172 



A HISTORY OF FISHES 



taken up by the walls of the canal, whence it passes into the 

 blood. The elaborate intercommunicating system of arteries 

 and veins, by means of which the nourishment is carried to 

 the hungry cells in every part of the body, is known as the 

 vascular system, and its principal features may now be 

 described. 



The essential organ of this system is the heart, in fishes a 

 stout, muscular pump of comparatively simple design and small 

 size, situated in a chamber known as the pericardium, which 

 generally lies below the pharynx and immediately behind the 

 gills (Figs. 68, 70), although in some of the Eels (Apodes) it is 

 placed some way behind the head. The heart consists of four 

 parts : a chamber or sinus into which the veins open ; an auricle 



brairv 



Fig. 70- 



Vascular system of a typical fish. (After Grote, Vogt and Hofer.) Arteries- 

 white : veins — black. 



or atrium; a deep red, thick- walled ventricle, the rhythmical 

 contractions of which serve to drive the blood round the body ; 

 and a bulb at the base of the main artery carrying the blood to 

 the gills. In the Selachians and more generalised Bony Fishes 

 this bulb is muscular, provided with special valves, and pulsates 

 like the ventricle, but in all higher forms these structures 

 degenerate and the walls are incapable of contraction. It is of 

 special interest to note that in the air-breathing Lung-fishes 

 there are traces of the beginnings of a division of the auricle 

 and ventricle into two by the development of a partition, thus 

 foreshadowing the four-chambered heart of the higher verte- 

 brates. From the ventricle the blood passes through the bulb 

 into the great ventral aorta, from both sides of which branches 

 carry it to the fine vessels in the gills, where the respiratory 



