Fig. 80. Tadpoles with external branchiæ. (From Huxley; after Ecker.)
A. Lateral view of a young tadpole.
B. Ventral view of a somewhat older tadpole.
kb. external branchiæ; m. mouth; n. nasal sack; a. eye; o. auditory vesicle; z. horny jaws; s. ventral sucker; d. opercular fold.
C. More advanced larva, in which the opercular fold has nearly covered the branchiæ.
s. ventral sucker; ks. external branchiæ; y. rudiment of hind limb.
Shortly after hatching, there grows out from the hyoid arch on each side an opercular fold of skin, which gradually covers over the posterior branchial arches and the external gills ([fig. 80] d). It fuses with the skin at the upper part of the gill arches, and also with that of the pericardial wall below them; but is free in the middle, and so assists in forming a cavity, known as the branchial cavity, in which the gills are placed. Each branchial cavity at first opens by a separate widish pore behind ([fig. 80]), and in Dactylethra both branchial apertures are preserved (Huxley). In the larva of Bombinator, and it would seem also that of Alytes and Pelodytes, the original widish openings of the two branchial chambers meet together in the ventral line, and form a single branchial opening or spiracle. In most other forms, i.e. Rana, Bufo, Pelobates, etc., the two branchial chambers become united by a transverse canal, and the opening of the right sack then vanishes, while that of the left remains as the single unsymmetrical spiracle. In breathing the water is taken in at the mouth, passes through the branchial clefts into the branchial cavities, and is thence carried out by the spiracle.
Immediately after the formation of the branchial cavities, the original external gills atrophy, but in their place fresh gills, usually called internal gills, appear on the outer side of the middle region of the four branchial arches.
Fig. 81. Tadpole of Bombinator from the ventral side, with the abdominal wall removed. (After Götte.)
Behind the mouth are placed the two suckers, and behind these are seen the gills projecting through the spiracles.
There is a single row of these on the first and fourth branchial arches, and two rows on the second and third. In addition to these gills, which are vascular processes of the mesoblast, covered, according to Götte, with an epiblastic (?) epithelium, branchial processes appear on the hypoblastic walls of the three branchial clefts. The last-named branchial processes would appear to be homologous with the gills of Lampreys. In Dactylethra no other gills but these are formed (Parker).
The mouth, even before the tadpole begins to feed, acquires a transversely oval form ([fig. 81]), and becomes armed with provisional structures in the form of a horny beak and teeth, which are in use during larval life.
The beak is formed of a pair of horny plates moulded on the upper and lower pairs of labial cartilages. The upper valve of the beak is the larger of the two, and covers the lower. The beak is surrounded by a projecting lip formed of a circular fold of skin, the free edge of which is covered by papillæ. Between the papillæ and the beak rows of horny teeth are placed on the inner surface of the lip. There are usually two rows of these on the upper side, the inner one not continuous across the middle line, and three or four rows on the lower side, the inner one or two divided into two lateral parts.
As the tadpole attains its full development, the suctorial organs behind the mouth gradually atrophy. The alimentary canal, which is ([fig. 81]) at first short, rapidly elongates, and fills up with its numerous coils the large body cavity. In the meantime, the lungs develop as outgrowths from the œsophagus.