Integument.—In all recent batrachians, the skin is naked, or if small scales are present, as in many of the Apoda, they are concealed in the skin. The extinct Stegocephalia, on the other hand, were mostly protected, on the ventral surface at least, by an armour of overlapping round, oval, or rhomboidal scales, often very similar to those of Crossopterygian or ganoid fishes, and likewise disposed in transverse oblique lines converging forwards on the middle line of the belly. Sometimes these scales assumed the importance of scutes and formed a carapace, as in the “batrachian armadillo” discovered by E.D. Cope. A few frogs have the skin of the back studded with stellate bony deposits (Phyllomedusa, Nototrema), whilst two genera are remarkable for possessing a bony dorsal shield, free from the vertebrae (Ceratorphrys) or ankylosed to them (Brachycephalus). None of the Stegocephalia appears to have been provided with claws, but some living batrachians (Onychodactylus, Xenopus, Hymenochirus) have the tips of some or all of the digits protected by a claw-like horny sheath.
The integument of tailed and tailless batrachians is remarkable for the great abundance of follicular glands, of which there may be two kinds, each having a special secretion, which is always more or less acrid and irritating, and affords a means of defence against the attacks of many carnivorous animals. A great deal has been published on the poisonous secretion of batrachians (34), which is utilized by the Indians of South America for poisoning their arrows. Some of the poison-secreting glands attain a greater complication of structure and are remarkable for their large size, such as the so-called “parotoid” glands on the back of the head in toads and salamanders.
| Fig. 13.—Ventral view of the head and trunk of Ichthyophis glutinosus. |
| Mn, Mandible. Hy, Hyoid. Br1 Br2, Br3, Branchial arches. Gl, Glottis. Tr, Trachea. Ivc, Inferior vena cava. V, Ventricle. Au, Auricles. Rsvc, Lsvc, right and left superior cavae. Ta, Truncus arteriosus. Ao, Left aortic arch. P.A. Right pulmonary artery. The pericardium (lightly shaded) extends as far as the bifurcation of the synangium. |
In all larval forms, in the Caudata, and in a few of the Ecaudata (Xenopus, for instance), the epidermis becomes modified in relation with the termination of sensory nerves, and gives rise to organs of the same nature as those of the lateral line of fishes. In addition to diffuse pigment (mostly in the epidermis), the skin contains granular pigment stored up in cells, the chromatophores, restricted to the cutis, which are highly mobile and send out branches which, by contraction and expansion, may rapidly alter the coloration, most batrachians being in this respect quite comparable to the famous chameleons. Besides white (guanine) cells, the pigment includes black, brown, yellow and red. The green and blue, so frequent in frogs and newts, are merely subjective colours, due to interference. On the mechanism of the change of colour, cf. W. Biedermann (35).
One of the interesting recent discoveries is that of the “hairy” frog (Trichobatrachus), in which the sides of the body and limbs are covered with long villosities, the function of which is still unknown (36).
The nuptial horny asperities with which the males of many batrachians are provided, for the purpose of clinging to the females, will be noticed below, under the heading Pairing and Oviposition.
Dentition.—In the Microsauria and Branchiosauria among the Stegocephalia, as in the other orders, the hollow, conical or slightly curved teeth exhibit simple or only slightly folded walls. But in the Labyrinthodonta, grooves are more or less marked along the teeth and give rise to folds of the wall which, extending inwards and ramifying, produce the complicated structure, exhibited by transverse sections, whence these batrachians derive their name; a somewhat similar complexity of structure is known in some holoptychian (dendrodont) Crossopterygian fishes. In the remarkable salamander Autodax, the teeth in the jaws are compressed, sharp-edged, lancet shaped. The teeth are not implanted in sockets, but become ankylosed with the bones that bear them, and are replaced by others developed at their bases. Teeth are present in the jaws of all known Stegocephalia and Apoda and of nearly all Caudata, Siren alone presenting plates of horn upon the gingival surfaces of the premaxillae and of the dentary elements of the mandible. But they are nearly always absent in the lower jaw of the Ecaudata (exceptions in Hemiphractus, Amphignathodon, Amphodus, Ceratobatrachus, the male of Dimorphognathus), many of which (toads, for instance) are entirely edentulous.
There is great variety in the distribution of the teeth on the palate. They may occur simultaneously on the vomers, the palatines, the pterygoids and the parasphenoid in some of the Stegocephalia (Dawsonia, Seeleya, Acanthostoma), on the vomers, palatines and parasphenoid in many salamandrids (Plethodontinae and Desmognathinae), on the vomers, pterygoids and parasphenoid (some Pelobates), on the vomers and parasphenoid (Triprion, Amphodus), whilst in the majority or other batrachians they are confined to the vomers and palatines or to the vomers alone (37).
As regards the alimentary organs, it will suffice to state, in this very brief sketch, that all batrachians being carnivorous in their perfect condition, the intestine is never very long and its convolutions are few and simple. But the larvae of the Ecaudata are mainly herbivorous and the digestive tract is accordingly extremely elongate and coiled up like the spring of a watch. The gullet is short, except in the Apoda. The tongue is rudimentary in the perennibranchiatea Caudata, well developed, and often protrusile, in the Salamandridae and most of the Ecaudata, totally absent in the Aglossa.
The organs of circulation cannot be dealt with here; the most important addition made to our knowledge in recent years being found in the contributions of F. Hochstetter (38) and of G.B. Howes (39), dealing with the azygous (posterior) cardinal veins in salamanders and some of the Ecaudata. The heart is situated quite forward, in the gular or pectoral region, even in those tailed batrachians which have a serpentiform body, whilst in the Apoda (fig. 13) it is moved back to a distance which is comparable to that it occupies in most of the snakes.