XII DEVELOPMENT 207 



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 stomodffium (Fig. 66, B — E) and, at a still earlier 

 period, the proctodaeum (p. 204) open into the enteric cavity, 

 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 which 

 the tadpole now browses upon the water-weeds which form 

 its staple food. It is on account of this diet that the in- 

 testine 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^) 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 quite like 

 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 com- 

 plete union of the operculum with the flank takes place on 

 the right side, the opercular aperture becoming closed : on the 



