Page:The American Cyclopædia (1879) Volume VII.djvu/516

 504 FROG are seated various colors, especially bright in the season of fecundation. The sense of smell is very imperfect ; the tongue is not an organ of taste but of prehension, soft and covered with a viscid mucus, its base attached to the concavity of the lower jaw, its bifurcated point extending backward, and the whole organ capable of being projected from the mouth in a reversed position for the seizure of its insect prey ; the organ of hearing has a tympanum, and an aerial cavity under it communicating with the throat. The mouth is very widely cleft, and some of the larger species have been known to swallow small mammals and birds ; like other amphibians they cannot drink. The structure of the heart, gills, and lungs, and the phenomena of the circulation in the tadpole and adults, and of the branchial, pulmonary, and cutaneous respirations, have been described in the article AMPHIBIA. The well known voice of the frog varies so much in intensity and tone as to render it difficult from the sound to as- certain the distance of the animal, far surpass- ing in this respect the efforts of the most skilful ventriloquist ; it can make a dull sound even under water. Among the many authors who have attempted to imitate in words the sounds of the frog, one of the most successful is Aris- tophanes, in whose comedy of the "Frogs" a frequent verse in the chorus is ^rekekekex, Icoax Tcoax, whose night-long repetition in spring and summer sometimes renders sleep impossible to those unaccustomed to it. By their power of retarding or accelerating the respiratory move- ments, and of aerating the blood through the vessels distributed to the skin, frogs are able to resist considerable changes of cold and heat, and to sustain life during their winter torpid- ity; the absorption and exhalation performed through the skin explain their occurrence and prolonged existence under circumstances where ordinary animals would soon perish, as under water and in air-tight places. The sexes are separate, and the reproductive functions are performed in the same mechanical and passion- less manner as in most fishes; the ova are fe- cundated at the moment of their exclusion. As the eggs are expelled they are enveloped in a glairy mass, in which the embryos are seen distributed like black dots ; the development is very rapid under favorable circumstances of temperature, the head and tail becoming per- ceptible in the course of the second day, the gills on the third, and the tadpole at the tem- perature of 80 F. (as in Rusconi's experi- ments) may leave the egg on the fourth or fifth day ; but in the ordinary seasons of temperate Europe and America, the young are not hatched until about a month after the deposit of the eggs. The tadpole is half an inch long when hatched ; the mouth is distinct, but small and without lips; the gills rapidly enlarge, and when at their maximum development afford beautiful objects for displaying the circulation ; the gills soon begin to decrease in size, and are finally withdrawn within the branchial cavity, as in fishes, and concealed by an opercular fold of integument ; the eyes are perfectly formed ; the mouth acquires movable lips, is placed nearer the end of the head, and is used for the intro- duction of vegetable food ; the caudal fin in- creases in size, and serves for rapid locomotion. Without any great change in form, the size is rapidly increased ; two small tubercles appear near the vent, the rudiments of the posterior legs, which are soon developed into the perfect limbs ; the anterior limbs are afterward formed under the skin in a similar manner ; as the legs are perfected the tail is gradually absorbed from the tip to the base, and progression is effected by the hind limbs. The lungs are now fitted for the respiration of air, and the little creatures come on land in search of worms and insects, Eight stages of development of the tadpole, from the re- cently hatched (1) to the adult form (8), as illustrated by St. George Mivart. and in such multitudes in damp weather as to give rise to the belief, still popularly adhered to in many places, that it has rained frogs. They grow rapidly during the summer and autumn, and in winter plunge into the mud to pass their stage of hibernation. In the tadpole state great numbers are devoured by fishes, other reptiles, and by each other ; and the adults furnish food for all classes of vertebrata from fishes up to man himself. It is probable that not more than one in a thousand of those which come from the egg in the spring live to reach their winter retreat ; if fortunate enough to escape from all enemies, frogs may live many years. Serpents among reptiles, pickerel among fishes, vultures, storks, herons, and cranes among birds, are the worst enemies of frogs ; were it