GENERAL CHARACTERS OF REPTILES. 543 



or less, according to the species. Frogs, Toads, and other 

 species formed almost on the same plan, are, of all the Batra- 

 chians, those whose metamorphoses are the most complete. At the 

 time when the young tadpole of the Frog comes from the egg, 

 it is very like a little fish, and can only live in water. Its 

 head is very large, its abdomen protuberant, and its body, 

 deprived of members, is terminated by a long and compressed 

 tail ; its mouth is as yet but a small aperture, hardly percep- 

 tible ; and its gills only consist of a tubercle placed at each side 

 of the posterior part of the head. These appendages are soon 

 elongated and divided into laminae ; the eyes are distinguishable 

 through the skin, and a transverse slit is seen under the neck, 

 so as to form there a kind of membranous operculum, analogous 

 to the gill-covers of Fishes. By degrees, the gills (Fig. 316) 

 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. 317) ; 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, envelops 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 themselves, and are deve- 

 loped little by little (Fig. 318) ; 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. 319). About the same time, the horny beak falls off, and 

 leaves the jaws bare; the tail begins to be atrophied (Fig. 320); 

 the lungs are .developed; and in proportion as these organs 

 become more exclusively the seat of respiration, the gills shrivel 



