396 



MANUAL OF ZOOLOGY. 



it is oxygenated. The water is constantly taken in at the 

 mouth by a movement analogous to swallowing, and it gains 

 admission to the branchial chambers by means of a series of 

 clefts or slits, the " branchial fissures," which are situated on 

 both sides of the pharynx. Having passed over the gills, the 

 deoxygenated water makes its escape posteriorly by an aperture 

 called the " gill-slit " or " opercular aperture," one of which is 

 situated on each side of the neck. As we have seen before, 

 the gill-slit is closed in front by a chain of flat bones collec- 

 tively constituting 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. 168, 

 a, 7/), 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 (7;). 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" (;). 

 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 () to 

 th * &**, through which it is dis- 

 tributcd by m ean s of the branchial 

 ves sels, the number of which varies 



venous blood to the gills (b b)\ (there are three on each side in a 

 [se"d 0rt bioo a d rr t y o n a1i *? ^w fishes, four in most of the bony 

 bod y- 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 



Bulbus arteriosus, at the base 



