350 



MANUAL OF ZOOLOGY. 



tuting the " gill-cover " or " operculum ; " and the gill-covers 

 are finally completed by a variable number of bony spines 

 the " branchiostegal rays" which articulate with the hyoid 

 arch, and support a membrane the 

 " branchiostegal membrane." 



The heart of Fishes is, properly 

 speaking, a branchial or respiratory 

 heart. It consists of two cavities, 

 an auricle and a ventricle (fig. 131, 

 a, v), and the course of the circulation 

 is as follows : The venous blood de- 

 rived from the liver and from the body 

 generally is poured by the vena cava 

 into the auricle (a), and from this it is 

 propelled into the ventricle (v). From 

 the ventricle arises a single aortic 

 arch (the right), and the base of this is 

 usually dilated into a cavity or sinus, 

 called the "bulbus arteriosus " (m). 

 The arterial bulb is sometimes covered 

 with a special coat of striated muscular 

 fibres, and is provided with several 

 transverse rows of valves. In these 

 cases, the bulbus acts as a kind of 

 continuation of the ventricle, being 

 capable of rhythmical contractions. 

 The blood is driven by the ventricle 

 through the branchial artery (ri) to 

 the gills, through which it is dis- 

 tributed by means of the branchial 

 vessels, the number of which varies 

 (there are three on each side in a few 

 fishes, four in most of the bony fishes, 

 five in the Skates and Sharks, and six or 

 seven in the Lampreys). The aerated 

 blood which has passed through the gills is not returned to the 

 heart, but is driven from the branchiae through all parts of 

 the body; the propulsive force necessary for this being derived 

 chiefly from the heart, assisted by the contractions of the vol- 

 untary muscles. In some fishes (as in the Eel) the return of 

 the blood to the heart is assisted by a rhythmically contractile 

 dilatation of the caudal vein. The essential peculiarity, then, 

 of the circulation of fishes depends upon this that the arterial- 

 ised blood returned from the gills is propelled through the sys- 

 temic vessels of the body, without being sent back to the heart. 



Fig. 131. Diagram of the cir- 

 culation in a fish. a. Auricle, 

 receiving venous blood from 

 the body ; v Ventricle ; in 

 Bulbus arteriosus, at the base 

 of the branchial artery; n 

 Branchial artery, carrying the 

 venous blood to the gills (bb~); 

 c Aorta, carrying the arterial- 

 ised blood to all parts of the 

 body. 



