544 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 
Fic. 286.—Upper part of the dorsal aorta in the skate. 
—After Monro. 
@.a., Dorsal aorta; ¢., coeliac artery; #., superior mesenteric; 
s.cl., subclavian ; ¢.d., efferent branchial vessels, three formed 
from the union of nine; w., 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. 
