52 



ZOOLOGY. 



where it is purified by the action of the air contained in 

 the water. This artery, at its base, where it leaves the 

 ventricle, usually swells out into 

 . a cavity furnished with muscular 



walls, termed the bulbus arteri- 

 OSUS. Its use is to assist the 

 ventricle in propelling the blood 

 to the gills. Instead of passing 

 immediately to the heart, as in 

 the other Yertebrata, the aerated 

 blood is conveyed from the gills 

 through the system, and is 

 changed into venous blood be- 

 fore it again reaches the auricle. 



The blood is generally of the 

 same temperature as the water 

 in which the fish lives. The 

 majority of the corpuscles are red 

 (except in the lancelet), oval, and 

 nucleated. 



64. Respiration. All fishes 

 are furnished with an apparatus 

 termed gills, adapted for breath- 

 er, system of veins, conveying ing the air contained in water. 



the blood to the heart; &, auri- qrL~ fxil^^ A e J.T, 



cle; c, ventricle; d, branchial llie tollOWlXlg description of the 



artery, corresponding to the structure and the arrangements 



pulmonary artery of the higher ,-, .,, ,. 



vertebrates; h, aorta, convey- OI the gills applies Only to the 



Fia. 10. CIRCULATION OF A 

 FISH. 



fishes - In the 



ous fishes the arrangements are 

 considerably modified. The gills consist of cartilagin- 

 ous leaflets arranged either in single or double rows, 

 covered with a mucous membrane which is abundantly 

 supplied with minute blood-vessels. These leaflets are 

 supported on bony or cartilaginous arches which are 

 separated from one another by slits, and are connected 

 with the hyoid or tongue bone. The arches are situ- 

 ated in cavities on each side of the neck, termed the 

 "branchial chambers." The fish takes a quantity of 



