TADPOLE, a term often, but wrongly, applied indiscriminately to all Batrachian larvae. It is absurd to call the larva of a newt or of a Caecilian a tadpole, nor is the free-swimming embryo of a frog as it leaves the egg a tadpole. A tadpole is the larva of a tailless Batrachian after the loss of the external gills and before the egress of the fore limbs (except in the aberrant Xenopus) and the resorption of the tail. What characterizes a tadpole is the conjoined globular head and body, so formed that it is practically impossible to discern the limit between the two, sharply set off from the more or less elongate compressed tail which is the organ of propulsion. In describing tadpoles, the term "body" is therefore used as meaning head and body. The tail consists of a fleshy muscular portion bordered above and below by membranous expansions, termed respectively the upper and lower crest, the former sometimes extending along the body.
Except in a few aberrant types, which are mentioned below, the mouth is surrounded by a much developed lip like a funnel directed downwards, and is armed with a horny beak not unlike that of a cuttle-fish. The characters offered by the circular lip are among the most important for the distinction of species. It may be entirely bordered by fleshy papillae, or these may be restricted to the sides, or to the sides and the lower border. Its inner surface is furnished with ridges beset with series of minute, bristle-like, erect, horny teeth, each of which, when strongly magnified, is seen to be formed of a column of superposed cones, hollowed out at the base and capping each other; the summit or crown of each of these cones is expanded, spatulate, hooked backwards, and often multicuspid. The number of these columns is very great. F. E. Schulze has counted as many as I roc) in the lip of Pelobates fuscus. The beak is made up of horny elements, like the labial teeth, fused together; its edge, when sufficiently magnified, is seen to be denticulate, each denticle representing the cusp of a single tooth. The gills, borne on four arches, are internal and enclosed in the branchial chambers. The arches bear on the convex outer side the delicate arborescent gills, and on the concave inner side develop a membranous septum with vermicular perforations, a special sifting or filtering contrivance through which the water absorbed by the mouth has to pass before reaching the respiratory organs of the branchial apparatus.
The water is expelled from the branchial chambers by one or two tubes opening by one orifice in most Batrachians. This orifice is the spiraculum, which is lateral, on the left side of the body, in most tadpoles, but median, on the breast or belly, in those of the Discoglossidae and of some of the Engystomatidae. All tadpoles are provided with more or less distinct lines of muciferous sensory crypts or canals, which stand in immediate relation to the nerve branches and are regarded as organs of a special sense possessed by aquatic vertebrates, feeling, in its broadest sense, having been admitted as their possible use, and the function of determining waves of vibration in the aqueous medium having been suggested. In addition to these lines, all tadpoles show more or less distinctly a small whitish gland in the middle of the head between the eyes, the so-called frontal gland or pineal gland, which in early stages is connected with the brain. A glandular streak extending from the nostril towards the eye is the lachrymal canal. The eyes are devoid of lids.
Owing to more or less herbivorous habits, the intestine is exceedingly elongate and much convoluted, being several times larger and of a greater calibre than after the metamorphosis. Its opening, the vent, is situated either on the middle line at the base of the tail, or on the right side, as if to balance the sinistral position of the spiraculum. The tail varies much in length and shape according to the species; sometimes it is rounded at the end, sometimes more or less acutely pointed, or even terminating in a filament. The skeleton is cartilaginous, and the skull is remarkable for the very elongate suspensorium of the lower jaw; the tail remains in the notochordal condition, no cartilages being formed in this organ, which is destined to disappear with the gills. The hind limbs appear as buds at the base of the tail, and gradually attain their full development during the tadpole life. The fore limbs grow simultaneously, and even more rapidly, but remain concealed within a diverticulum of the branchial chambers until fully formed, when they burst through the skin (unless the left spiraculum be utilized for the egress of the corresponding limb).
The above description applies to all European and North American tadpoles, and to the great majority of those known from the tropics. The following types are exceptional.
The circular lip is extremely developed in Megalophrys montana, and its funnel-shaped expansion, beset on the inner side with radiating series of horny teeth, acts as a surface-float, when the tadpole rests in a vertical position; the moment the tadpole sinks in the water the funnel collapses, taking on the form of a pair of horns, curling backwards along the side of the head; but, as they touch the surface again, it re-expands into a regular parachute.
In some species of Rana and Staurois inhabiting mountainous districts in south-eastern Asia, the larvae are adapted for life in torrents, being provided with a circular adhesive disk on the ventral surface behind the mouth, by means of which they are able to anchor themselves to stones.
In some Indian and Malay Engystomatids of the genera Callula and Microhyla, the tadpoles are remarkably transparent, and differ markedly in the structure of the buccal apparatus. There is no funnel-shaped lip, no horny teeth, and no beak. The spiraculum is median and opens far back, in front of the vent.
In the Aglossal Xenopus, the tadpoles are likewise devoid of circular lip, horny teeth, and beak, and they are further remarkable in the following respects: There is a long tentacle or barbel on each side of the mouth, which appears to represent the "balancer" of Urodele larvae; the spiraculum is paired, one on each side; the fore limbs develop externally, like the hind limbs.
Some tadpoles reach a very great size. The largest, that of Pseudis paradoxa, may measure a foot, the body being as large as a turkey's egg. The perfect frog, after transformation, is smaller than the larva. Pseudis was first described by Marie Sibylle de Merlon (1647-1717), in her work on the fauna of Surinam (published first in 1705 at Amsterdam, republished in Latin in 1719), as a frog changing into a fish. Among European forms, some tadpoles of Pelobates attain a length of seven inches, the body being of the size of a hen's egg. The tadpole of the North American bull-frog measures six inches, and that of the Chilian Calyptocephalus gayi seven and a half inches.
L. F. Heron-Royer and C. Van Bambeke, "Le vestibule de la bouche chez les tetards des batraciens anoures d'Europe," Arch. Biol., ix. 1889, p. 185; F. E. Schulze, "Uber die inneren Kiemen der Batrachierlarven," Abh. Ak. Berl., 1888 and 1892; G. A. Boulenger, "A Synopsis of the Tadpoles of the European Batrachians," P.Z.S., 1891, p. 593; F. E. Beddard, "Notes upon the Tadpole of Xenopus laevis," P.Z.S., 1894, p. tot; S. Flower, "Batrachians of the Malay Peninsula and Siam," P.Z.S., 1899, p. 885; H. S. Ferguson, "Travancore Batrachians," J. Bombay N.H. Soc., xv. 1904, p. 499. (G. A. B.)
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