The membranes of the brain and spinal cord have an abundance of pigment cells in their visceral laminae, and deposits of crystalline calcium carbonate are observable on the exterior of these membranes, and especially of their prolongations upon the spinal nerves (p. 76). The nasal capsules are supported by a solid (Siren) or fenestrate (Proteus, Menobranchus) cartilage tube: they open internally within the lip in front of the ethmopalatine, there being no maxillae. The mucous membrane, as in Fish, is disposed in radial folds. In other Amphibia there are as a rule cartilaginous turbinal ridges, and the internal nares lie between the maxillae and vomers. The Urodela possess an internasal gland opening into the mouth and lodged in the septum of the nose. It lies more anteriorly under the praemaxillae in Anura. Other glands open into the narial chambers, and in Anura on the oral mucous membrane as well. In the Gymnophiona the cartilaginous ridges are replaced by bony ridges developed on the bones surrounding the narial chamber. The tentacles are structures peculiar to this group. They are essentially fibrous sacs lying in the cavity of the orbit, and opening anteriorly in front of it. They contain large glands and a retractor muscle, which is attached anteriorly close to the external aperture.
At this spot there is a ridge or papilla which can be protruded and retracted. A gland lying in the nasal cavity opens into the sac near its external aperture. The eye is small in comparison with that of Fish, but as in that class, the lens is spheroidal, and the cornea, except in the Land Salamander (S. terrestris), flat. The eye is hidden beneath the integument in Proteus and Gymnophiona. An upper and lower lid is usually developed in Urodela: the latter in Anura forms a large nictitating membrane, connected at its angles with the retractor bulbi muscle. The lacrymal gland is absent, but there is a Harderian gland in Anura. A lacrymal duct leads to the narial chamber, except in Gymnophiona. The ciliary muscle is feebly indicated. Colourless or faintly yellow oil globules are found in the cones of Anura. There is no tympanic cavity in Urodela, and the same is true of some Anura, but a simple stapes is present, attached to the membrane of the fenestra ovalis. Other Anura have a tympanic membrane attached to a tympanic annulus, the homologue of the spiracular cartilage of Elasmobranchii, a tympanic cavity, and a well-formed and complex stapes or columella auris (supra, p. 339), and a fenestra rotunda in addition to the f. ovalis.
In the aglossal Anura (Pipa, Dactylethra) the two Eustachian tubes have a single median pharyngeal orifice. The semicircular canals are well curved in Anura, flattish in Urodela. The sacculus is large in Urodela, but becomes reduced with the increase in the size of the cochlea in Anura. The internal ear is very rudimentary in Siphonops annulatus, and probably in all other Gymnophiona, and contains no sensory nerve-endings.
Amphibia are sometimes edentulous, e.g. Pipa and the Bufonidae. The Tadpole of the Anura has the jaws covered by a horny sheath. The same is the case with the praemaxillae and dentary bones of Siren lacertina with the addition, however, of true teeth on the vomero-palatines and splenials. The adult Anura rarely have teeth on the dentaries, and those structures are confined in them to the praemaxillae, maxillae and vomers. On the latter, however, they are sometimes wanting. In Urodela teeth occur on the praemaxillae, maxillae, vomero-palatines, rarely on the parasphenoid, as in Spelerpes rubra and Batrachoseps, and in the lower jaw on the dentary and splenial. They usually form a single or nearly single row on the praemaxillae, maxillae, and bones of the mandible, but the vomerine, palatine, and parasphenoidal teeth are numerous and thick set. The points of the teeth only just protrude above the mucous membrane. In the Anura and Salamandrine Urodela they are double pointed, one point external, the other internal. The bulk of the tooth consists of tubular dentine surrounding a pulp-cavity, and coated with a thin layer of enamel in the Frog, or capped by enamel in the Newt, etc. The teeth are affixed to the jaws by pillars of true bone.
In Ceratophrys (Anura) the dentine at the base of the teeth is folded (as in some Lizards, e. g. Vara7ius). So too in many Stegocephali, and in the genus Labyrinthodon the folds are deep, of great extent, secondarily folded, and the intervals between them are filled with cement. The Stegocephali have teeth on the praemaxillae, maxillae, vomers, palatines, on the dentary, and apparently sometimes on the splenial, and they are lodged in shallow depressions. The tongue is rudimentary in the lower Urodela. It is wanting in the Aglossa ( = Pipa, Dactylethra) among Anura, but in this order, and in Salamanders it is of fair size, fleshy, attached anteriorly, but with a free thickened posterior border which is thrown forward in the act of protrusion. In the Anuran Rhinophrynus it is free anteriorly, and in the Urodele Spelerpes it is fungiform and can be protruded to a great distance. As in higher Vertebrata the tongue has a musculature of its own; it contains numerous glands and -has special sense-cells interspersed in the epithelium.
The alimentary canal takes a direct antero-posterior course, and the division into stomach, intestine and rectum is scarcely marked externally in Proteus. It is marked in other Urodela where there is a distinct pylorus, and the intestine (= midgut) is thrown into more or fewer coils except in Gymnophiona. The rectum is dilated. The Anura are characterised by the form of the stomach, which has its cardiac end dilated on the left side and its pyloric end conical and contracted. The convolutions of the intestine are numerous, and lie almost entirely on the right side. In the vegetable-feeding Anuran Tadpole the intestine is of great length and spirally coiled. The rectum opens into the anterior end of a cloaca. The mouth, pharynx, oesophagus, stomach in part, and in Iriton the rectum are ciliated. The intestinal epithelium has a striated border and there are many glands. The liver is two lobed in Urodela, elongated in the lower forms and in Gymnophiona, broader in the higher. In the Anura the left lobe is subdivided into two more or less completely, and in the Aglossa the three lobes thus formed are united only by peritoneum. A gall bladder is always present.