In the Batrachia

In the Batrachia, the posterior limbs are much longer than the anterior. The radius and the ulna in the fore-limb, and the tibia and fibula in the hind-limb, are fused together into one bone. The carpal bones no longer present the typical arrangement; and, in the tarsus, there are two proximal, greatly elongated, cylindrical bones, which take the place of a calcaneum and an astragalus, while the distal series is reduced.

The limbs of the Labyrinthodonts were feeble in comparison with the size of the body. In the genera Archegosaurus Keraterpeton, Urocordylus, Lepterpeton, each foot possessed five digits, and the carpus and tarsus were unossified.

The Amphibia usually possess teeth on the vomers, premaxillae, maxillae, and dentary pieces of the mandible, but rarely on the palatine and pterygoid bones. The premaxillary and vomerine teeth are disposed in concentric semicircles, an arrangement which is very characteristic of the group. In the larvae of the Batrachia, and in Siren, the premaxillae and mandibles are ensheathed in horny beaks, as in the Chelonia and Aves. In addition, Siren has teeth in the vomers, and on the splenial piece of the mandible; Menobranchus and Siredon have pterygoid teeth. Many of the Labyrinthodonts possess palatine teeth. In some Gymnophiona the mandible has a double row of teeth, and there is an approximation to this structure in the Labyrinthodonts.

The teeth usually become anchylosed with the adjacent bones. In existing Amphihia their structure is simple, but in the Labyrinthodonts, the parietes of the teeth, at a certain distance below the summit, become longitudinally folded, and each fold may be again longitudinally plaited, so that the transverse section of the tooth acquires a very complicated structure, the pulp-cavity being subdivided into a great many radiating and branching segments. The structure is similar in principle to that exhibited by the teeth of many of the Ganoidei. In many of the Labyrinthodonts, again, two of the anterior mandibular teeth take on the form of long tusks, which are received into fossae, or foramina, of the upper jaw, as in most existing Crocodilia. The tongue is fixed to the floor of the mouth in Urodela and Gymnophiona, and remains undeveloped in the genera Pipa and Dactylethra, which have thence been termed Aglossa. In other Batrachia, the tongue, which is usually long, and fixed by its anterior end to the symphysis of the mandible, can be rapidly protruded and used as an organ of prehension. No distinct salivary glands have been observed in the Amphibia. Many male Batrachia have the mucous membrane of the floor of the mouth produced into pouches which can be distended with air.

The simple alimentary canal is usually short, and much longer in the larvae (which are vegetable-feeders) than in the adults. A gall-bladder is always present.

The heart presents two auricles, a single ventricle and a bulbus arteriosus. A venous sinus, the walls of which are rhythmically contractile, receives the venous blood from the body, and opens into the right auricle. In Proteus, Menobranchus, and Siren, the septum of the auricles is less complete than in the other Amphibia. The left auricle is much smaller than the right, and a single pulmonary vein opens into it. The interior of the ventricle is more like a sponge than a chamber with well-defined parietes. The walls of the long bulbus arteriosus contain striated muscular fibres, and are rhythmically contractile. Valves are sometimes placed at each end of it, and it may be imperfectly divided into two cavities by an incomplete longitudinal partition. It terminates, upon each side, in either three, or four, trunks, which ascend upon the branchial arches. The most anterior of these trunks give off the carotid arteries, the most posterior the pulmonary arteries, and arteries to the integument; the middle trunks form the principal roots of the dorsal aorta.