[Illustration: FIG. 12.--Ventral view of the hyoid of _Rana esculenta_. a, Anterior; b, lateral; c, posterior processes; d, thyro-hyals.]
_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.
[Illustration: FIG. 13.--Ventral view of the head and trunk of _Ichthyophis glutinosus_.
Mn, Mandible. Hy, Hyoid. Br^1 Br^2, Br^3, 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.]