524 



PISCES FISHES. 



there are four and a half gills in all. There is no operculum 

 or gill cover. 



Circulatory system. --The impure blood from the body 

 enters the heart by a bow-shaped sinus venosus, opening 

 into a large thin-walled auricle. Thence through a bivalved 

 aperture the blood passes into the smaller muscular ventricle, 

 and from this it is driven through a contractile conus 

 arteriosus, with three longitudinal rows of five valves, into 

 the ventral aorta. 



FIG. 252. Heart and adjacent vessels of skate. In part 



after Monro. 



:. Ventricle; c.a., conus arteriosus; /.?'., posterior innominate; 

 y.rt., ventral aorta; ./., anterior innominate; T/i., thyroid; 

 >/i., mouth ; a., auricle ; s.v., sinus venosus ; s.c., precaval 

 sinus or sinus of Cuvier ; h.s. , hepatic sinus ; j. , jugular ; br., 

 brachials ; cd., cardinal ; t'pg., epigastric. 



The ventral aorta gives off a pair of posterior innominate arteries, 

 which take blood to the three posterior gills, and a pair of anterior 

 innominate arteries, which supply the anterior gill and the hyoid half 

 gill on each side. 



The purified blood passes from each half gill by an efferent branchial 

 artery. To begin with, there are nine of these on each side, but by 

 union they are reduced first to four and then to three efferent trunks, 

 which combine to form the dorsal aorta. 



From the efferent branchial of the hyoid arch a carotid arises, which 

 divides into internal and external branches supplying the brain and 

 head. The two internal carotids unite, and pass through a small hole 



