Fig. 10.–1, Front view of the mouth of a tadpole of Rana temporaria, showing the transverse rows of tiny horny teeth; 2, three successive horny teeth, much magnified. (After Gutzeit.)
The mouth of the tadpoles of Anura is furnished with horny armaments, substitutes for teeth. Their development and that of the mouth in general has been well described by Gutzeit.[[30]] In the young larvae of Rana temporaria, one or two days after hatching, a shallow groove appears above the conspicuous pair of adhesive organs. The groove becomes rhombic in outline, and when the mouth has been formed in its centre, the jaws appear in the median corners of the rhombus. The epidermis then rises like a circular wall around the jaws, and divides into an upper and lower lip; furrows appear on them, and between these various papillae and comb-like transverse plates of teeth. The papillae are possibly tactile organs, but although nerves enter them, nerve-endings of a sensory nature have not yet been discovered. On the fourth day the jaws become black, by the tenth day horny teeth have appeared upon all the plates of the mouth-armature, and on the seventeenth day the mouth-apparatus has reached the configuration typical of the tadpole, which is now about 14 mm. long. The number of horny teeth in R. temporaria amounts to about 640. These teeth are not cuticular products, but cornified cells; they are very small, and consist each of one horny cell, which is shaped like a nightcap, the apex of which is curved back and serrated. The little teeth are shed continuously, the renewal taking place by successive cells growing into the bases of the older series. The shape and size differ much in the various genera and species. The comb-like plates, composed of those teeth which surround the lips, seem to be used chiefly for the fixing or hooking of the food, while those which compose the horny beak proper, the armature of the jaws, are used like the radulae of snails. These beaks are likewise composed of a great number of individual teeth, closely packed together in several rows, but the teeth themselves are simple and not serrated.
In Hyla arborea there are in all about 560 teeth. The development of the mouth does not begin before the eleventh day; the horny teeth break through, and the jaws get black edges, on the eighteenth. In Pelobates fuscus the number of horny teeth is increased to about 1100. In Borborocoetes taeniatus the horny teeth form series of five bells, which fit into each other like the joints of a rattlesnake's tail.
One of the most extraordinary kinds of tadpoles is that of Megalophrys montana.[[31]] Mr. Annandale (Skeat Expedition) found it at Bukit Besar, Malay Peninsula, from 2000 to 3000 feet above the level of the sea. The tadpoles (Fig. 11) were found in the beginning of the month of May 1899 in sandy streams and in pools of rain-water; they floated in a vertical position, the peculiar membranous funnel-shaped expansion of the lips acting as surface-floats. The inside of the funnel is beset with radiating series of little horny teeth, and the whole apparatus is possibly used for scraping the under-surface of the leaves of water-plants in search of food. Total length of the tadpoles 1 inch.[[32]]
The gills, the formation of the operculum, and the modifications of the branchial arterial arches have been described fully on p. [43]; those of the hyo-branchial skeleton on p. [31]. Fusion of the opercular fold with the skin of the neck, across the branchial region, causes the head to become confluent with the trunk (cf. Fig. 9, 3, p. [57]). The body becomes oval, more or less globular, and the alimentary canal is greatly elongated and stowed away in the shape of a neat, very regular spiral, shining through the ventral wall of the body; the anus opens at the end of a somewhat protruding tube, either in the median line, just in front of the ventral fin (Discoglossidae, Pelobates, Bufo), or it assumes an asymmetrical position by turning to the right side (Hyla, Rana).
Fig. 11.–Tadpoles of Megalophrys montana from Bukit Besar, Malay Peninsula. × 3.
Although both pairs of limbs begin to bud simultaneously, or the fore-limbs even earlier, the hind-limbs are hurried on, and appear first, long before the fore-limbs. The latter lie ready beneath the skin of the gill-chamber, and the right always breaks through the skin, while the left does the same in the Mediogyrinidae, while in the Laevogyrinidae it is generally pushed through the left-sided spiracular opening, immediately behind the outer gills. According to Barfurth the right limb appears, in about 80 per cent. of Rana esculenta, from two to eight hours before the left.
Meanwhile the lungs are being developed, and the tadpole occasionally rises to the surface to breathe air. The gills, which, as has been explained elsewhere, are less ancestral than they are larval organs, degenerate, and all the organs are modified for the coming terrestrial life. The fins of the tail are absorbed, the horny armature of the mouth and lips is shed in pieces and makes room for the true teeth, the eyes receive lids, and the whole cranium, especially the apparatus of the jaws, undergoes the final modifications–widening and lengthening of the mouth, arresting of the mento-Meckelian cartilages, elongation of the Meckelian cartilages or lower jaw proper, shifting backwards of the suspensorium, and lengthening of its orbital process to form the pterygo-palatine bridge.