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FROG, the common name of an extensive group of Batrachians forming, along with the toads, the amphibian order A mourn. They are divided into 9 families, containing 92 genera and 440 species, and are found in all quarters of the globe, being most abundant in the tropical and subtropical regions, but also occurring within the Arctic circle. Most of the families have a very limited distribution, and only two of them, the true frogs (Ranidce), of which there are 150 species, and the Polype7latidw, a family of tree-frogs containing 124 species, can be regarded as almost cosmopolitan. The neotropical or South American region is richest in peculiar forms, while it possesses some only found beyond it in the widely remote Australian region ; thus the Pelodrycola', a family of tree-frogs, is peculiar to the two; the genus Litwia is confined to Australia, with the exception of a single. species occurring in Paraguay ; while the only frog known in New Zealand has its nearest allies in South America. Those regions bear also a negative resemblance in the total absence from both of the genus Rana, the 60 species of which are distributed throughout the other quarters of the globe. These facts, among others, have been adduced in support of the thoory that at one time the continents of South America and Australia had a land connexion. Frogs are almost totally absent • from oceanic islands, a single species (Liopelma Hochstetteri) occurring in New Zealand, and one or two others in the Pacific islands, as far east as the Fijis, beyond which they are unknown. On the assumption that those islands obtained their present fauna from the nearest continental land, the absence of frogs can be readily explained by the fact that salt water is alike fatal to the adult frog and to its spawn, and thus formed an insuperable barrier to their migration.
Frogs, as is shown by their wide distribution, are capable of enduring a considerable degree of both heat and cold ; they are, however, altogether intolerant of long-continued drought, a desert forming as certain a barrier to their migration as an ocean. Both during their larval stage and afterwards, for the purpose of cutaneous respiration, abundant moisture is a necessity of their existence ; consequently, whether they live on the ground or on trees, they are never found far from rivers, marshes, or lakes. In winter the frogs of northern climates hibernate, burying themselves in the mud at the bottom of pools, and lying clustered together in a state of complete torpidity. In hot climates they are said to go into a similar condition, known as " sestivation," during periods of exceptional heat and drought, in order to retard the dissipation of the moisture in their bodies. On reappearing from their lung winter sleep the work of reproduction is at once entered upon, the males snaking their presence known to the females by the vigorous exercise of their vocal organs. The croaking of the common frog can only be regarded as pleasant from its association with the welcome advent of spring ; still more unpleasant, however, is the much louder croak of the edible frog of the Continent, the species to which Horace probably refers in the lines "rameque palustres Avertunt somnos."
The eggs of the frog, consisting of little black specks surrounded by an albuminous envelope, are fertilized during their extrusion from the body of the female, and are generally deposited at the bottom of the water, ascending, however, soon after to the surface, owing to the swelling and partial decomposition of the glairy substance surrounding the ova. There are several species known in which the eggs are deposited in an exceptional manner. Those of a small frog (Alytes obstetricans) found in France and Germany form a long chain, which the male twines round his thighs, retiring with them into seclusion until the young are ready to leave, when he enters the water, and the tadpoles immediately make their escape. The female of an American tree-frog (Nototrema marsvpialum) has a pouch along the whole extent of its back for the reception of its eggs ; and Professor Peters of Berlin has recently- drawn attention to a tree-frog of the genus Polypedates found in tropical West Africa, in which the female, after depositing her eggs in the usual mass of albuminous jelly, attaches them to the leaves of trees overhanging a dry water-hole or pool. The albumen speedily dries and forms a horny coating on the leaf, under which lie the unimpregnated eggs. On the advent of the rainy season the albumen becomes softened, and the eggs are washed into the pool below, now filled with water, where they are fecundated by the male.
The development of the egg after impregnation proceeds more or less rapidly according to the temperature, the young of the common frog being hatched, according to Rusconi, in 4 days, in a temperature varying from 70° to SO' Fahr., while in the climate of England this does not take place in less than a month. The creature which emerges from the egg is altogether unlike a frog, consisting mainly of a bulky head and tail, and wholly destitute of limbs. This is the tadpole or larval stage in the development of the frog, when it is essentially a fish, capable only of existing in water, breathing by gills, and having like a fish a two-chambered heart. At first the gills or branchise are external, but they are soon withdrawn within the bronchial cavity, and concealed by an opercular membrane. As the process of development proceeds, the limbs begin to bud. forth, the posterior pair appearing first ; and with their growth the tail begins to dwindle, not falling off, but being gradually absorbed. At the same time vast changes aro taking place in the blood-vascular system, the gills gradually disappearing, two lungs being developed, and the heart becoming three-chambered as in reptiles. The young frog must now come to the surface to breathe, and soon leaves the water altogether. On emerging from the egg, the tadpole at first feeds upon the gelatinous mass which before had formed a protective covering. It is improvided with teeth, but has two minute horny jaws, which enable it to feed on decaying animal and vegetable matter. According to Bell (British Reptiles), tadpoles sometimes kill and feed upon each other. "I observed," he says, "that almost as soon as one had acquired its limbs it was found dead at the buttons of the water, and the remaining tadpoles feeding upon it. This took place with all of them successively, excepting 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 description, 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, experiment 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 themselves 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 receptacle 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 Geratophrys (fig. 1), in certain species of which a few bony plates 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 forward, 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 been introduced from the Continent. It has since been introduced into various parts of the south of England, but apparently without much success, the summer heat there being, it is said, insufficient to enable the tadpole to attain its full development before the advent of autumnal cold. It may be readily distinguished from the common species by the yellowish mesial line which runs down the whole length of its back, by the absence of the characteristic brown spot behind the eyes, and by the presence in the males of "vocal saes." It is also more aquatic than the common frog, seldom leaving the banks of its native pond or stream, into which it is always ready to dip on the slightest appearance of danger. It is very abundant throughout central and southern Europe, and forms, especially in France, a valued article of food, the hind legs when cooked being regarded as a luxury. Regarding a dish of these, Mr F. Buckland says, " Most excellent eating they were, tasting more like the delicate flesh of the rabbit than any thing else I can think of." The edible frog has been lately introduced into Ireland. The Bull-frog (Rana mugiens1 is one of the largest species, measuring sometimes 8 inches in length, exclusive of the hind legs, and having a gape sufficiently wide to swallow ducklings whole. It inhabits North America, where it is said to haunt the pools formed at the origin of springs, the waters of which it was supposed to keep pure - a belief which long afforded it considerable protection ; lately, however, the Americans have taken to frog-eating, and the bull-frog, in the absence of R. eseulenta, has been selected among others for this purpose.
The Tree-frogs (fig. 3) are readily distinguished from all others by having the ends of their toes dilated into knobs or discs, generally provided with a sticky secretion, by means of which they can cling to the leaves and branches of trees. They are small, elegant, and exceedingly active creatures, the males possessing loud voices, of which they make copious use during the breeding season and on the approach of rain. Frogs have from remote times been regarded as weather prophets, and at the present day, in sonic parts of Germany, the European Tree-frog (Ifyla arborea) is used as a barometer. A few of them arc placed in a tall bottle provided with miniature ladders, the steps of which they ascend during fine weather, seeking the bottom again on the approach of rain. All frogs, whether arboreal or not, have their hind feet webbed, and in at least one tree-frog (Rhacophorus) the webs on all the feet are so largely developed as to render it probable that by their means the frog is able to execute flying leaps. This "flying frog " was brought to Mr Wallace, while travelling in Borneo, by a Chinaman, who assured him " that he had seen it come down in a slanting direction as if it flew." Its body was about 4 inches in length, and the expanded webs of each hind foot covered a space of 4 square inches, while its forelegs were bordered by a membrane - features highly suc,- gestive of aerial locomotion.
Fossil remains of the frog do not occur in strata older than the Tertiary, being found in greatest abundance in the Miocene deposits. See AMPHIBIA. (J. G