510 MANUAL OF ZOOLOGY. 



which are retained in those forms in which the branchiae are 

 permanent (Perennibranchiata). In the tailless Amphibians 

 (Anoura), with hardly an exception, two sets of gills are de- 

 veloped an external set, which is very soon lost, and an inter- 

 nal set, which is retained for a longer or shorter period. As 



Fig. 2j6.Anoura. Hyla leucotcenia, one of the Tree-frogs (after Gunther). 



maturity is approached, true lungs adapted for breathing air 

 are developed. The development, however, of the lungs varies 

 with the completeness with which aerial respiration has to be 

 accomplished ; being highest in those forms which lose their 

 gills when grown up ( Caducibranchiata), and lowest in those in 

 which the branchiae are retained throughout life (Perennibran- 

 chiata] ; while even in the highest forms of the class the struct- 

 ure of the lung is very simple. 



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 



