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MANUAL OF ZOOLOGY. 



ated. 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, collectively consti- 

 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. 139, 

 a, v), and the course of the circulation 

 is as follows : — The venous blood de- 

 rived from the liver and firom 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 («) 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, ^wr 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 



Fig. 159. — Diagram of the cir- 

 culation in a fish, a Auricle, 

 receiving venous blood from 

 the hoiy; v Ventricle; m 

 Bulbus arteriosus, at' the base 

 of the branchial artery ; « 

 Branchial artery, carrying the 

 venous blood to the gills {6 h) ; 

 c Aorta, carrying the arterial- 

 ised blood to all parts of the 

 body. 



