VERTEBRATA : FISHES. 



473 



ba 



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 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 " branchi- 

 ostegal membrane." 



The heart of fishes is, pro- 

 perly speaking, a branchial 

 or respiratory heart. It con-i 

 sists of two cavities, an aur- 

 icle and a ventricle (fig. 257), 

 and the course of the circu- 

 lation is as follows : The ven- 

 ous blood derived from the 

 liver and from the body gen- 

 erally is poured by the vena 

 cava into the auricle (ait), 

 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 arterios- 

 us " (ab). The arterial bulb 

 is sometimes covered with a 

 special coat of striated mus- 

 cular fibres, and may be pro- 

 vided 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 (b) to the 

 gills, through which it is distributed 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 



Fig. 257. Diagram of the circulatory system 

 in a Fish, the vessels containing venous blood 

 being longitudinally shaded, and those con- 

 taining arterial blood being cross-shaded, vc 

 Vena cava ; vp Vena portse ; au Auricle ; v 

 Ventricle ; ab Bulbus arteriosus ; b Branchial 

 artery ; ba One of the divisions of the bran- 

 chial artery going to the gills, from which 

 the c 



esponding branchial 

 veins, by the union of which the subvertebral 



proceeds one of t 

 veins, by the uni 

 aorta (so) is formed ; z Intestine ; k Kidney. 



