The Amphibia are usually divided by modern writers into four orders, the old order Lepidota, comprising the Mud Fishes, being now placed at the head of the Fishes, under the name of Dipnoi. Whilst there is a general agreement as to the number and characters of the Amphibian orders, the names employed to designate them are very various, and it really matters little which are adopted.
Order I. Ophiomorpha, Owen (= Gymnophiona, Huxley; Apoda, Peromela, Ophidobatrachid): - Serpentiform or vermiform Amphibians, without limbs; anus terminal; skin mostly with horny scales imbedded in it. Eyes rudimentary or absent.
This is a small order, including only certain snake-like, vermiform animals (fig. 278) which are found in various tropical countries, burrowing in marshy ground, something like gigantic earth-worms ; while some of them seem to temporarily inhabit water. They form the family Caeciliadae (so called by
Fig. 278. - Ophiomorpha. a Siphonops annulatus, one of the Caecilians, much reduced : b Head ; c Mouth, showing the tongue, teeth, and internal openings of the nostrils ; d Tail and cloacal aperture. (After Dumeril and Bibron.)
Linnaeus from their supposed blindness), and are characterised by their snake-like form, and by having the anus placed almost at the extremity of the body. The body is cylindrical and worm-like, and is completely destitute of limbs. The skin is glandular, naked, and viscous, thrown into numerous folds, and containing numerous delicate, rounded, horny scales, which are dermal in their character, and are wanting in Siphonops annulatus. The mandibular rami are short, and are united in front by a symphysis. The teeth are long, sharp, and generally recurved; and a row of palatine teeth forms a concentric series with the maxillary teeth. The tongue is fleshy, fixed to the concavity of the lower jaw, and not pro-trusible (fig. 278). The ribs are numerous, but there is no sternum. The adult possesses lungs, one of which is smaller than the other, and the nose opens behind into the mouth. The eyes are rudimentary, nearly concealed beneath the skin, or altogether wanting.
The position of the Caeciliae was long doubtful; but their Amphibian character was ultimately proved by the discovery that whilst the adult breathes by lungs, the young, in some cases, possess internal branchiae, communicating with the external world by a branchial aperture on each side of the neck; while in other cases the adult is viviparous, and the young, prior to birth, are furnished with vesicular external branchiae placed on the side of the neck. In addition to the presence of branchiae in the larva, the Caecilians are further connected with the Amphibia by their possession of a double occipital condyle and of a glandular skin; whilst they approach the Fishes in generally having small horny scales embedded in the integument, and in the fact that the vertebrae are amphicoelous, and the cavities formed by their apposition are filled with the gelatinous remains of the notochord. As regards their distribution in space, the species of Caecilia are found in India, Africa, and South America; Siphonops and Rhinatrema are exclusively American ; and Epicrium is exclusively Asiatic.
Order II. Urodela ( = Ichthyomorpha, Owen; Sauroba-trachid). - This order is commonly spoken of collectively as that of the "Tailed " Amphibians, from the fact that the larval tail is always retained in the adult. The Urodela are characterised by having the skin naked, and (except in Salamandra ungniculata) destitute of any exoskeleton. The body is elongated posteriorly to form a compressed or cylindrical tail, which is permanently retained throughout life. The dorsal vertebrae are biconcave (amphicoelous), or concave behind and convex in front (opisthocoelous), and they have short ribs attached to the transverse processes. The bones of the fore-arm (radius and ulna) on the one hand, and those of the shank (tibia and fibula) on the other, are not anchylosed to form single bones.
In one section of the order - formerly called Amphipneusta - the gills are retained throughout life, and the animal is therefore "perennibranchiate." In this section are the Proteus, Siren, Menobranchus, etc. In the remaining members of the order the gills disappear at maturity, and the animal is therefore "caducibranchiate." In this section are the Land and Water Salamanders. One form, however - the Axolotl of Mexico - appears to be sometimes caducibranchiate, though generally perennibranchiate. The genera Amphiuma and Menopoma, also, exhibit a partially intermediate state of parts; for though they lose their branchiae when adult, they nevertheless retain the branchial apertures behind the head.
Fig. 279. - Head and fore-part of the body of Proteus anguinus, showing the external branchiae and tridactylous fore-limb.
Of the perennibranchiate Urodela, one of the best known is the singular Proteus (Hypochthon) anguinus (fig. 279), which is only found inhabiting pools in certain caves in Illyria and Dalmatia. It is of a pale flesh-colour, or nearly white, with three branchial tufts on each side of the neck, while the gill-slits are also persistent. It attains a length of about a foot, and has two pairs of weak limbs, of which the anterior have three toes, and the posterior only two. From its habitat, the power of vision must be quite unnecessary, and, as a matter of fact, the eyes are altogether rudimentary, and are covered by the skin. Several varieties of Proteus are known, and the one figured above has been described as a distinct species (P. xantho-stictus). The blood-corpuscles in Proteus are oval in shape, and are larger than those of any other vertebrate animal.