The sternum or hyposternum (see p. 82) in Urodela consists of a shovel-shaped plate of cartilage formed by the union of two cartilaginous rods lying in tendinous intersections of the recti abdominis muscles, with cartilage derived from the coracoid. It is deeply grooved on either side for the reception of the epicoracoidal edges of the coracoid. It has a somewhat similar structure and origin in some Anura, e. g. Bombinator, but in the majority it has an ossified base bearing a cartilaginous expansion, and, as in the Frog, is applied to the posterior edge of the united coracoids. It is doubtfully homologous with the sternum of higher Vertebrata.
The limbs with their respective girdles are absent in Gymnophiona, and the Aistopoda among Stegocephali; the pelvic limb and girdle in Siren lacertina among Ichthyoidea. The shoulder-girdle in Urodela has broad scapular, coracoidal and large clavicular processes. The latter projects somewhat forwards, and is free distally except in Menopoma, where it fuses with the coracoid as in all Anura, inclosing a fenestra. Ossification takes place in the glenoidal regions of both scapula and coracoid, and in the base of the clavicular process. The amount of ossification varies, and is greatest in the- Anura. The clavicular ossification is in this order purely perichondria!. Many of the Anura possess an anterior prolongation of the interclavicular region (see pp. 81-2). The ventral ends of the coracoids overlap in the Urodela and a few Anura, but in others of the last-named order are connected by intervening interclavicular cartilage. Bony scapulae, coracoids, and clavicles are found in most Stegocephali, but there was evidently also a large amount of cartilage.
Each half of the pelvis consists of a continuous cartilage in Urodela. The ilium is partially ossified, as is the ischium, but the pubic region is more or less permanently cartilaginous. In Salamandra perspicillata there is, however, a continuously bony ischio-pubic region, and some Stegocephali have well-formed and separate pubic and ischiadic bones. There is a median ventral symphysis. The obturator nerve perforates the pubic region, but neither in Urodela nor in other Amphibia does an obturator, i. e. thyroid, foramen exist. The ilium of the Anura is of remarkable length, inclined backwards, and the distal ends of the two ilia, together with the ischo-pubic region, form a thin vertical plate. The acetabulum is a deep depression in this plate. There is an ischium, but a pubic bone is generally suppressed and its region ossified apparently by the ilium. It has, however, been found in Dactylethra capensis, a Toad which also possesses a simple epipubic cartilage. This cartilage, which projects forwards medianly from the symphysis pubis, is absent in other Anura, but large in Urodela, where it is Y-shaped.
The limbs consist of the same parts as in higher Vertebrata. They are relatively small as compared with the bulk of the body in the lower Urodela and the Stegocephali, but are large and long, especially the hind extremities, in Anura. The humerus, radius, and ulna are cylindrical bones with cartilaginous ends in Urodela, and the same is true of the corresponding sections of the hind-limb. The cartilage contains much calcareous deposit in Anura, in which also the radius is fused with the ulna, the tibia with the fibula. The bones of the carpus and tarsus are usually cartilaginous in Urodela, ossified in Anura. There are many differences observable in the carpal and tarsal bones, but in Anura the astragalus and calcaneum are always lengthened like the other bones of the limb, and are united at each end. A double centrale is often present in adult Urodela. In this order there are generally four, in Anura five toes to the fore-foot, in both five toes to the hind-foot. But reduction of the numbers occurs in Urodela. Indications of a sixth ulnar digit and a sixth tibial digit are found in both Urodela and Anura, and in the latter the sixth toe may be well developed.
The toes are webbed in Anura. The carpus and tarsus appear to have been cartilaginous with rare exceptions in Stegocephali.
The olfactory lobes are sessile, and in Anura connected at their bases (see p. J5). The cerebral hemispheres are relatively large, largest of all in Gymnophiona, and are usually elongated. They are connected by an anterior commissure. The thalami optici and the paired optic lobes are not so clearly marked off from one another in Urodela as in Anura, where the latter are large, and contain ventricles of considerable size. The cerebellum is always a transverse bridge of variable breadth. The pineal gland is saccular in the young Anuran, perforates the roof of the skull, and is attached anteriorly to the skin. In the adult the cavity of the sac is obliterated, and its walls become fibrous. The gland does not perforate the cranium in Urodela, but its apex is attached to the cranial roof. The infundibulum is of great length in Epicrium. All the parts of the brain lie in the same plane. The most notable features in the cranial nerves are the following. The roots of the olfactory nerves unite only distally in Pipa, and not at all in Gymnophiona, where the dorsal and ventral bundles pierce the ethmoid separately.
The Gasserian ganglion of the trigeminus and the ganglion of the seventh nerve remain distinct in Urodela, though united by a nerve: they are closely approximated in a few Anura, but fused in the majority. The auditory and facial have a common root. The ganglia of the glossopharyngeus and vagus are united, and the nerves generally pass out through the same foramen in the exoccipital. There is no spinal accessory nerve, and the area supplied by the hypoglossal is supplied by the first spinal nerve in the majority: by the first and second (Salamandra): by the second (Pipa) or the second and third (Menobranchus). There is a cutaneous branch of the trigeminus to the dorsal aspect of the head in some Urodela, and in the tadpole of the Frog. Cutaneous branches of the vagus exist also, taking a lateral course along the body. There are three such nerves in the perennibranchiate Urodela, in Derotremata, and even in Triton, but they are lost in Salamanders (? all) and Anura. In the latter, however, a branch accompanies the pulmo-cutaneous artery.