Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/832

796 FROG upon it. This took place with all of them successively, ex cepting the last, which lived on to complete its change." After leaving the water they feed almost entirely on insects and slugs. The form of the frog is too well known to require de scription, but there are many almost unique features in its organization that may be noticed. Respiration in the adult frog is partly pulmonary ; but as it is destitute of ribs, this operation in the frog cannot be performed by the alternate expansion and contraction of the chest, as in other air- breathing animals. The air has to be swallowed in order to be conveyed to the lungs, and the mechanism by which this operation is performed necessitates the closing of its mouth and the admission of air by the nostrils, so that a frog can be most readily suffocated by having its mouth gagged open. Respiration is also partly cutaneous, experi ment having shown that the skin gives off carbonic acid gas in sufficient quantity to enable the creature to live for a very considerable time after pulmonary respiration has been stopped. Moisture is as necessary to the skin in the performance of this function as it is to the gills of a fish, and in order to preserve to the utmost its humidity, frogs avoid as much as possible the hot sunlight, sheltering them selves beneath stones or under loose turf, and reappearing on the advent of rain, sometimes in such numbers in a single locality as to have given rise to stories of frog showers. The skin of the frog, however, readily absorbs water ; and this it stores up in an internal reservoir, from which it can in seasons of drought moisten the surface of its skin. When a frog is suddenly caught it frequently ejects a quantity of water, and thus suddenly diminishes its volume. The water avoided is not urine, nor is the recep tacle containing it the urinary bladder, as was at one time supposed. The skin of frogs is perfectly smooth, having neither plates nor scales, except in the American genus Cera- tophrys (fig. 1), in certain species of which a few bony plates FIG. 1 Ceratophrys granosn. are enclosed in the skin of the back. The tongue in frogs is probably employed more for the capture of its insect prey than as the organ of taste. It is fixed in front of the mouth, and free behind, and in seizing its prey, the free end, which possesses a viscid secretion, is darted suddenly for ward, the captured insect being as suddenly transported to the back part of the mouth. Nothing can exceed the rapidity with which this motion is performed. Minute teeth are present in the upper jaw, and on the palate, in the true frogs. The vocal organs by which the characteristic croaking is produced differ somewhat in different species, a similar variety appearing in the quantity and quality of their song. The male of the edible frog is provided with bladder-like cheek pouches the so-called vocal sacs which it distends with air when in the act of croaking, an operation which it performs to such purpose as to have received the name of " Dutch nightingale " on the Continent, and " Cambridgeshire nightingale " in England. The bull-fro" 1 (liana mugiens) has a laryngeal mechanism which Cuvier compared to a kettledrum, by which it produces a sound not unlike the bellowing of a bull. The sound produced by the tree frogs is both loud and shrill, but in certain circum stances it seems to be somewhat pleasing. Thus Darwin says, " Near Rio de Janeiro I used often to sit in the even ing to listen to a number of little Ifylcc, which, perched on blades of grass close to the water, sent forth sweet chirping notes in harmony." The voice of another tree-frog (Hyla crepitans) has been compared to the sound produced " by the cracking of a large piece of wood;" while another, belouging to Surinam, has an extremely disagreeable voice, and unfortunately so much of it that, when a number of them combine, they at times drown the orchestra of the Paramaribo theatre. No frog, so far as yet known, pos sesses any poison organs. A species found in France (1 elo- batfs fuscus), when disturbed, emits a strong odour, some what resembling garlic, and of sufficient pungency to make the eyes water ; another (Hyla micans) exudes from the surface of its body a slimy substance having luminous pro perties, which probably acts as a defence by frightening its enemies. Many of the tree-frogs are of a green colour, while others are brown, "and these," says Mr Wallace (Tropical Nature, 1878), " usually feed at night, sitting quietly dur ing the day so as to be almost invisible, owing to their colour and their moist shining skins so closely resembling vegetable substances." The majority of tree-frogs have their colour thus adapted to their surroundings, and are thus enabled the more readily to elude their enemies ; for, so far as yet known, all the species protectively coloured are edible, forming the chief food of many mammals, birds, and reptiles. A few, however, are brilliantly and conspicu ously coloured, as if courting observation ; and such species, there is reason to believe (the reader will find evidence of this in Belt s Naturalist in Nicaragua), are rejected by frog-eating animals on account of their nauseous secretions, or some other unknown property which renders them un palatable. The bright colours thus become directly useful to the species by making them readily recognizable as un eatable. The Common Frog (Rana temporaria) is the most widely distributed species of the group, occurring throughout tho temperate regions of both hemispheres, including North Africa, and Asia as far east as Japan. It does not occur in Iceland. Both in England and in Scotland it is abundant, nor is it uncommon in Ireland, although popularly supposed to be absent from that island. It varies very considerably in colour, being either a reddish, yellowish, or greenish- brown above, with irregular spots and fasciae, and of a lighter colour beneath, but having almost invariably an oblong patch of brown behind the eyes, by which it may be readily distinguished from other European species. Although not an article of human food, the common frog in its various stages of growth forms the staple diet of many other animals. The tadpoles are eaten by newts and the smaller fishes, and frogs of all ages by weasels, waterfowl, pike, and snakes. By those agencies their numbers, which other wise would be enormous, are greatly reduced. This species is said to take five years in attaining its full growth, and to live for about fifteen years. Like the toad it can be ren dered tame and domesticated, having been known to take up its abode in a moist corner of a kitchen, and to come forth regularly at meal time to be fed. The Edible .Frog (Rana esculenta) is very widely distributed throughout the temperate regions of the earth, but is not found in Americn, where, however, a closely allied species occurs (fig. 2). It is found in England, where it was first observed in a Cambridgeshire fen is 1843, to which probably it hail