432 MANUAL OF ZOOLOGY. 



(Urodela], and in the Frogs and Toads (Anoura), two sets of 

 gills are developed an external set, which is very soon lost, 

 and an internal set, which is retained for a longer or shorter 



Fig. T.%4. A>toura. Hyla leucota-Hia, one of the Tree-frogs (after Giiuther). 



period. As maturity is approached, true lungs adapted for 

 breathing air are developed. The development, however, of 

 the lungs varies with the completeness with which aerial respi- 

 ration has to be accomplished ; being highest in those forms 

 which lose their gills when grown up (Cadutibranchiatd), and 

 lowest in those in which the branchiae are retained throughout 

 life (Perennibranchiata). 



In accordance with the change from an aquatic or branchial 

 to a more or less completely aerial or pulmonary mode of 

 respiration, considerable changes are effected in the course 

 and distribution of the blood-vessels. In the larval condition, 

 when the respiration is entirely effected by means of the gills, 

 the circulation is carried on very much as it is in Fishes. 

 The heart is composed of a single auricle and ventricle, and 

 the blood is propelled through a bulbus arteriosus and bran- 

 chial artery to the gills. The aerated blood is then collected 

 in the branchial veins, and instead of being returned to the 

 heart, is forthwith propelled to all parts of the body, the de- 

 scending aorta being formed out of the branchial veins. At 

 this stage, therefore, the heart is a branchial one, and the single 

 contraction of the heart is sufficient to drive the blood through 

 both the branchial and systemic circulations, just as we saw 

 was permanently the case with all the Fishes except the Lepi- 



