Ichthyopsida with an integument rich in glands or glandular cells and devoid of an epidermic exo-skeleton. The azygos system of fins is always present in the larva and in the adult of certain sub-groups, and is never supported by fin-rays. The limbs are borne by well-developed shoulder- and pelvic-girdles, and the latter is connected to a sacral vertebra: they consist as contrasted with the limbs of Pisces of an upper and lower limb, with a hand and foot, thus agreeing with those of higher Vertebrata. The alimentary canal always terminates with a cloaca common to the rectum and urogenital ducts, to which is appended ventrally an allantoid bladder.
The epidermis in the young larva is ciliated. In older larvae it consists of only two layers of cells, an outer with a striated cuticular border, and an inner, in which mucous cells ( = Leydig cells) are to be found scattered. In the adult the outer layer is cornified and is cast off periodically in the Anura and the Urodela; but the inner layer forms a thick stratum mucosum, and certain of its cells are enlarged and flask-shaped, and are supposed to yield a secretion which facilitates the moult of the cornified layer. Some of the cells contain pigment. The surface of the epidermis is often rough, and the roughness is due in part to processes and ridges of the corneous cells, in part to local thickenings of the epidermis itself. The lower layers of the corium are loose in texture and contain lymph-spaces continuous with the subcutaneous spaces so largely developed in this class. Smooth muscle cells are found mixed with the fibrous tissue, and chro-matophores or pigment cells in abundance. The colours of the pigment are various, black, white, orange, metallic or iridescent: its state of concentration in the cells depends on the activity of the nervous system, and nerve fibres have been traced into direct continuity with the cells. The glands are round or tubular.
The former vary much in size; the small are scattered, the large are found chiefly on the head, neck and flanks. Masses of them, known as 'Parotids,' are found on the neck in Bufo -and Salamandra. The tubular glands occur chiefly on the hand and foot, and on the head in many tropical Salamanders. The gland-secretion is milk-white and poisonous. The young larva or tadpole of the Anura has two collections of unicellular glands beneath the head. Their secretion is sticky and serves to attach the larva to foreign objects. Variations from the ordinary structure are seen in the development of epidermic nails in the Japanese Salamander Onychodactylus and the Toad Dactylethra capensis. Calcareous concretions are found in the dermis of the dorsum and dorsal aspect of the limbs in the common Toad Bufo vulgaris and B.japonicus. Bony dorsal plates are developed in certain species of Ceratophrys and Ephippifer (=Brachycephalus) among Anura. The Gymnophiona (Caecilia and its allies) have semicircular cutaneous lamellae in which are imbedded, except in Siphonops annulatus, cycloid dermal scales containing calcareous concretions.
This dermal skeleton is well developed and ossified in the extinct Stegocephali ( = Labyrinthdontia, &c). It generally consists of a median anterior thoracic plate, rhomboidal in shape, with the addition of two antero-lateral pieces in some instances, and of a series of ventral scales usually arranged in oblique lines which meet one another ventrally at an angle pointing forwards. These scales, as a rule, only cover the abdominal surface. Sometimes, however, the back, tail and limbs are also protected, but by scales which differ much in shape, etc, from the ventral, e. g. in the Hylonomidae. The organs of the lateral line are not known to exist in any Gymnophiona, except the larval Epicrium glutinosum, but are found in the larvae of every other order. They are persistent in all Urodela which lead an aquatic life, e.g. Proteus, Triton, but are lost in those which are terrestrial when adult, e.g. Salamandra, as well as in all adult Anura. On the head it is generally possible to distinguish the supra- and infraorbital lines of organs: the mandibular is generally broken up into an anterior and a posterior part, and the commissure across the supra-occipital region occurs only in Menopoma. But in some instances the number of organs is great, and it is difficult to trace the typical arrangement.
There are three lateral lines; a median which extends to the tip of the tail; a dorsal which always unites near the tip of the tail with the median line, sometimes also anteriorly; and a ventro-lateral which extends only between the fore- and hind-limbs. Each organ has a protective hyaline tube in the larva, and there is at first one organ to each somite, but the number increases subsequently by division.
Cartilage persists largely in the chondrocranium of Anura: to a much less extent in other Amphibia, especially in Gymnophiona. There is a large basicranial and superior cranial fontanelle in Urodela, a sub-frontal and two small sub-parietal fontanelles in Anura. The occipital region contains two large exoccipitals which form each a cranial condyle 1: a small supra- and basi-occipital have been detected in some Anura. The ear capsule of Anura contains a pro-, or a pro-sphen-, otic bone, to which is added in Urodela an opisth-pter-otic. Proteus and one or two others have an epiotic ossification as well. The middle trabecular region is ossified by a sphen-ethmoid. The olfactory capsules do not unite with the ethmo-nasal cartilages in Siren, Proteus, Menopoma, and some Salamanders. The chondrocranium is invested by paired parietals, frontals, prefrontals, nasals (absent in Siren and Proteus), paired vomers and a parasphenoid. The Anura have a paired fronto-parietal, sometimes united medianly, nasals, vomers and a parasphenoid.
The vomer is single in Dactylethra, absent in Pipa and Hylaplesia. The palato-pterygoid of Urodela appears as a continuous membrane-bone subsequently divided, except in Proteus and Menobranchus, into palatine and pterygoid bones - the former becoming continuous with the vomer of its side. The latter is absent in Siren. The cartilage element appears later: the palatal element from the ethmoid, the pterygoidal from the quadrate. The palato-pterygoid of Anura arises in continuity with Meckel's arch, fuses with the ethmoid anteriorly and remains in continuity with the quadrate posteriorly. Ossification takes place from the perichondrium. The palatine is sometimes absent, sometimes subdivided. Two praemaxillae are always present, fused into one, however in some Salatnandrina. Maxillae are absent in Proteus, very rudimentary in Siren and Menobranchus. A quadrato-jugal, absent in all Urodela, unites them to the quadrate or suspensorium in Anura with few exceptions. The quadrate is more or less ossified in Urodela, and united to the cranium by three processes. It is as a rule cartilaginous in Anura and is united to the auditory capsule by an otic process, and articulated to it by a pedicle. A squamosal lies on its external aspect. The lower jaw is a persistent Meckel's arch.