54* 



PISCES FISHES. 



spiracular cartilage. Through the spiracles water may enter 

 or leave the mouth. 



There are other five pairs of gill-clefts, separated by com- 

 plete partitions (Elasmobranch), and with ventral apertures. 

 The first is bounded anteriorly by the hyoid arch, posteriorly 

 by the first branchial arch. The hyoid bears branchial 

 filaments on its posterior surface ; the first four branchials 

 bear gill filaments on both surfaces ; the fifth branchial 

 bears none. Each set of branchial filaments is called a half- 

 gill ; and as the first four branchial arches bear a half-gill on 



FIG. 286. Upper part of the dorsal aorta in the skate. 

 After Monro. 



d.a. t Dorsal aorta; c., coeliac artery; m., superior mesenteric ; 

 s.cl.y subclavian ; e.&., efferent branchial vessels, three formed 

 from the union of nine ; &., vertebral ; c., carotid. 



each side, and the hyoid arch a half on its posterior surface, 

 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. 



