494 PISCES— FISHES. 



spiracles — open dorsally behind the eyes. Each contains 

 a rudimentary gill on the anterior wall, supported by a 

 spiracular cartilage. Through the spiracles water may enter 

 or leave the mouth. 



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

 titions, and with ventral apertures. The first is bounded 

 anteriorly by the hyoid arch, posteriorly by the first branchial 

 arch. The hyoid arch bears branchial filaments on its 

 posterior surface ; the first four branchial arches bear gill 

 filaments on both surfaces ; the fifth branchial arch bears 



Fig. 209. — Upper part of the dorsal aorta in the skate. 

 — After Monro. 

 d. a., Dorsal aorta: c, cceliac artery; in., superior mesenteric; 

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

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



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 

 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. 



