5/8 METAMORPHOSES OF BATRACHIA. 



of membranous operculum, analogous to the gill-covers of Fishes. 

 By degrees, the gills (Fig. 332) ramify ; and the lips are covered 

 with a kind of horny beak ; by the aid of which the animal fixes 

 itself to the vegetables, on which it is chiefly nourished. This 

 state, however, only endures for a short time. At the end of 

 some days, the gill- fringes, which float on each side of the neck, 

 disappear (Fig. 333) ; and respiration is performed by the aid 

 of small vascular tufts, fixed along four cartilaginous arches, 

 situated under the throat, and fastened to the hyoid bone. A 

 membranous tunic, covered by the skin, envelopes these internal 

 gills ; to which the water comes through the mouth, by passing 

 through the intervals of the arches of the hyoid ; after having 

 bathed these organs, the liquid escapes by one or two external 

 slits, whose position varies a little according to the species. The 

 respiratory apparatus then presents, as we see (ANIM. PHYSIOL. 

 317), a most exact resemblance to that of Fishes. A little 

 time afterwards, the posterior feet of the tadpole present them- 

 selves, and are developed little by little (Fig. 334) ; and they 

 attain considerable length, before the anterior feet can be seen. 

 These last are developed beneath the skin, which they pierce 

 more slowly (Fig. 335). About the same time, the horny beak 

 falls off, and leaves the jaws bare ; the tail begins to be atrophied 

 (Fig. 336) ; the lungs are developed ; and in proportion as these 

 organs become more exclusively the seat of respiration, the gills 

 shrivel up and disappear, the cartilaginous arches which sup- 

 ported them being also in part absorbed ; finally the tail entirely 

 disappears, the little animal takes the form which it must ulti- 

 mately preserve (Fig. 337), and completely changes its regimen. 

 Herbivorous at first, it becomes by degrees exclusively carnivo- 

 rous ; and whilst these metamorphoses are taking place, its in- 

 testinal canal, at first long and twisted in a spiral form, becomes 

 short, almost straight, and enlarged in parts corresponding with 

 the stomach and colon. 



522. The apparatus of circulation undergoes changes corre- 

 sponding to those which the organs of respiration experience. 

 The heart of the adult Batrachian is composed, like that of the 

 Reptiles, of two auricles and a single ventricle ; whence arises 



