The hind-limbs usually have the digits webbed for swimming, and are generally much larger and longer than the fore-limbs. The vertebral column is short (of ten vertebras in the Frogs, but only eight in Pipa). The tongue is soft and fleshy, not supported by an os hyoides, but fixed to the symphysis of the lower jaw in front.
In the adult Anoura, respiration is purely aerial, and is carried on by means of lungs, which are, comparatively speaking, well developed. As there are no movable ribs by which the thoracic cavity can be expanded, the process of respiration is somewhat peculiar. The animal first closes its mouth, and fills the whole buccal cavity with air taken in through the nostrils. The posterior nares are then closed, and by the contraction of the muscles of the cheeks and pharynx the inspired air is forcibly driven into the windpipe through the open glottis. The process, in fact, is one of swallowing; and it is possible to suffocate a frog simply by holding its mouth open, and thereby preventing the performance of the above-mentioned actions. There can be no doubt, also, that the skin in these animals plays a very important part in the aeration of the blood, and that the frogs especially can carry on their respiration cutaneously, without the assistance of the lungs, for a very lengthened period. This undoubted fact, however, should not lead to any credence being given to the often-repeated stories of the occurrence of frogs and toads in cavities in solid rock, no authenticated instance of such a phenomenon being as yet known to science.
The young or larvae of the Frogs and Toads are familiarly known as "Tadpoles." The ova of the Frog are deposited in masses in water, and the young form, upon exclusion from the egg, presents itself as a "tailed" Amphibian, completely fish-like in form, with a broad rounded head, a sac-like abdomen, and a compressed swimming-tail (fig. 283, a). Behind the mouth are placed little "holders" or organs of adhesion; and the upper and lower jaws acquire horny sheaths and constitute a kind of beak. There are at first two sets of gills, one external and the other internal. The external branchiae (fig. 283, a) have the form of filaments attached to the side of the neck, and they disappear very shortly after birth. The internal branchiae are attached to cartilaginous arches, which are connected with the hyoid bone, and they are contained in a gill-cavity, protected by a flap of integument, which differs from the gill-cover of fishes in never developing any opercular bones or branchiostegal rays. Within the branchial chamber thus formed the fore-limbs are budded forth, but the hind-limbs are the first to appear externally, instead of the fore-limbs as is the case with the Urodela. Even after the first appearance of the limbs, the tail is still retained as an instrument of progression; but as the limbs become fully developed, the tail is gradually absorbed (fig. 283, d), until in the adult it has wholly disappeared.
Fig. 283. - Development of the common Frog (Rana temporaria). a Tadpole viewed from above, showing the external branchiae (g); b Side view of a somewhat older specimen, showing the fish-like tail; c Older specimen, in which the hind-legs have appeared; d Specimen in which all the limbs are present, but the tail has not been wholly absorbed. (After Bell.)
The development of the Frog is thus a good illustration of the general zoological law that the transient embryonic stages of the higher members of any division of the animal kingdom are often represented by the permanent condition of the lower members of the same division. Thus the transitory condition of the young Frog in its earliest stage, when the branchiae are external, is permanently represented by the adult perenni-branchiate Urodela, such as the Proteus or the Siren. The final stage, again, when the gills have disappeared and the limbs have been developed, but the tail has not been wholly absorbed, is represented by the caducibranchiate Urodela, such as the common Newt. In some of the Tree-frogs, however, there appears to be no true metamorphosis. Thus in the larvae of Hylodes the branchiae are absent or evanescent, the anterior and posterior limbs are developed contemporaneously, and the tail is absorbed within the first day after emergence from the egg.
The order Anoara comprises a considerable number of forms, but may be divided into the three principal sections of the Pipidae, Bufonidae, and Ranidae. In the Pipidae, or Surinam Toads, there are rarely teeth, and the mouth is destitute of a tongue. A singular and hideous species (Pipa Americana) is the best known, and it inhabits Brazil and Surinam. In this curious Amphibian the eggs are placed by the male on the back of the female, in the soft integument of which, in cell-like cavities, the eggs are hatched and the young developed. The larvae possess external branchiae, which are very early absorbed. In the aberrant form Dactylethra, the upper jaw is furnished with small teeth, and the three inner toes of the hind-feet are furnished with nails, as is the case with no other Amphibian, except Salamandra unguiculata amongst the Urodela. This curious form is found at the Cape of Good Hope and in Mozambique, and its larvae appear to be destitute of external branchiae.
In the Toads, or Bufonidae, a tongue is present, but the jaws are not armed with teeth. The tongue agrees with that of the Frogs in being fixed to the front of the mouth, whilst it is free behind, so that it can be protruded for some distance from the mouth. In one Toad only (Rhinophrynus) is there a tongue which is free in front. The hind-limbs are not disproportionately developed, whilst the toes are only imperfectly webbed, and the toes of the fore-limb are free. The skin is warty and glandular. The common Toad (Bufo vulgaris) is an excellent example of this family. The Natter-jack Toad is the only other British species, but about fifty other forms are known, of which many are American.
In the Ranidae the tongue has the same form as in the Toads, but the upper jaw always carries teeth. The hind-limbs are much larger than the fore-limbs, and are fitted for leaping, whilst the toes are webbed. The toes of the fore-limbs are free. The common Frog (Rana temporaria) is a good example of the typical Ranidae. This familiar species (fig. 284) is found over nearly the whole of Europe, North
Fig. 284. - The common Frog (Rana temporaria).
Africa, Northern Asia, and North America. It hibernates, passing its winter sleep buried in mud at the bottom of ponds. Larger than the common Frog is the Eatable Frog (Rana esculenta) of Europe, and larger again than this is the Bull-frog (Rana pipiens) of North America. The Tree-frogs (Hylidae) are adapted for a wholly different mode of life, having the toes of all the feet furnished with terminal suckers (fig. 276), by the help of which they climb with ease. They are mostly found in warm countries, especially in America, but one species (Hyla arborea) is European.
In the curious American Tree-frogs forming the genus Opis-thodelphys, the females have a dorsal brood-pouch, which extends over the back and opens posteriorly, and into which the eggs are introduced prior to hatching.