xii DEVELOPMENT 207 



aerated water for purposes of respiration. A thin 

 median fold of the skin, the tail-fin (Fig. i), arises 

 all round the tail, both above and below, and the tail 

 now forms a powerful swimming organ. The stomo- 

 daeum and, at a still earlier period, the proctodaeum 

 (p. 204) open into the enteric cavity (Fig. 66, B E), so 

 that there is now a complete enteric canal : it grows 

 much faster than the body generally, and becomes coiled 

 like a watch-spring (D, E). The mouth is very small, 

 and is bounded by lips beset with little horny projections 

 or papillae and provided with a pair of horny jaws 

 (C E), with v/hich the tadpole now browses upon the 

 water- weeds which form its staple food. It is on account 

 of this diet that the intestine is of such great proportionate 

 length : vegetable diet contains less nourishment, bulk 

 for bulk, than animal, and a longer intestine is required 

 to extract all the nutriment from it. 



Soon the external gills show signs of shrivelling, and 

 on the inner portions of the branchial arches internal gills 

 (Fig. 66, D 1 ) are developed, like those of a fish. A fold 

 of skin appears on either side in front of the gills and 

 gradually grows backwards, covering the branchial clefts 

 and the external gills, which latter soon disappear 

 entirely ; this is the operculum or gill-cover (B, C) ; it is 

 I similar to that of an ordinary fish, such as a cod or a 

 perch, except that it is not supported by bone. The 

 current of water from the pharynx now, of course, makes 

 its final exit by a wide cleft between the edge of the 

 operculum and the flank. The operculum gradually 

 ! extends, and its free edge unites with the wall of the 

 body posterior to the gills, so that the originally large 

 aperture through which the water from the pharynx 

 passes is much reduced. After some time a complete 

 union of the operculum with the flank takes place on 

 the right side, the opercular aperture becoming closed : 



