the active increase in number of the cells on the ventral side of the cephalic and primary trunk- segments. The two longitudinal swellings thus produced appear at the time when the limbs become sharply marked off from the body, and develop from before backward. According to Sedgwick, each of these swellings passes directly into a corre- sponding thickening of the cephalic (antennal) segment, but this, according to v. Kennel, is not the case, the swellings ending bluntly where the cephalic section commences (Fig. 92), so that the part of the central nervous system pertaining to the cephalic segment arises separately from the rest.* A much slighter thickening of the ectoderm does, however, occur, according to v. Kennel, between the cephalic and the trunk portions of the longitudinal swellings at the time when the latter appear. And this, since it denotes the formation of a commissure, might be regarded as indicative of a continuity between the cephalic and the trunk portions of the longitudinal swellings. This question as to the continuous origin of the cephalic and the trunk portions of the central nervous system has already been discussed in connection with the Annelida (Vol. i., p. 287). It was not indeed there finally settled, but it is in connection with them that a decision of the question can best be expected. The paired thickening on the ventral surface just described gives origin not only to the nervous system, but also to the ventral organs (v. Kennel, No. 4). Transverse sections of the embryo show that the thickening projects both externally and internally (Figs. 100, p. 200, and 101, p. 202). In the middle of the cell-mass Avhich forms it, a horizontal fissure then arises extending from before backwards and separating the mass into an outer and an inner portion (Fig. 100 B). The inner mass of cells represents the rudiment of the nervous system (»), the outer, remaining in connection with the epidermis, represents that of the so-called ventral organs (vo), the development and significance of which must now be discussed. The ventral organs. As the cleft, which in each segment divides the rudiment of the nervous system from that of the corresponding ventral organ, is interrupted by intersegmental cellular strands con- necting its two walls (Fig. 101 B, p. 202), a segmentation of the ventral organs takes place which is visible even externally. The connecting strands between the ventral organ and the nerve-cord, which occur between the consecutive pairs of limbs, are retained * An entirely distinct origin for the brain and for the ventral chain of ganglia cannot here be asserted, inasmuch as the ganglion of the maxillary segment is also drawn into the brain, as will be shown presently. On this account and also because of the relation above pointed out, of the antennae to the limbs, there is room for doubt as to the true cephalic nature of the brain in Peripatus. 190 ONYCHOPHORA. until the embryo is mature, and are even found in the adult (v. Kennel). As development proceeds, the ventral organs shift together and finally unite in the middle line ; they become flattened and a slight depression is seen on their outer surface. Whereas at first they were very massive (Figs. 100 and 101 B), they now appear much smaller as compared with the size of the embryo (Fig. 102, p. 205). As the embryo develops, they become less and less con- spicuous, and, in the adult, are represented merely by a small unpaired follicular depression of the epidermis situated medianly between the bases of the limbs on each segment (v. Kennel), and until recently overlooked. an. - Fig. 96.— Transverse sections through the head of an embryo of P. Edwardsii (after v. Kennel). In A, only half the section is drawn, an, antennal nerve ; n, brain (consisting of cell- and iibre-substance) ; us, primitive segment of the head ; co, ventral organ. The ventral organs of the anterior segments differ from the rest. Those belonging to the segment of the oral papillae, as well as those of the maxillary segment, are drawn into the buccal cavity, and can still for a time be recognised on its floor (Fig. 94 A, vo 1 and vo 2 ). Of these, the two posterior unite to form the ventral wall of the oesophagus, while the anterior organs remain distinct. Consequently each of these latter develops further independently, and in both the external depressions are more marked than the other trunk ventral organs (thus retaining, according to v. Kennel, the more primitive character). This is still more the case with those structures which must no doubt be regarded as ventral organs of the cephalic segment. These are two deep epidermal depressions lying near one THE NERVOUS SYSTEM AND THE YEXTRAL ORGAN-. 191 another on the ventral side of the cephalic segment (Fig. 96 B, which have arisen, like the ventral organs of the trunk, by the splitting up of the ectodermal layer into an outer and an inner portion, and the subsequent invagination of the former. These depressions, -which at first are quite open, but later close almost completely, can be recognised even in surface view, first as pits, and later as irregular slits on the ventral surface of the cephalic lobes (Figs. 93, p. 18-1, and 94 B, p. 185). At a later stage the ventral organs close completely and lose their connection with the epidermis. As the two vesicles sink down deep into the mass of the brain (Fig. 96 B) and thus become closely connected with this latter, it is clear that, when the brain becomes small in comparison to the head and shifts to the dorsal side of the latter, the vesicles follow the brain, and remain connected with it in the form of a thick-walled vesicle, the so-called brain-appendage of Peripcdu*. The ventral organ of the cephalic segment, if, indeed, this vesicle is to be considered as such, would be distinguished from those of the trunk by the complete loss of its connection with the epidermis. The significance of the ventral organs has until now remained obscure. Their great development in the early part of embryonic life, and their reduction in the adult, indicates that they are organs which were more highly developed in the ancestors of Peripatus. From their position it might be concluded that perhaps the greater part of the ventral surface, by means of its strong ciliation, functioned for locomotion, like the ventral ciliated area of the Annelida. The connection of the ventral organs with the nervous system is not surprising, considering the origin of the latter out of these ectodermal masses. It is possible that during ontogeny the ventral organs may be concerned in supply- ing the cell-material for the development of the ventral chain of ganglia, v. Kexxel's statement that the gradually diminishing cell-mass of the ventral organs is used in the further development of the epidermis seems in keeping with the original connection of these organs with the ectoderm, especially as, with the exception of the ventral organ of the cephalic segment, the greater j>art of each organ retains this connection. The similarity between these cephalic ventral organs and the "cephalic pits" of the Arachnida, which are in the closest connection with the nervous system (pp. 12 and 53), is very striking. Fig. 96 B shows how closely the "ventral organs" of the head of Peripatus become applied to the rudiment of the brain, and comparison of Figs. 93 and 94 B, with Fig. i C, p. 6, Fig. 7, p. 10, and Fig. 28, p. 52, shows that a marked agreement exists even in the external position of the depressions in the two groups. In the present state of our knowledge, however, we are not justified in carrying this comparison further.* The Nervous System. When the rudiments of the two longi- tudinal nerve-trunks first separated from those of the ventral organs, * [Willey (App. to Lit. on Onychophora, Xo. II.) finds what he believes to be ventral organs persisting in the adult on the anal segment. — En.] 192 ONYCHOPHORA. a thin layer of fibres had already appeared on the dorsal side of the former. As development proceeds this gradually thickens (Figs. 101, p. 202, and 102, p. 205). The position of this fibrous layer on the mass of the ganglion-cells is practically retained in the adult, for even there the fibrous mass lies dorsally to the ganglionic cells (Balfour, No. 1), and only a very few of the latter attain a position dorsally to the fibrous mass. This feature must be regarded as a primitive one. In more highly differentiated forms, e.g., the Crustacea and the Arachnida, the fibrous mass is indeed peripheral when it first appears, but is soon covered by ganglionic cells, and comes to lie within the mass of the ganglion. It has already been pointed out, in connection with the Crustacea (Vol. ii., p. 162), that the appearance of the fibrous substance on the inner periphery of (i.e., dorsally to) the ventral strands might represent a primitive condition. The transverse commissures which are, in Peripatus, found in large numhers connecting the longitudinal nerve-trunks, grow out from the latter like the peripheral nerves, which are said to be formed by the outgrowth of nerve- fibres (v. Kennel). The brain arises in a manner agreeing with the origin of the rest of the nervous system, but certain complications are caused by the fact that it is formed by the fusion of the ganglia of two distinct segments. The separation of the ganglionic rudiment of the cephalic segment from the epidermal thickening (ventral organ) takes place somewhat as in the trunk-segments, but the fibrous tissue here lies much deeper in the mass of ganglionic cells, and is partly covered dorsally by the latter (Fig. 96 B). From this dorsal cell- mass a strand of cells is continued into the antennal rudiment, and forms the rudiment of the antennal nerve (v. Kennel, No. 4, Sedgwick, Xo. 10, Pts. iii. and iv.). The latter therefore appears as a direct continuation of the cerebral ganglion, and is in this way distinguished from all the other peripheral nerves, which are merely outgrowths of nerve-fibres (without participation of ganglionic cells). The nerve-mass yielded by the cephalic segment soon grows to such a size as to occupy the greater part of the head; the two masses of ganglion cells, from which the antennal nerves proceed, shift towards the middle dorsal line, where they form a pair of large egg-shaped swellings (Fig. 97, g). The pair of ganglia composing the brain are at first separated by a deep slit. This becomes bridged over later, the fibrous mass of the two halves of the brain uniting THE NERVOUS SYSTEM. 193 to form a commissure (the so-called supra-oesophageal commissure, v. Kennel). This commissure is thus of secondary origin, and seems also to involve parts of the brain lying further back. These parts, however, do not belong to the cephalic segment, but are formed by the (maxillary) segment that follows it. When the jaws become enclosed in the developing buccal cavity, the ganglia of the maxillary segment also sink below the surface, and pass toward the dorsal surface, so that they can soon be recognised in viewing the embryo from the dorsal side (v. Kennel, Fig. 97, g m ). It must be assumed that this upward displacement takes place along the oesophageal commissures which are already present. The maxillary ganglia are thus approximated to those of the cephalic segment, with which they subsequently fuse. This fusion is very intimate, and the maxillary ganglia can be recognised as two moderate prominences behind the antennal ganglia (Fig. 97, g m ). The fibrous masses of the maxillary ganglia from the two sides unite to form a com- missure, the sub - oesophageal commissure, this being facilitated by the downward slope of the posterior ends of these ganglia. This method of formation of the sub-oesophageal commissure renders it very improbable that it is a primitive structure. A commissure lying further back and consisting of cells (Fig. 97, c) might rather be considered as such. This latter commissure connects two ganglionic swellings, which may perhaps be attributed to the segment of the oral papillae. All the commissures which follow this are, as already mentioned, said to be derivatives of the fibrous substance. The point that must be regarded as of most importance in the formation of the brain in Peripatus is the fusion of the maxillary ganglia with the ganglia of the cephalic segment, for this feature distinguishes Peripatus from the Myriopoda and the Insecta so far as is at present known, and connects it rather with the Crustacea, in which the ganglia of the segment of the second antennae unite with the brain (Vol. ii., p. 165). It therefore seems likely that o Fig. 97. — Anterior part of the central nervous system of an embryo of P. Edwardsii at a somewhat earlier stage than that depicted in Fig. 94 B, dorsal aspect (after v. Kennel), at, antenna ; au, eye ; c, first commissure after the sub-oesophageal com- missure ; gj and g/j, cephalic portion of the brain ; gm, portion belonging to the maxillary segment ; g IV , the next follow- ing ganglion ; op, oral papilla ; p /t first foot ; s, passage for the oesophagus ; sd, slime-gland. 194 ONYCHOPHORA. the jaws of Peripatus are to be homologised, not with the mandibles of the Insecta, but rather with the second antennae of the Crustacea. The question which naturally arises as to whether the corresponding segment has been lost in the Insecta, or, in other words, as to the relation to it of the mandibular segment, can hardly, in the present state of our knowledge, be profitably discussed. The close connection brought about between the maxillary segment and the cephalic segment increases the probability of the view expressed above, that the antennal segment also (now known as the cephalic segment) may have been united with a cephalic section formerly present, and now to a great extent degenerated. We were led to this assumption by the presence of the two prominences in front of the antennal rudiments (Fig. 94, x), and by the close agreement in manner of formation between the antennae and the feet. It is, indeed, difficult to reconcile with this view the statement that the antennal nerve forms in a manner essentially different from the peripheral nerves, but this point has as yet received too little attention to be considered as of decisive importance. The Eyes. The rudiments of the eyes have already appeared before the separation of the nervous system from the ventral organs. On the dorsal boundary of the ectodermal thickening in the cephalic segment, a small pit is formed on each side, behind and somewhat ventrally to the rudiments of the antennae ; the floor of this pit is at first connected with the ectodermal thickening, but soon becomes detached from it. The pit closes to form a vesicle, which becomes constricted off from the ectoderm. Outwardly, i.e., towards the epidermis, this vesicle is unilaminar, but on the inner side it is multilaminar. Pigment appears on the inner boundaries of its cells, and in its cavity the lens is secreted. The cells of the inner and lateral walls yield the rods of the retina. A differentiation into cell- and fibre-substance has already taken place in the thickened inner wall of the optic vesicle, and a connection which occurs between this part and a process sent out by the brain gives rise to the optic nerve, which is thus a secondary formation (v. Kennel). Sedgwick's account of the origin of the eyes in Peripatus is somewhat different. According to him, the region in which they originate still belongs to the brain, and they do not lose their connection with the latter, the inner wall of the optic vesicles remaining united with the cell-mass of the brain. The optic nerve arises at this point later by simple constriction. The eyes thus originate chiefly from the brain, and are covered merely by the ectoderm of the surface ; they are "cerebral eyes," according to Sedgwick, in opposition to v. Kennel, who believes, as stated above, that they arise independently of the brain. It is possible that the observations made on the origin of the eyes in Perijmtus can be harmonised with those of the development of the eyes in the Annelida. The eyes of Peripatus agree closely with more highly organised Annelidan eyes, THE SLIME- AND THE CRURAL GLANDS. 195 such as those of the Alciopinac. According to Kleinenbeeg (A r ol. i., p. 289, Annelidan Lit., No. 26), the eyes in this family arise independently of the cephalic ganglion as two ectodermal invaginations, but the inner wall of the optic vesicle is said to become closely connected with the brain, giving off cell- material direct to the latter. The elements of the two organs, in any case, seem for a time to be closely united, at the very point where the optic nerve forms later. If Kleinenbeeg's observations are confirmed, a similar condition might be thought to prevail in Pcripatus also, and the opposing views of v. Kennel and Sedgwick might thus be explained. The Slime- and the Crural Glands. The slime-glands are of ectodermal origin, arising as depressions on the tips of the oral papillae (Fig. 93). At first the pits are shallow, but they gradually deepen and their blind ends grow inwards and backwards. The pit has thus, at the stage depicted in Fig. 94 B, become a conical tube (Fig. 97, sd), which has grown back to the intestine. This growth continues in the following stages, so that the glands eventually attain a considerable length. They retain their simple tubular form; the branches which occur in them in the adult appear as outgrowths shortly before the embryo is mature and ready for birth (v. Kennel). The slime-glands are no doubt to be regarded as modifications of the crural glands, which, as sac-like structures, lie in the lateral divisions of the body-cavity and open on the ventral side of the feet. In the different species of Peripatus they differ in number and in distribution. These glands first appear at a late stage of embryonic development as ectodermal invaginations lying at the bases of the feet distally to the apertures of the nephridia (Fig. 102, c, Sedgwick). In the male (P. capensis) the crural glands of the last pair of feet are transformed into long glandular tubes (Balfour). The Alimentary Canal. With the exception of the short stomodaeum and proctodaeum, which are ectodermal derivatives, the alimentary canal is of ento- dermal origin. The following account is derived principally from the description given by Sedgwick of P. capensis, this form being chosen because we must regard it as more primitive than the American species examined by v. Kennel. The two forms vary principally in the first stages of the development of the intestine, the later stages showing great similarity. In order to understand the formation of the intestine, we must revert to the gastrula stage of P. capensis. The blastopore there leads into a cavity, which is lined by a thick protoplasmic 196 OXYCHOPHOEA. syncytium containing nuclei and rich in vacuoles. This voluminous nucleated mass must no doubt be regarded as corresponding to the yolk with its nuclei found in P. novae-zealandlae. In the latter form, the nucleated yolk forms part of the boundary of the arch- enteric cavity. In both forms the blastopore lengthens (Fig. 99 .4), and is constricted in the middle of its length, where its edges become approximated and fused ; thus the original single blastopore becomes divided into two apertures (Fig. 99 A-C). During this process, the vacuolated entodermal syncytium becomes arranged into a regular epithelium which, where the blastopore is still patent, passes over into the ectoderm, but in the region of the closed blastopore forms a tube that is said at first to be connected with the mesoderm-bands lying in that region, but to become isolated later, thenceforth form- ing a distinct entodermal tube. In P. novae-zealandiae, in consequence of the large amount of yolk present, this process is somewhat different. The entoderm-cells are here said to become arranged at the periphery of the yolk into an epithelium which thus surrounds the yolk. The latter would then he gradually absorbed during the further development of the intestine. The mouth and anus form as in P. capensis (Sheldon). The two apertures derived from the elongation and constriction of the blastopore (Fig. 99 D) are the primitive mouth and anus. They do not, however, persist as those organs in the adult, owing to the appearance of a depression of the ectoderm at each of the openings, so that the point of union between the ectoderm and the entoderm is shifted inward, and an ectodermal stomodaeum and proctodaeum are formed. The changes of form undergone by the embryo have their influence on the rudiment of the intestine. As a consequence of the curvature of the embryo, the entoderm extends anteriorly and posteriorly above the mouth and the anus (Fig. 98 ^4). The anterior wall of the stomodaeum thus runs forward. During the further development of the embryo, however, the course changes. When the mouth is shifted more to the anterior end, the anterior entodermal sac degenerates, and the stomodaeum now appears directed posteriorly (Fig. 98 B). The dorsal wall of the anterior portion of the intestine up to this point was closely apposed to the body-wall (Fig. 98 A and B), but the latter now separates from the gut and forms the swollen anterior end of the embryo (Fig. 98 C). It is followed in this course by a diverticulum of the entoderm, while the stomodaeum retains its former position. This diverticulum is THE ALIMENTARY CANAL. 197 also obliterated in the further course of development, and the intestine then runs straight back. The stomodaeum gives rise in Peripatus to the muscular oesophageal swelling (pharynx), meso- dermal tissue also contributing to its formation. The external changes in the mouth have already been described in connection with the external form of the body (Fig. 94, p. 185). The growth of the embryo produces similar changes at the posterior end of the intestine. A B. C. Fig. OS. — Median longitudinal sections through embryos of P. capensis at various ages (after Sedgwick), are, anus; di, anterior entodennal diverticulum; ■rut, entoderm; m, mouth; st, stomodaeum. In the American species of Peripatus, the intestine even at its first appearance differs from that of P. capensis, as no elongation of the blastopore occurs in these forms (v. Kenuel). The rudiment of the enteron, which is completely closed to the exterior and has been produced by the ingrowth of cells (Figs. SO, p. 171, and SI, p. 173), is here sac-shaped. As the embryo lengthens, the enteron also extends in the form of a tube. Its connection with the ectoderm is brought about through the fusion of the entoderm with the ectoderm, an invagination of the latter at this point forming the oral aperture. The mouth arises ventrally ou the boundary between the head and the trunk, and the anus in front of the blastopore (Fig. 89 A). It has already been pointed out (p. 178) that these two apertures occupy the same positions as in P. cwpensis, and that they perhaps 198 OXYCHOPHORA. B. were originally related to the blastopore. v. Kennel, however, does not believe this, and, further, seems little inclined to attribute much value to the observations on this point made in other species of Peripatus. He attributes an altogether different significance to the groove in the blastoderm observed by himself and described by us iu accordance with the views of English authors as the blastopore. The further development of the pharynx takes a course similar to that above described, the primary oral aperture shifting inwards, while an anterior ento- dermal diverticulum appears. The anal aperture, on the contrary, which arose in front of the blastopore through the formation of a slit (Fig. 89 A), is said not to coincide with the adult anus. The former closes by the approximation of its edges, and an ectodermal invagination arises a little distance in front of it, grows inward towards the entoderm and fuses with it. The rectum and anus are thus formed, the latter then shifting more to the posterior end of the embryo in consequence of the unequal growth of the latter (v. Kennel). The Mesodermal Structures. The formation of the chief mass of the meso- derm proceeds from a zone of growth lying at the posterior end of the blastopore, and extends forward from this point in the form of two bands (mesoderm - bands) lying symmetrically to the ven- tral median line. Where a slit -like blastopore is present, as in the African and Australian species, the mesoderm -bands lie in close contact with it, and are thus situated in the region where the ectoderm passes into the entoderm. After the blastopore has partially closed, the posterior (anal) aperture lies in front of the growing zone, and its position is the same in Fig. 99.— Ventral aspect of embryos of P. capensis, to ,, . . . illustrate the segmentation of the mesoderm (after the American species, 111 Balfour and Sedgwick), a, anus ; hi, blastopore ; which the blastopore is to, mouth.; its, primitive segments; w, zone of . growth. not slit-like. THE MESODERMAL STRUCTURES. 199 English authors, in accordance with the terminology used in the Vertebrata, have called the growing zone the primitive streak, and the groove-like depression that occurs in it the primitive groove. If such a groove occurs, it must no doubt be regarded as a continuation of the blastopore, and we must assume that it is not the most posterior part which is retained as the anus. The growing point itself must be considered as lying on the posterior margin of the blastopore. At this point, a great accumulation of cells takes place, and the germ-layers are here still fused and undifferentiated. In so far as the mesoderm-bands extend forward from this undifferentiated cell -mass, the condition here to a certain degree resembles that in the Annelida. Sedgwick even speaks of polar cells of the mesoderm, but of these nothing definite is known. There can be no doubt that the mesoderm is chiefly produced from behind, i.e., from the growing zone, but in consequence of the close apposition of the mesoderm-bands to the edges of the blastopore, the participation of the latter in their growth cannot be excluded (Sedgwick). In the American species, it appears certain that no such partici- pation occurs. The forward growth of the mesoderm-bands takes place from the point of ingrowth, which must be regarded as the blastopore, and their growth determines the lengthening of the whole embryo. The mesoderm-mass here separates from the sac-like rudiment of the enteron (Figs. 100 and 101), but not so sharply as to exclude a connection of the former with the ectoderm on the one side and with the entoderm on the other, which can be proved to exist even at later stages, when the mesodei'm has become far more highly differentiated. The mesoderm may thus be regarded even in this case as arising on the boundary between the ectoderm and the entoderm. The further development of the mesoderm-bands takes place in a manner very similar to that in which they develop in the Annelida. Before they have reached the anterior end of the blastopore, they break up into paired, regularly arranged segments (Fig. 99 A-C, us). Cavities then appear in these, and, as these gradually widen, the cell-material of the separate segments becomes arranged into a regular epithelium. The paired primitive segments thus arise. As they extend further, the outer wall becomes applied to the ectoderm and the inner to the entoderm (Fig. 100), like the somatic and splanchnic layers of the Annelida. A pair of primitive segments belongs to each body-segment. The differentiation of the primitive segments commences in the most anterior part of the mesoderm-bands and extends backward, their number increasing with the growth of the body ; the first pair of primitive segments to develop is thus that belonging to the cephalic segment, and this is also much larger than any other pair. It extends almost to the ventral and dorsal middle lines ; the two halves, however, do not come into contact, and con- sequently no mesentery is formed. Transverse sections through the body of an embryo at the stage when the primitive segments are being differentiated closely resemble, especially in the anterior and posterior regions, similar sections through an Annelidan embryo ; 200 ONYCHOPHORA. as. they show the ectoderm with its ventral thickenings, and the two niesoderm-segments containing the primitive body-cavity, bounded by the epithelial walls, applied to the ectoderm and the entoderm (Fig. 100). Such anatomical and histological differentiation is present in the embryo represented in Fig. 88, and no further essential change appears until twelve to fifteen segments are visible externally, together with the full (adult) number of internal segments (v. Kennel). When the mesoderm-bands have broken up into the series of consecutive primitive segments, the resemblance with the Annelida is very striking, but the further course of development differs, inasmuch as it is not the segmental cavities which yield the A. 1. nus. us. vor.tn Fig. 100. — Transverse sections through embryos of P. capensis (A) and P. Edwardsii (P.) (after Sedgwick and v. Kennel). A, transverse section through the region of the oral papillae in an embryo at about the stage depicted in Fig. 91 A. B, transverse section through a trunk- segment of a young embryo, d, intestine (entoderm) ; Ih, dorsal and ventral spaces between ectoderm and entoderm (parts of the primary and adult body-cavity) ; I, lateral, m, median portions of the segmental cavities ; mes, portions of mesoderm detached from the primitive segments; n, rudiment of the ventral cord; us, primitive segment; vo, ventral organ ; vo+n, common thickening of the ventral organ and the ventral nerve-cord. body-cavity of the adult, for, in Pervpatus, the latter arises as a pseudocoele independent of the primitive segments. All that is retained of these segments enters into the formation of the nephridia and the genital organs (v. Kennel, Sedgwick). The formation of the future body-cavitj' and of the nephridia is commenced by a thickening of the ventral Avail of the primitive segments ; and subsequently, by an ingrowth of the cells of this thickening, a separation of the segmental cavity into two spaces is brought about, one dorso-median and the other lateral (Fig. 100 B, m and 1) ; these are at first connected, but become completely separated later (Fig. 104 A, p. 210). The dorsal portion shifts THE BODY-CAVITY AND THE BLOOD-VASCULAR SYSTEM. 201 towards the dorsal median line, while the greater part of the lateral portion is withdrawn into the rudiments of the limbs (Fig. 104, v. Kennel, Sedgwick). Even before this separation has commenced, while the primitive segments still retain their sac-like shape, the antero-dorsal portion of each grows forward over a part of the preceding primitive segment, and thus extends into the preceding body-segment. This explains the fact that in transverse sections we not only see the segmental cavity of the segment through which the section passes, but also a portion of that belonging to the next segment, and that this latter lies above the ventral portion of the segmental cavity of the preceding segment. The lateral portions of the primitive segments yield the nephridia, and the dorso-median the genital glands in the segments which contain these organs ; in the other segments these portions disappear, their cell-elements being used in the formation of the blood vascular system and the musculature, and for the further development of the pseudocoele, which now comes under consideration. The Body-cavity and the Blood-vascular System. Even before the division of the primitive segments into two portions, the ectoderm had separated from the entoderm with which it was until then in close contiguity, thus giving rise to a free space dorsally and ventrally to the intestine. These spaces are the first indication of the body-cavity of the adult (Fig. 100 A and B, 111), and into them the mesoderm-cells which become detached from the primitive segments wander. As these cells become applied to the entoderm and ectoderm, the cavity which is at first bounded merely by these two germ-layers, and is therefore to be regarded as the primary body-cavity, becomes lined with mesodermal elements (Fig. 101 A, Hi). These spaces, in consequence of their origin, are not segmented, but the other and lateral portions of the future body-cavity, which arise by separation of the cell-elements in the inner thickened somatic wall of the lateral portions of the primitive segments, exhibit a segmental arrangement (Fig. 101 A, l.lh). These cavities, at first distinct from one another, fuse together later, and give rise to the two spaces, the lateral sinus of Sedgwick, which later run continuously through the body, and in which the nerve- strands lie in the adult. Another space on each side of the body agreeing in origin with these latter spaces, develops still more peripherally in the limb-rudiments and surrounds the nephridia (Figs. 101 and 102, p.lli). This last part of the body-cavity, which may best be described as the pedal body-cavity, unites later in some 202 OXYCHOPHORA. places with the lateral spaces, so that, where this is the case, the nephridia and the longitudinal nerves come to lie in one common cavity. Several cavities unite to form the central space which, in the adult, contains the intestine and the genital organs. According to n. us n 3* ***«' Wi>v '^$m?m & Fig. 101. — Transverse sections through embryos of P. capensis at different ages, A being taken through the segment of the oral papillae (somewhat diagrammatic, after Sedgwick), d, intestine; lh, dorsal and ventral median portions of the body-cavity; m.lh, lateral portion of the median body-cavity; n, rudiment of the ventral cord; ne, nephridia (in A, rudi- ment of the salivary glands) ; oc, external aperture of the same ; p, limb ; pe, pericardial cavity; p.lh, pedal body-cavity; sh, segmental cavity; its, dorsal portion of the primitive segments ; vo, ventral organs. Sedgwick, two new spaces (Fig. 101 B, pe and m.lh) appear on the outer side of each of the dorsal portions of the primitive segments (sh), the wall of which to some extent forms their inner boundary. THE BODY-CAVITY AND THE BLOOD-VASCULAR SYSTEM. 203 The lower of these spaces (m.lh) at a later stage grows above the intestine, and unites with its fellow and with the space which has already appeared beneath the intestine (Ih) to form the greater part of the permanent median cavity, the so-called central compartment of the body-cavity, while the upper one represents (pe) the rudiment of the pericardial cavity. The pericardial spaces on each side extend towards the median line, the remains of the primitive segments being thus displaced downwards. The cavity (lit) that arose early above the intestine thus appears confined, together with the dorso-median portions of the primitive segments (s7i), between the two pericardial spaces (pe) ; the latter grow above the pseudocoele (Ih), and also between the latter and the dorsal portions (sh) of the primitive segments, and unite with one another in the middle line. Thus the common pericardial cavity is formed, surrounding the dorsal pseudocoele (Ih) ; the latter now assumes a tubular form and becomes the definitive heart (Fig. 102, h). According to Sedgwick, the primitive segments take no part in the formation of the heart. The ostia of the heart, the formation of which has not been closely observed, do not arise until later, when the embryo is ready for birth. Detached mesoderm-cells, which become applied to the outer wall of the heart, give rise to the cell-mass within the pericardial cavity, which has been compared to the fat-body of the Insecta. It involuntarily reminds us of the cell-growth on the dorsal vessel of the Annelida, which is probably homologous with the pericardial gland of the Mollusca ; but we are prevented from homologising the two structures because the pericardial gland, as an outgrowth of the peritoneal epithelium, lies within the secondary body-cavity, while the cell- mass in Peripatus lies outside the latter. The pericardial space in Peripatus, like that of other Arthropods, does not correspond to the pericardium of the Mollusca or the coelom of the Annelida. Only its ventral wall (the pericardial septum, Fig. 101 B, and 102, ps) is perhaps in part formed by the somatic wall of the primitive segments, as is also the case in the Insecta. In Peripatus, as in the Arthropoda, the dorsal vessel is in direct communication, in the adult, with the body-cavity, and this fact is explained by the similarity in the development of this system of organs in the two divisions. In the two anterior (cephalic and maxillary) segments, the trans- formation of the primitive somites undergoes certain modifications determined by the special form of these parts. In the maxillary segment, the inner or dorsal part of the primitive somite is not extensive, and fuses with the corresponding part of the succeeding segment which projects into this segment. The different spaces of the permanent body-cavity are here less distinctly developed. The lateral parts of the primitive segments which occupy the rudi- 204 ONYCHOPHORA. merits of the jaws undergo considerable thickening of their outer walls, the formation of the strong musculature of the jaws being thus brought about, the inner wall supplies the cells which form the muscles of the pharynx and stomodaeum. The primitive somites in the cephalic segment are at first very large and occupy the greater part of the segment. As the ventral organs and the brain increase in size, the primitive segments are, however, pressed towards the dorsal surface, and thus become less extensive. Parts of the primitive segments pass into the antennae (as elsewhere into the feet), so that these latter at first appear to be hollow, though the cavity degenerates later (Fig. 96 A, us). The wall of the first primitive segment gives off cells for the formation of the musculature of the oesophagus. According to Sedgwick, the anterior primitive segment, like the rest, is divided into a dorsal and a lateral portion, the significance of which will be discussed below (cf. the Nephridia). The Musculature. Even in early stages, before any differentiation had taken place in the primitive segments, cells became detached from them and became applied to the ectoderm. These cells, and others which follow them during the further development of the mesoderm, give rise, immediately below the ectoderm, to a layer of circular muscle-fibres, which at first is thin, but in later stages becomes much thicker (Sedgwick). The longitudinal muscles arise later than the circular fibres, their fibres appearing in the cell-layer that covers the latter internally. According to Sedgwick, they are distributed into various com- plexes, one ventral, two ventro-lateral, two lateral and two dorsal, corresponding to the longitudinal muscle-bundles of the adult. The musculature of the intestine and of the inner organs generally is derived from the wandering cells which become detached from the primitive segments and applied to these organs. The Nephridia. The nephridia arise in the following way from the lateral portions of the primitive segments, the greater part of which occupy the bases of the limbs. Each primitive segment has a conical outgrowth directed towards the ventral side, which lengthens and, at the base of the foot, fuses with the ectoderm, which becomes perforated at this point, and thus the cavity of the primitive segment opens on to the exterior (Fig. 101 A); this aperture persists as the external opening of the nephridium (Sedgwick). The nephridium is now essentially complete (Fig. 101), for it does not possess a funnel opening into the adult body-cavity, as was formerly believed by Balfour and Gaffron, but, according to the latest observations of Sedgwick, throughout life ends blindly in this direction, the canal THE NEPHRIDIA. 205 of the nephridium being continued into tlie terminal saccular enlargement (Fig. 102, es). "We must thus assume that the nephrostome of the Annelida is represented by the opening of the nephridium into the terminal sac. The terminal sac, therefore, corresponds to the coelom (secondary body-cavity of the Annelida), a view which is confirmed by the manner in which the nephridia arise. A part of the coelom has thus come into direct relation to the kidney, and a state of things is found very similar to what has already been met with in the Crustacea (Vol. ii. , p. ISO), and, with certain modifications, will be found to recur in the Mollusca. jnes Fig. 102.— Transverse section through the posterior region of the body of an advanced embryo of P. capensis (after Sedgwick, somewhat diagrammatic), c, rudiments of the crural glands ; d, intestine ; e.?, end-sacs of the nephridia; h, heart ; l.lh, lateral, m.lh, median, p.lh, pedal portions of the adult body-cavity ; mes, mesodermal tissue ; n, ventral nerve-trunk ; ne, nephridial canal ; oc, external aperture of the nephridium ; p, foot ; pe, pericardial cavity ; ps, pericardial septum ; sb, collecting vesicle (urinary bladder) of the nephridium ; sd, slime- gland ; so, sole of the foot (thickening of the ectoderm) ; st, transverse commissure connecting the nerve-trunks (n) and the ventral organ (vo) ; g, gonad. The above description of the simple formation of the nephridia applies specially to those of the segments carrying the first to third limbs (of P. capensis). Those of the following limbs are distinguished by the fact that the canal becomes much coiled in later stages and widens towards its outer end (Sedgwick, Fig. 102, sb), like the urinary bladder in the nephridia (antennal glands) of the Malacostraca. Apart from the transformation which we shall find in the nephridia during the formation of the salivary glands and the genital organs, there are im- 206 ONYCHOPHORA. portant changes to be observed in the cephalic and maxillary segments. In the latter the nephridia have degenerated ; traces of them only are said to be found (v. Kennel). In the cephalic segment, on the contrary, the two segmental cavities (in early stages) are said still to open outward through canals (Sheldon, No. 12, Pt. ii.). v. Kennel and Sedgwick describe a (canal-like) continuation of the primitive cephalic cavity which descends on the outer side of the ectodermal thickening (rudiment of the nervous system) and fuses with the ectoderm, immediately in front of the jaws (Sedgwick) ; according to L. Sheldon, indeed, it even opens outward at this point. This canal has been considered homologous to the canal of the nephridia. According to Sedgwick, it therefore belongs to the lateral portion of the first primitive segment. We cannot clearly make out, from the figures given, the relation of this lateral portion to the coelomic cavity of the antennae. We therefore refrain from discussing the position of this efferent canal as conrpared with those of the other nephridia, and merely point out that a remarkable change in the position of the nephridium towards the limb must have taken place if this canal is really the nephridial canal of the so-called cephalic segment, and if our former assumption that the antennae of Per (pectus are transformed limbs is correct (c/. p. 186). The Salivary Glands. According to v. Kennel and Sedgw t ick, who agree on this point, there can be no doubt that the paired gland -which opens into the buccal cavity through a short, common duct arises from the nephridia of the segment carrying the oral papillae. These ducts develop in the same way as the undoubted nephridia. They originate, after the separation of the dorso-median part, from the lateral portions of the primi- tive segments which develop an external aperture (Fig. 92, no, and 93, sp, p. 184). Their furtherdevelop- ment is peculiar only in so far that the canal, at the point where it passes into the terminal sac, begins to lengthen posteriorly (Fig. 103 .4), so that a long, blind tube arises at this point (Fig. 103 D, k). This tube gives origin to the principal part of the salivary gland ; it, however, retains through- out life the vesicular portion of the rudiment (s) corresponding to the terminal sac (v. Kennel, Sedgwick). The connection of the Fig. 103.— Formation of the salivary glands of 7'. capensis (after Sedgwick), k, canal of the gland; n, nephridial canal ; s, terminal sac, the walls of which in A appear thinner than in 1'.. THE ANAL GLANDS. 207 latter with the glandular tube becomes drawn out into a short canal (Fig. 103 B), which enters the latter dorsally (Sedgwick). The two external apertures of the nephridia (Fig. 103, sp) are displaced into the buccal cavity by the fold which encloses the mouth. They here come to lie in a transverse groove, which, as the buccal cavity develops further, becomes deeper and shorter. This groove eventually becomes a short canal with a slit-like lumen, into which the two nephridial canals (salivary glands) open. This is the common efferent duct of the salivary glands opening into the buccal cavity (v. Kennel). The Anal Glands. The so-called anal glands, a pair of glandular tubes which, in the male of P. Edwardsii, open ventrally on either side of the anus, and, in P. capensis, open through a short, common efferent duct at the genital aperture,* and are evidently related to the genital apparatus, are shown by their development to be modified nephridia (v. Kennel). They arise in P. Edwardsii from the primitive segments of the last (limbless) segment upon which the anus opens ventrally. The anal glands occur as rudiments in both sexes ; in the male only, however, do they attain the functional tubular form ; in the female they degenerate. In P. capensis, at the male genital aperture, a pair of glands open which are apparently the homologue of the anal glands of the American species. But since the nephridia of the segment which carries the genital aperture give rise to the efferent ducts of the genital apparatus (see below, p. 209), these glands must have a different origin. It appears probable that they are derived from one of the two additional pairs of primitive segments found by Sedgwick in P. capensis behind the primitive segments of the anal papillae. In this form, the genital aperture has shifted to a position quite near the anus, lying in front of it on the segment carrying the anal papillae. In P. Edwardsii, on the contrary, the genital aperture is found two segments further forward, on the penultimate limb-bearing segment. Since, according to Sedgwick, there are still two segments which remain in an undeveloped condition behind the last fully formed primitive segment (that of the efferent genital ducts), it might be assumed that these corresponded to the last limb-bearing segment and to the so-called anal segment of the American species. The latter would thus have two well-developed segments (the genital segment and that following it) in a region where in the African and New Zealand species a degeneration occurred, which led to the genital and anal apertures coming to lie on apparently one and the same segment. This would also explain the approximation of one of the last pairs of nephridia (the anal glands) to the antepenultimate pair (the efferent genital ducts). This assumption seems to be confirmed by the fact recently made * [In P. novac-britanniae (Willet), the pygidial (anal) glands open by a median aperture situated immediately above the anal orifice. — Ed.] 208 ONYCHOPHORA. known by L. Sheldon (No. 13) that, in P. novae -zealandiae, in the so-called anal segment, there are two coiled glandular tubes, each of which opens independently at the side of the body and laterally to the nerve-trunks, i.e., at a spot where normally the nephridial apertures open. These two glands are the equivalents of the anal glands (Sedgwick, Sheldon), and are more correctly called accessory glands of the male genital apparatus ; from their position, they may safely be regarded as modified nephridia. It should be mentioned further that the American species, which thus shows the more primitive condition in the segmentation of the posterior end of the body, shows on the other hand a less primitive method of reproduction. The shifting of the anus forward from the terminal segment must, indeed, in any case be regarded as secondary. The Genital Organs. In the fifteen anterior segments of the embryo of P. capensis, the dorso-median portions of the primitive segments are concerned in the formation of the pericardium and heart, but in the following segments their fate is quite different. After their separation from the lateral or nephridial portions, they shift towards the dorsal median line, and, decreasing in size, come to lie as small triangular sacs between the wall of the intestine and the pericardium (Fig. 102, g). It is these, according to Sedgwick, Avhich yield the genital glands. Cells appear in them at a very early stage ; these, which are distinguished by their size and specially large nuclei, are the primitive genital cells. We might assume with v. Kennel, that these arise in the Avail of the primitive segments themselves, or in the mesoderm-mass, before it breaks up into primitive segments, as will be described later in connection with the Insecta. On the other hand, Sedgwick ascribes an entodermal origin to the genital cells. [See Editorial Preface, Vol. II.] By the fusion of the dorsal portions of the primitive somites pertaining to consecutive body-segments and the breaking through of their transverse walls, two tubes are formed, and these come to lie in the middle division of the body-cavity. Up to this point, the rudiments of the genital organs are alike in the two sexes, but a histological differentiation now takes place, inasmuch as the genital cells increase more rapidly in the male, and become smaller, whereas in the female the germ-cells retain their large size. There is also an anatomical differentiation, the genital rudiments in the female fusing at the anterior end, while in the male they remain distinct in corre- spondence with the form of the genital apparatus in the adult. We must assume that the median portions of these posterior primitive segments yield the genital glands, while the efferent ducts are derived from the lateral portions of that primitive segment ORIGIN" OF THE MESODERMAL STRUCTURES. 209 which develops the genital aperture (in P. capensis this is the segment of the anal papillae, and in the American species the antepenultimate segment).* An actual separation into a lateral and a dorso-median section, such as takes place in the other primitive segments, does not, however, occur in the genital segment; here, indeed, the primitive somite extends dorsally, but its widened dorsal portion remains connected with the lateral portion. After this primitive segment, like the rudiments of the nephridia, has acquired an external aperture, its dorsal part fuses with the posterior ends of each of the tubular genital glands, and the rudiment of the genital organs is thus essentially completed. The two external apertures shift towards the middle line so as to lie beside one another. An invagination of the ectoderm then yields the unpaired terminal region (ductus ejaculatorius, vagina) of the genital apparatus. The development of the genital organs makes it evident that the cavity of the genital glands is homologous with the secondary body-cavity (or coelom). Its cellular lining thus corresponds to the peritoneal epithelium of the Annelida ; in both cases the genital products become detached from this, fall into the secondary body-cavity, i.e., in Peripatus the cavity of the genital glands, and pass out of the body through the nephridia. That the efferent ducts of the genital apparatus in Peripatus are homologous with the nephridia cannot be doubted. This is not only proved by their manner of developing, but is also confirmed by the fact that in the American species the nephridia are wanting in the antepenultimate segment, which carries the genital aper