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.
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 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.