Of the Sirenidae, the most familiar are the Sirens and the Axolotls. The Siren, or Mud-eel (fig. 277, A), is found abundantly in the rice-swamps of South Carolina, and attains a length of three feet. The branchiae are persistent, and the hinder pair of legs wholly wanting. Two other species are known, but they are likewise confined to North America.
The Mexican Axolotl (Siredon pisciforme, fig. 280) is a native of the Mexican lakes, and attains a length of about a foot or fourteen inches. It possesses both pairs of limbs, the anterior pair having four toes, and the hinder pair five toes.
Fig. 280. - The Axolotl (Siredon pisci-forme) - after Tegetmeier. The ordinary form, with persistent branchiae.
As ordinarily known in its native country, the Axolotl is certainly perennibranchiate, and they breed in this condition freely. There is no doubt, however, that individual specimens may lose their gills, without thereby suffering any apparent change, except it be one of colour. The Axolotl, therefore, is in the singular position of being sometimes "caducibranchiate," whilst it is ordinarily "perennibranchiate" * Nearly allied to the Axolotl is the Menobranchus (fig. 277, C) of North America, in which the branchiae are persistent. Amphiuma (fig. 277, B) and Menopoma, as already remarked, differ from the forms just mentioned in losing the gills when adult, but in retaining the external branchial apertures on the side of the neck. The species of Amphiuma are North American, and have both pairs of limbs, though these are of very small size.
The species of Menopoma, as now restricted, are also confined to the fresh waters of the United States, and likewise possess both pairs of limbs. The Giant-salamanders of Java form the genus Sieboldia or Cryptobranchus, and differ from Menopoma chiefly in the fact that the gill-slits are closed in the adult. They reach a length of several feet, and are the nearest living allies of the extinct Andrias of the Miocene Tertiary.
In the second section of the Urodela, comprising those forms in which the gills are caducous, and both pairs of limbs are always present, are the Water-salamanders or Tritons, and the Land-salamanders. The Tritons are the only examples of the aquatic Salamanders which occur in Britain, and every one, probably, is acquainted with the common Newt.
The Water-salamanders or Newts (fig. 281) are distinguished from the terrestrial forms by being furnished with a compressed fish-like tail, and by being strictly oviparous. The larvae are tadpole-like, with external branchiae, which they retain till about the third month. The adult is destitute of gills, and breathes by lungs alone, but the larval tail is retained throughout the life of the animal. The tongue is small, free, and pointed behind, and there are two rows of palatine teeth. The fore-feet are four-toed, the hind-feet five-toed; and the males have a crest on the back and tail.
* Professor Marsh of New Haven has shown that the Siredon lichenoides of the western States of America, when kept in confinement, loses its gills, and dorsal and caudal fins, whilst it changes much in colour, and undergoes various minor modifications in structure. Its habits, also, become less aquatic, and it becomes apparently absolutely identical with Ambly-stoma mavortium, a Salamandroid. This discovery has thrown considerable doubt upon the value of the distinction between perennibranchiate and caducibranchiate Amphibians, and has rendered it probable that all the species of Siredon are merely larval Salamanders, as long ago suspected by Cuvier. At the same time, the Axolotls certainly breed freely whilst in possession of their branchiae, and there is as yet no proof that they lose their gills whilst in a state of nature. It is, in fact, still uncertain whether all the species of Siredon are merely larval stages of Amblystoma, or whether all the Amblystomae possess a larval Siredon-stage.
Fig. 281. - Great Water-newt (Triton cristatus) - after Bell.
The development of the Newts is so like that of the Frogs that it is unnecessary to dilate further upon it here; but there are these two points of difference to be noticed: 1stly, That the embryonic tail is not cast off in the adult; and, 2dly, That the fore-limbs appear externally sooner than the hind-limbs - the reverse of this being the case amongst the Anoura.
The Land-salamanders form the genus Salamandra, and are distinguished from their aquatic brethren by having a cylindrical instead of a compressed tail, and by bringing forth their young alive, or by being ovo-viviparous, in which case the larvae have sometimes shed their external branchiae prior to birth. The head is thick, the tongue broad, and the palatine teeth in two long series. The skin is warty, with many glands secreting a watery fluid. The best-known species is the S. maculosa of Southern Europe. Another species (S. alpina) lives upon lofty mountains. The Salamanders of the Old World are represented in North America by various species of Amblystoma.
Order III. Anoura ( = Batrachia, Huxley; Theriomorpha, Owen; Chelonobatrachia, Batrachia salientia, etc.) - This order includes the Frogs and Toads, and is characterised by the following points : The adult is destitute of both gills and tail, both of which structures exist in the larva, whilst the two pairs of limbs are always present. The skin is soft, and there are rarely any traces of an exoskeleton. The dorsal vertebrae are "procoelous" or concave in front, and are furnished with long transverse processes, 7vhich take the place of ribs, which are only present in a rudimentary form. The radius and ulna in the fore-limb, and the tibia and fibula in the hind-limb, are anchylosed to form single bones (fig. 282). The month is sometimes edentulous, but the upper jaw has usually small teeth, and the lower jaw sometimes.