544 GENERAL CHARACTERS OF REPTILES. 



up and disappear, the cartilaginous arches which supported 

 them being also in part absorbed ; finally the tail entirely dis- 

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

 mately preserve (Fig. 321), and completely changes its regimen. 

 Herbivorous at first, it becomes by degrees exclusively carnivo- 

 rous; and whilst these metamorphoses are taking place, its 

 intestinal canal, at first long and twisted in a spiral form, 

 becomes short, almost straight, and enlarged in parts correspond- 

 ing with the stomach and colon. 



482. The apparatus of circulation undergoes changes cor- 

 responding to those, which the organs of respiration experience. 

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



1 t o t o ab 



vb 3 a "/> av c ab 2 



FIG. 322. BLOOD-VESSELS or THE TADPOLE m ITS FIRST STATB. 



, artery originating from the single ventricle, and dividing into six branchial arteries, 

 ab ; brl, br2, br3, the three pairs of gills, from which return the branchial veins vb. 

 The second and third branchial veins on each side form a trunk, c, which unites with 

 the opposite one to form the great dorsal artery, av ; the first pair of branchial veina 

 send off the trunks, t, t, to the head. 1, 2, 3, communicating branches, connecting the 

 branchial arteries with the branchial veins, in this stage very small ; ap, pulmonary 

 arteries, as yet but little developed. 



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

 a large artery, which swells at its base into a contractile bulb, 

 and which is sometimes bifurcated to form the two arches of the 

 aorta. But when the young animal respires by the gills only, 

 the blood forced out of the ventricle is distributed to these organs ; 

 and thence the greater part is returned into a dorsal artery, 

 whose branches convey it to the rest of the body (Fig. 322). 



