VASCULAR SYSTEM OF SKATE 603 



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 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 



Fig. 340. — Upper part of the dorsal aorta in the skate. 



— After Monro. 



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

 s.cL, subclavian ; e.b., efferent branchial vessels, three formed 

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



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

 valved 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. 



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 



