VERTEBRATA: AMPHIBIA. 511 



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 

 Dipnoi. The pulmonary arteries are at first very small, and 

 take their origin from the last pair of branchial arteries. When 

 the lungs, however, are developed, and the respiration com- 

 mences to be aerial, the pulmonary arteries increase propor- 

 tionately in size, and more, and more blood is gradually 

 diverted from the gills and carried to the lungs, so that the 

 branchiae suffer a proportionate diminution in size. In those 

 Amphibians in which branchiae are permanently retained 

 (Perennibranchiata), this state of affairs remains throughout 

 life that is to say, a portion of the venous blood is sent by 

 the pulmonary artery to the lungs, and a portion goes to the 

 gills. In those Amphibians, however, in which the adult 

 breathes by lungs alone (Cadiicibranchiata), further changes 

 ensue. In these the pulmonary arteries increase so much in 

 size that they ultimately divert all the blood from the branchiae, 

 and these organs, having fulfilled their temporary function, 

 become atrophied and disappear. The vessels which return 

 the aerated blood from the lungs (the pulmonary veins) increase 

 in size proportionately with their increased work, and ultimately 

 come to open into a second auricle formed at their point of 

 union. The heart, therefore, of the Amphibia in their adult 

 state consists si two auricles and a common ventricle. The right 

 auricle receives the venous blood from the body, and the left 

 receives the arterial blood from the lungs, and both empty their 

 contents into the single ventricle. As in Reptiles, therefore, 

 the ventricular cavity of the heart in adult Amphibians contains 

 a mixed fluid, partly venous and partly arterial, and from this 

 both the body and the lungs are supplied with blood. 



The larval Amphibians are furnished with a more or less 

 extensively developed caudal appendage or tail, which may or 

 may not be retained throughout life. In the so-called " tailed " 

 Amphibians, such as the Newts, the larval tail is permanently 

 retained (fig. 277) ; whereas in the "Tail-less " forms, such as 

 the Frogs (fig. 276), the tail is absorbed before maturity is 

 attained. In a few cases, it seems questionable if the larvae 

 possess branchiae, and there is no metamorphosis properly so 

 called, since the young animal resembles the adult in all 

 except size almost immediately after exclusion from the egg. 

 In one of these cases (Hylodes) the larval tail appears to offici- 

 ate as a breathing-organ, before emergence from the egg, but 

 is absorbed within the first day after hatching. In other 

 cases, again e.g., in Pipa and Nototrema though a metamor- 



