CHAPTER IV

STEGOCEPHALI OR LABYRINTHODONTS–LISSAMPHIBIA–APODA

_SUB-CLASS I. STEGOCEPHALI OR PHRACTAMPHIBIA_

_With a considerable amount of dermal armour, especially on the head._

The earliest known terrestrial four-footed creatures occur in the Carboniferous strata of Europe and North America. They and their immediate allies, which extend through the Permian into the Upper Trias, are now comprised under the name of STEGOCEPHALI, so called because the whole of the dorsal side of the cranium is covered, or roofed over, by dermal bones (στέγος, roof; κεφαλή, head). That these creatures, of which naturally only the skeletal parts are known, were not fishes, is shown by the typically pentadactyloid limbs; but to recognise them as Amphibia, and as distinct from Reptiles, is difficult, especially if the incipient Reptilia, which have sprung from some members of this Stegocephalous stock, are taken into account. However, they possess either two occipital condyles, or none, and their vertebrae are either pseudocentrous or notocentrous, but not gastrocentrous. Moreover, the whole skeletal organisation is still so ideally generalised, that it is easy to derive directly from it the arrangement prevailing in the Apoda and Urodela.

The vertebral column always comprises a well-developed, sometimes a very long tail. The vertebrae exhibit three types, two of which are fundamentally distinct, while the third is a further development of the second.

1. _Lepospondylous and pseudocentrous._–The vertebra consists of a thin shell of bone surrounding the chorda dorsalis, and is composed of two pairs of arcualia, which meet each other, {79}forming a suture, along the lateral side of the vertebra, both partaking in the formation of a transverse process which carries the rib.

2_a_. _Temnospondylous._–The vertebra is composed of three pairs of units, which remain in a separate, unfused state. Two of them are dorsal arcualia, one of which tends to form the centrum of the vertebra, which then carries the neural arch.

2_b_. _Stereospondylous._–The three component units fuse by co-ossification into a solid, amphicoelous vertebra.

The ribs are one- or two-headed, rather strong, but short, rarely reaching half-way round the body. They occur on all the vertebrae of the trunk and on most of those of the tail. One pair of ribs connects one vertebra, the sacral, with the pelvis, of which the ilium and ischium are generally ossified, rarely also a portion of the pubic region.

The shoulder-girdle is very primitive, greatly resembling that of the Crossopterygian fishes. It consists of the following bones:–a median, rhombic, or T-shaped interclavicle, a pair of clavicles, of cleithra, of coracoids, and of scapulae. The limbs show the typical pentadactyle plan, but even in these earliest Tetrapoda the hand possesses only four fingers, with 2, 2, 3, 2 phalanges respectively. The foot has five toes, with 2, 3, 4, 4, 3, or 2, 2, 3, 4, 3 phalanges.

Many Stegocephali were possessed of a dermal armour, covering either the whole body or only the under parts. Hence the term Phractamphibia (φρακτός, armoured). The armour consists of a great number of small cutaneous scales,

## partly calcified, or perhaps ossified, and arranged in many more or less

transverse rows. We can only surmise that these scales were covered by corresponding epidermal sheaths. The skull is ideally complete in the number of separate bones which appear on its surface. Besides the outer nares and the orbits there is always an unpaired, small, interparietal foramen. The whole temporal region is completely roofed over. The following bones are present:–nasals, frontals, parietals, supra- and latero-occipitals; lacrymals (unless fused with the jugals?), prefrontals, postfrontals, postorbitals, squamosals, and epi-(or opisth-)otics; premaxillaries, maxillaries, jugals, quadrato-jugals, and supra-temporals; quadrates, pterygoids, palatines, vomers, and an unpaired parasphenoid.–The lower jaw is composed of a pair of dentaries, {80}articulars, angulars, and splenials. The dentaries and apparently sometimes the splenials, the palatines, maxillae, and vomers carry teeth. The eyes possess a ring of sclerotic bones.

ORDER I. STEGOCEPHALI LEPOSPONDYLI.

Vertebrae pseudocentrous.

SUB-ORDER 1. BRANCHIOSAURI.–The young had several pairs of gill-arches, which, to judge from their size and from the fact that they are beset with numerous nodules, denticles, or irregular little processes like gill-rakers–seem to have been exposed to the surface and to have carried gills. In the adult the arches and gills seem to be absent.

[Illustration: FIG. 12.–A, Dorsal and B, ventral views of the cranium of _Branchiosaurus salamandroides_, × about 4. (After Fritsch.) C, Posterior view of the cranium of _Trematosaurus_, × about ½. (After Fraas.) _Br_, Branchial arches; _C_, condyle; _Ep_, epiotic; _F_, frontal; _J_, jugal; _L.O_, lateral occipital; _M_, maxillary; _N_, nasal; _No_, nostril; _Pa_, parietal; _Pl_, palatine; _Pm_, premaxillary; _P.o_, postorbital; _Pr.f_, prefrontal; _Ps_, parasphenoid; _Pt_, pterygoid; _Ptf_, postfrontal; _Q_, quadrate; _Qj_, quadrato-jugal; _S.o_, supraoccipital; _Sq_, squamosal; _St_, supratemporal; _V_, vomer.]

One of the commonest genera is _Branchiosaurus_, including _Protriton_. _B. salamandroides_ of the Lower Red Sandstone of Europe is known in every stage, from larvae of 16 mm. to the full grown animal of 64 mm. in length. The whole body was {81}covered with little cutaneous scales. _Pelosaurus_ and perhaps _Melanerpeton_ are allied genera.

The following genera are small newt-like creatures of the Carboniferous age of Europe and North America. In _Keraterpeton_ of Bohemia, Ireland, and Ohio, the dermal scales were restricted to the under parts; and the ribs were rather long, reaching half way round the body. Gills have not been observed. _K. crassum_, a European species, reached more than one foot in length, two-thirds of which fall to the tail. The ventral side is covered with a most elaborate armour, which consists of about eighty chevron-shaped rows of little scale-shaped nodules. The epiotic bones end in strange processes, carrying a pair of spikes, giving the skull a "horned" appearance, hence the generic name. _Urocordylus_ is an allied genus.

SUB-ORDER 2. AISTOPODES.–Body snake-like and without any limbs, hence the name ἄιστος, unseen; ribs long, and reaching half way round the body; from Carboniferous strata in Ireland and Bohemia, with allied, or perhaps identical forms in Ohio. _Dolichosoma longissimum_ possessed more than 150 vertebrae, and was about a yard long. The epiotics end in obtuse projections, recalling those of _Keraterpeton_. These marvellous creatures had strange appendages, extending from behind the sides of the head, which were possibly the supports of external gills; since the upper end of one of the visceral arches, probably the hyoidean arch, is attached to the labyrinthic region, and from this arch starts a bony rod which carries long skeletal filaments. The body seems to have been naked.

_Ophiderpeton_ had a compound ventral shield, while the skin of the back contained granular scutes. Although the Aistopodes have, not without reason, been looked upon as greatly resembling the Coeciliae or Apoda in organisation, especially in that of the vertebral column, the total absence of any other fossils which might bridge over the enormous gulf between the Coal Age and recent times, makes the attempt to derive the Apoda from these creatures very hazardous.

ORDER II. STEGOCEPHALI TEMNOSPONDYLI.

Mostly with rather long ribs and with chiefly ventral armour.

{82}_Chelydosaurus_ from the Lower Red Sandstone of Bohemia was 3 feet long, and possessed a beautiful, complicated, ventral armour, consisting of about sixty chevron-shaped rows, about three times as numerous as the vertebrae in the corresponding region. _Sphenosaurus_ from the same strata and localities must have been 2 yards long. The trunk-vertebrae of both these genera were composed of four pairs of arcualia. _Trimerorhachis_ from the Permian of Texas is very imperfectly known, but its trunk-vertebrae, as the name implies, consist of three pairs of separate arcualia, one of which, the interdorsal pair, tends to form a kind of centrum.

_Dissorophus multicinctus_, also from the Permian of Texas, has been described by Cope[40] as a "Batrachian Armadillo," and considered allied to _Trimerorhachis_. Ten vertebrae are known, of an aggregate length of 93 mm.; the length of the creature was perhaps one yard. The neural spines are elevated, and the apex of each extends in an arch on each side to the ribs. These spinous branches touch each other, forming a carapace. Above, and corresponding to each of them, is a similar dermal and osseous element, which extends from side to side without interruption in the median line, forming a dermal layer of transverse bands which correspond to the skeletal carapace beneath it. This creature remotely approaches the genus _Zatachys_, Cope, where a dermosteous scute is co-ossified with the apex of the neural spine. The systematic position of this genus is at least doubtful.

_Archegosaurus decheni_ from the Lower Red of Germany, known by many well-preserved specimens, reached a length of 4 or 5 feet. The trunk vertebrae are tri-partite, those of the tail quadri-partite, like those of the trunk of _Chelydosaurus_. Young specimens show traces of gill-arches. The thoroughly terrestrial walking limbs have four fingers and four toes; the arrangement of the tarsalia, most of which are ossified, lend support to the view that the morphological axis went through femur, fibula, intermedium, the centralia, the second distal tarsale, and the second toe. The dentine and enamel of the teeth are much folded, and this feature, which applies to most members of this Order, to a lesser degree also to others, has caused them to be comprised under the name of LABYRINTHODONTA. The upper {83}surface of the head shows very characteristically arranged grooves, which probably contained slime-canals and possibly sensory organs.

_Actinodon_ and _Euchirosaurus_ are closely allied forms, chiefly from the Lower Red Sandstone of France; _Gondwanosaurus_ occurs in the Permian of India.

ORDER III. STEGOCEPHALI STEREOSPONDYLI.

These are the most highly developed members of the typical Labyrinthodonta, characterised by their much-folded teeth, and by their solid, bi-concave vertebrae. _Loxomma_ occurs in the Upper Carboniferous of England and in the Lower Red of Bohemia: _Trematosaurus_, _Capitosaurus_, and _Metopias_ from the New Red or Lower Trias to the Keuper of Germany. _Mastodonsaurus_ from the Trias of England and Germany is the most gigantic Amphibian known, with a skull of nearly 1 yard in length.

_Labyrinthodon_ from the Keuper of Warwickshire is one of the latest members of the group. Labyrinthodont creatures have also been described from the Trias of South Africa, _e.g._ _Rhytidosteus_; those from North America are insufficiently preserved.

Many of these and allied genera have left their footprints in slabs of Sandstone, both Lower and New Red, in Europe, Africa, and America. But although their spoors are common enough, only a few can with certainty be referred to Stegocephali, _e.g._ _Saurichnites salamandroides_ of the Lower Red of Germany. The spoors of _Chirotherium_, common in the New Red of Germany and England, for instance in Cheshire, belong to unknown owners; both the large hind feet (which measure nearly half a foot in length) and the much smaller fore feet, had five digits, the first of which stood off like a thumb. Five-fingered Stegocephali are unknown.

There is an almost complete absence of fossil Amphibia from the Upper Trias to the Oligocene. The Stegocephali as such seem to have died out with the Trias. The recent Amphibia, of course, must have had ancestors in the Mesozoic age. There is one little skeleton, from the Wealden of Belgium, which belonged to a newt-like creature, called _Hylaeobatrachus croyi_. Scarce fragments, described as _Megalotriton_, are known from the Oligocene of France, and _Triton_ itself seems to be indicated by {84}remnants in the Lower Miocene of France and Germany. But fairly complete specimens of large creatures, much resembling _Cryptobranchus_, have been found in the Upper Miocene of Oeningen, Canton Solothurn, Switzerland. The first known specimen, now at Haarlem, indicating a total length of 3 feet or more, was described and figured in the year 1726 by Scheuchzer, in a learned dissertation entitled "_Homo diluvii testis_."

Betrübtes Beingerüst von einem alten Sünder Erweiche Herz und Sinn der neuen Bosheitskinder.

Which may be rendered as follows:–

Oh, sad remains of bone, frame of poor Man of sin Soften the heart and mind of sinful recent kin.

This was the motto attached to the illustration, and it remained a warning to mankind until Cuvier declared the skeleton to be that of some large newt. Tschudi named it _Andrias scheuchzeri_, but it is scarcely generically distinct from _Cryptobranchus_, being almost intermediate between _C. alleghaniensis_ and _C. japonicus_, see p. 97.

_SUB-CLASS II. LISSAMPHIBIA._

_Amphibia without dermal armour._

ORDER I. APODA OR LIMBLESS AMPHIBIA.

The Amphibia Apoda, Coeciliae or Gymnophiona, are a small group of worm-shaped, burrowing creatures, restricted to the Neotropical and Palaeotropical regions, excluding Madagascar. They have no limbs and no girdles. The tail is extremely short; the vertebrae are pseudo-centrous, and most of them carry rather long ribs, none of which, however, meet to form a sternum. The whole snake-like body is covered with a smooth and slimy skin which forms numerous transverse folds or rings.

The most remarkable feature of the SKULL is its solid compactness, which stands in direct correlation with the burrowing habits of these creatures. The whole dorsal surface of the cranium is practically roofed in by bone, so that, in this respect, it greatly resembles that of the Stegocephali; but this resemblance is produced chiefly by a broadening of those bones which exist also in the other Lissamphibia, while supratemporals and supra-occipitals are absent.

{85}[Illustration: FIG. 13.–Skull of _Ichthyophis glutinosa_. × 3. (After Sarasin.) A, Lateral, B, ventral, C, dorsal view. _A_, Posterior process of the os articulare; _Ca_, carotid foramen; _Ch_, choana or posterior nasal opening; _F_, frontal; _J_, jugal; _Lo_, lateral occipital; _Mx_, maxillary; _N_, nasal; _No_, nostril; _O_, orbit; _P_, parietal; _Pa_, palatine; _Pm_, premaxillary; _Pof_, postfrontal; _Prf_, prefrontal; _Pt_, pterygoid; _Q_, quadrate; _S_, squamosal; _St_, stapes; _T_, tentacular groove; _Vo_, vomer; _X_, exit of vagus nerve.]

There is, however, a pair of bones which represent either the postorbitals or the postfrontals, perhaps both, of the Stegocephali. The quadrato-jugal arch is enormously developed, and by reaching the parietal, frontal, and postorbito-frontal bones (which latter occur only in _Ichthyophis_ and _Uraeotyphlus_) and the maxilla, extends over the whole of the orbito-temporal fossa. The squamosal is completely fused with the quadrato-jugal. The stapes has the typical stirrup-shape, is even perforated by an artery, and articulates distally with the shaft of the quadrate (as in the snakes). The maxilla is very large and broad. Owing to its broad junction with the quadrato-jugal arch, the prefrontal and frontal, the orbital fossa is reduced to a very small hole, or the maxilla completely covers the eye. Somewhere between the latter and the nares the maxilla is perforated by the tentacular groove. The periotic bones are represented by the prootics and epiotics; they fuse with the lateral occipitals and with the parasphenoid. The whole {86}orbito-ethmoidal region of the primordial skull is also turned into one mass of bone.

The angular element of the lower jaw forms a thick and large process which projects upwards and backwards from the mandibular joint. The former possession of a splenial bone is indicated by the occurrence of a second series of teeth in the mandibles of _Ichthyophis_ and _Uraeotyphlus_. Other genera have vestiges of this second row, or it may be completely lost.

The hyoid and branchial apparatus is more primitive than in any other recent Amphibia. In the larva the hyoid and the first and second branchial arches are connected with each other by a median copular piece. The third branchial arches are free from the rest, but are fused in the middle line, the fourth are loosely attached to the previous pair. In the adult both fuse into one transverse, curved bar, and the second pair of branchials lose their connexion with the basal longitudinal piece and likewise form a transverse bar.

The vertebrae are built upon the pseudocentrous type, are amphicoelous, and the chorda is intravertebrally destroyed by cartilage, as in the majority of the Urodela. The number of vertebrae is great, amounting in some species to between 200 and 300, of which a few belong to the tail. The first vertebra is devoid of an odontoid process. The ribs are proximally bifurcated as in the Urodela.

The eyes are practically useless, being either more or less concealed under the skin, or they are covered by the maxillary bones. All Coecilians possess a peculiar tentacular sensory apparatus, which consists of a conical flap-shaped or globular soft tentacle, which is lodged in a special groove or canal of the maxilla, between the eye and the nose, whence it is frequently protruded while the animal is crawling about. These tentacles in the young _Siphonops_ lie, according to the Sarasins, quite close to the eyes, but are later transferred nearer to the nose. The organ consists of a peculiarly rolled up and pointed fold which arises from the bottom of the sac or pit, where it receives a nerve. It is protruded by becoming turgid with blood, and is retracted by a strong muscle. Into the lumen of the sac are poured secretions from the large orbital (Harderian) gland, to keep the apparatus clean. Hence arose the mistaken {87}notion of its being a poison-organ. The whole structure is possibly an offshoot of the naso-lacrymal duct.

The skin is most remarkable. In the ripe embryo the epidermis passes smoothly over the surface. Beneath follow two layers of soft cutaneous connective tissue, bound together by transverse or vertical lamellae, so that ring-shaped compartments are formed, and in these are embedded slime-glands. In the adult each compartment is modified into an anterior glandular belt and a posterior space, from the bottom of which grow several scales. The number of cutaneous rings agrees originally with that of the vertebrae; but later, and especially in the hinder portion of the trunk, each ring breaks up into two or more secondary segments, and these no longer agree with those of the skeleton. Each scale is beset with numerous smaller scales which consist of hardened cell-secretions infiltrated with calcareous matter. The whole scale is consequently an entirely mesodermal product of the deeper layers of the cutis. The usual statement that the skin forms imbricating lamellae, on the inner side of which appear the scales, is wrong. The "lamellae" can be lifted up only after the general epidermal sheath has been broken artificially in the constrictions between the rings. No scales exist in the Indian genus _Gegenophis_ and in the American _Siphonops_, _Typhlonectes_, and _Chthonerpeton_, a secondary loss which does not indicate relationship. The scales develop late in embryonic life, and they are reasonably looked upon as inheritances from the Stegocephali. The glands either produce slime, whose function seems to be the keeping clean of the surface of the body, or they are squirt-glands. The latter kind are also numerous and are filled with a fluid which is squeezed out by muscular contraction, and seems to be poisonous, as it causes sneezing to those who handle or dissect fresh specimens.

The Coecilians live in moist ground and lead a burrowing life. Their developmental history has only recently been studied, and in but a few species, see _Ichthyophis_, p. 91, and _Hypogeophis_, p. 92. The female is fertilised internally, copulation taking place by means of eversion of the cloacal walls in the shape of a tube. The spermatozoa possess an undulating membrane; the eggs undergo meroblastic division and the embryos have three pairs of long external gills. Some are viviparous.

The snake-like, limbless shape of the body (Fig. 15) is, as in {88}snakes, correlated with an asymmetrical development of the lungs; the left is reduced, while the right is drawn out into a long cylindrical sac. The liver is likewise very long, and partly constricted into a great number of lobes. Owing to the great reduction of the ribs progression is effected in an almost earthworm-like fashion by the peristaltic motion of the skin, assisted by its numerous ring-shaped constrictions.

The systematic position of the Coeciliae has been, and is still, a controversial matter. The Sarasins took up Cope's suggestion, that their nearest allies are the Urodela, especially _Amphiuma_, and they went so far as to look upon _Amphiuma_ as a neotenic form of the "Coecilioidea," which they divided into Amphiumidae and Coeciliidae; the Coecilioidea and Salamandroidea forming the two sub-orders of the Urodela. They based this startling conclusion chiefly upon remarkable resemblances between _Amphiuma_ and _Ichthyophis_, namely, (1) the mode of laying the eggs on land and coiling themselves around them; (2) the existence of remnants of a tentacular apparatus in _Amphiuma_; (3) Cope's statement that _Amphiuma_ alone among the Urodela possesses an ethmoid like the Coeciliae. This latter point is, however, erroneous; it has since been shown by Davison[41] that _Amphiuma_ possesses no ethmoid bone, but that, instead of it, descending plates of the frontals join below the premaxilla and function as a nasal septum, with a canal for the olfactory nerves.

We look upon the Apoda with more reason as creatures which of all the Lissamphibia have retained most Stegocephalous characters and at the same time form a highly specialised group equivalent to the Urodela and the Anura. The following are Stegocephalous inheritances peculiar to the Apoda in opposition to the other recent Amphibia: retention of cutaneous scales with calcareous incrustations, greatly resembling the scales of the Carboniferous Microsauri; occasional retention of post-frontal and lateral nasal or lacrymal bones, and of a second row of teeth in the mandible. To these may be added the presence of epiotic bones, and the primitive character of the branchial arches. The loss of all these characters would turn the present Apoda into limbless Urodela, but this assumption does not justify their inclusion in this Order. The possible homology of the tentacular apparatus has been discussed elsewhere, p. 45.

{89}Fossil Apoda are not known; their subterranean life does not favour preservation.

[Illustration: FIG. 14.–Map showing the distribution of the Coeciliae or Amphibia Apoda.]

Only family, COECILIIDAE. About forty species are known. These have been placed in seventeen genera, mostly on comparatively slight grounds, and several of these genera are probably unnatural, the distinctive characters having undoubtedly been developed independently in various countries. We have to remember that the recent species are the remainder of a formerly much more numerous group; it is also likely that more will be discovered in the tropical forests of South America and Sumatra.

Boulenger[42] has distinguished them as follows:–

I. Cycloid scales embedded in the skin. _A_. Eyes distinct, or concealed under the skin. _a_. Two series of teeth in the lower jaw. α. Quadrato-jugal (squamosal) and parietal bones in contact. Tentacle between eye and nostril. _Ichthyophis_, 2 species, India and Malay islands, p. 90. " below and behind nostril. _Hypogeophis_, 3 species, East Africa and Seychelles, p. 92. " below and in front of eye. _Dermophis_, 5 species, America and Africa, p. 93. " below the nostril. _Coecilia_, 6 species, America. β. Quadrato-jugal separated from parietal. Tentacle close to the eye. _Rhinatrema_, 2 species, America. " below and behind nostril. _Geotrypetes_, 1 species, West Africa. " below nostril. _Uraeotyphlus_, 3 species, West Africa and India. _b_. One series of teeth in the lower jaw. Tentacle in front of the eye. _Cryptopsophis_, 1 species, Seychelles. _B_. Eyes below the cranial bones. Quadrato-jugal in contact with{90} parietal. Tentacle near the nostril. _Gymnophis_, 4 species, South America. _Herpele_, 2 species, Panama and Gaboon. II. Without scales. _A_. Eyes distinct, or concealed under the skin. _a_. Two series of teeth in the lower jaw. α. Quadrato-jugal in contact with parietal. Tentacle behind nostril; end of body laterally compressed. _Typhlonectes_, 3 species, America, p. 93. β. Quadrato-jugal separated from parietal. Tentacle between eye and nostril. _Chthonerpeton_, 2 species, America. _b_. One series of teeth. α. Quadrato-jugal and parietal in contact; tentacle in front of the eye. _Siphonops_, 4 species, America. β. Quadrato-jugal separated from parietal. _Bdellophis_, 1 species, East Africa. _B_. Eyes below the cranial bones. _a_. Two series of teeth. Quadrato-jugal and parietal in contact; tentacle behind and below nostril. _Gegenophis_, 1 species, India. _b_. One series of teeth. Quadrato-jugal separated from parietal. _Scolecomorphus_, 1 species, East Africa. _Boulengerula_, 1 species, East Africa.

_Ichthyophis glutinosa_ extends from the slopes of the Himalayas to Ceylon, the Malay islands, and into Siam. A second species, _I. monochrous_, occurs in Malabar, Malacca, Borneo, and Java. _I. glutinosa_ reaches about one foot in length, with a greatest thickness of a little more than half an inch. The general colour is dark brown or bluish black, with a yellow band along each side of the body.

This species has been studied extensively by the Sarasins.[43] It breeds in Ceylon after the spring monsoon. The ovarian egg is oval, measuring 9 by 6 mm. The yolk is yellow; the blastoderm lies towards one of the poles. The strong vitelline membrane becomes surrounded in the oviduct by a dense albuminous membrane, which forms twisted chalazae, just like those of birds' eggs, and by these two cords the eggs are strung together. Around all this lies another mantle of albumen. The female digs a hole close to the surface in moist ground near {91}running water, and there lays about two dozen eggs. The egg-strings become glued together, entangled into a bunch, and the female coils herself round the bunch and remains in that position, probably to protect the eggs against other burrowing creatures, as blind snakes (_Typhlops_ and _Rhinophis_) and certain limbless lizards, with which the ground literally swarms. During this kind of incubation the eggs assume a round shape, and grow to twice their original size, and the mature embryo weighs four times as much as the newly laid egg.

[Illustration: FIG. 15.–_Ichthyophis glutinosa_ × 1. (After P. and F. Sarasin.) 1, A nearly ripe embryo, with gills, tail-fin, and still with a considerable amount of yolk; 2, female guarding her eggs, coiled up in a hole underground; 3, a bunch of newly laid eggs; 4, a single egg, enlarged, schematised to show the twisted albuminous strings or chalazae within the outer membrane, which surrounds the white of the egg.]

The external gills are delicately fringed and red, and they move up and down in the fluid of the egg. The body of the embryo is at first white, but becomes pigmented with dark grey. A strong line of lateral sense-organs is formed, and a ring of them lies around the eye and others on other parts of the head. The short tail develops a fin. Of the three pairs of gills the third is the shortest, and is generally turned dorsalwards. In embryos of 4 cm. in length the longest gill measures as much as 2 cm. Yolk is still present in embryos which have reached the surprising length of 7 cm. Then the gills begin to shrink a little, and at this time one pair of gill-clefts breaks through at the base of the third external gill.

When the larvae are hatched the gills are lost. The young larva takes to the water in a gill-less state, and moves about like an eel. At the bottom of the gill-hole on each side two arches are visible, and there are at this stage neither inner nor {92}outer gills. The larvae frequently come up to the surface to breathe. The eyes are large and clearly visible, but the tentacles are still undeveloped. The epidermal sense-organs are numerous, and appear as white spots in the grey skin; about fifty extend from the gill-opening to the tip of the tail.

_Ichthyophis_ seems to live a long time in the larval state. At last the gill-clefts close, the tail-fin disappears, and the tentacles come to the surface. The whole skin assumes a totally new structure, and the fish-like larva turns into a burrowing, subterranean creature so terrestrial that it gets drowned when made to remain in the water.

_Hypogeophis._–According to A. Brauer[44] three species of Coecilians are found in the Seychelles: _Cryptopsophis multiplicatus_, which is rare, _Hypogeophis rostratus_ and _H. alternans_. They live in moist ground, near the coast in swamps, higher up in humus, under rotten trees and rocks, down to the depth of one foot. In the island of Silhouette, Brauer found them in brooks, at least during the dry season, from May to September. The natives call them "vers de terre." They seem to propagate during the greater part of the year, provided there is sufficient moisture. The female coils round the eggs, which vary from half a dozen to thirty in number, those of _H. rostratus_ measuring 7-8 mm., those of _H. alternans_ only 4-5 mm.

The embryos undergo their whole development in the egg. Four pairs of gill-clefts break through, the first between the hyoid and the first branchial arch, the fourth between the third and fourth branchial arches. There appears also a spiracular cleft between the quadrate and the hyoid arch; this cleft is, however, only developed dorsally, and persists for a shorter time. The external gills appear at the same time as the clefts, upon the first three branchial arches; the third gill is the latest, and remains in a vestigial condition covered up by the two others. The gills, of which the second is the longest, are not (as stated by the Sarasins) direct prolongations of the gill-arches, but they begin as button-like growths upon the arches. They begin to disappear with the absorption of the yolk, getting actually smaller. In embryos of 6 cm. they are 6 mm. long, while in embryos of 6.5 cm. they are reduced to 4.5 mm. in length. The {93}first to disappear is the third gill, of course by being resorbed; and the clefts are closed before the creature leaves the egg. _Hypogeophis_ not leading an aquatic larval life possesses no tail-fin in the embryonic state, the gill-holes are closed, and the epidermal sensory organs disappear long before the time of hatching.

Vestiges of gills appear also on the hyoid and on the mandibular arch, but on the latter they are of very short duration. Those of the hyoid gradually fuse with the first of the branchial gills, and these also concentrate with their bases so that they ultimately seem to spring from one common stem. Brauer remarks that the distinction between internal and external gills seems to be one of degree only; the hyoidean and mandibular gills namely start from the hinder margin of the arches, just like the internal gills of _Torpedo_ according to Ziegler, while the other gills start from the sides of the branchial arches. He also found a pair of little swellings behind the last gill-cleft, and an unpaired swelling (corresponding with a double one in _Ichthyophis_) in front of the vent. Not unreasonably he sees in these swellings the last, very transitional vestiges of the paired limbs.

_Typhlonectes compressicauda_ of Guiana and Venezuela is one of the largest Coecilians, reaching a length of 18 inches, with a body-diameter of ¾ inch. The general colour, as in most of these creatures, is olive brown to black. A sort of adhesive disc surrounding the vent occurs in this genus. Peters, who described this species, found in one female six embryos of comparatively enormous size, one of them being 157 mm. (more than 6 inches) long, and 12 mm. thick, and devoid of a tail-fin. Instead of lateral gill-openings there is a "bag" on each side 55 mm. long, upon which is distributed a blood-vessel. The Sarasins have examined the same specimen: The gills are not a bag, but consist of two flat, unbroken membranes which are closely connected with each other. In fact the outer gills of all Amphibia may be said to begin in the shape of small bags, whence sprout secondarily the gill-fringes; but in _Typhlonectes_ they form these flaps instead of growing into the usual three gills. The embryos have no epidermal sense-organs, but plenty of skin-glands. Probably when born they take at once to terrestrial life, the flaps are possibly shed at birth, and there remains a little cicatrix.

_Dermophis thomensis_ of West Africa (its other relations live in East Africa, South and Central America) is also viviparous.

{94}