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 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 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].
Fossil Apoda are not known; their subterranean life does not favour preservation.
Fig. 14.–Map showing the distribution of the Coeciliae or Amphibia Apoda.