408 MANUAL OF ZOOLOGY. 



dosiren. 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 

 commences to be aerial, the pulmonary arteries increase pro- 

 portionately in size, and more and more blood is gradually 

 diverted from the gills and carried to the lungSj so that the 

 branchiae suffer a proportionate diminution in size. In those 

 Amphibians in which branchiae are permanently retained {Per- 

 ennibranchiatd), 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 {Caducibranchiata), further changes ensue. In 

 these the pulmonary arteries increase so much in size that they 

 ultimately divert all the blood from the branchise, and these or- 

 gans, 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 of two 

 auricles and a common ventricle. The right auricle receives 

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

 arterial blood from their lungs, and both empty their contents 

 into the single ventricle. As in Reptiles, therefore, the ventri- 

 cular 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. 



As regards the digestive system of the Amphibia there is little 

 to say, except that the rectum opens, as it does in Reptiles, 

 into a common chamber or " cloaca," into which are also dis- 

 charged the secretions of the kidneys and generative organs. 

 A liver, gall-bladder, spleen, and pancreas are always present. 

 Singular pulsating cavities, belonging to the lymphatic system, 

 and known as " lymph-hearts," are also present in the higher 

 Amphibians. 



CHAPTER LXI. 

 ORDERS OF AMPHIBIA. 



The Amphibia are usually divided by modem writers into four 

 orders, the old order Lepidota, comprising the Lepidosiren, 



