CIRCULATORY SYSTEM. 489 



branchial arch bears none. Each of the first four branchial 

 arches bears a half gill on each side; thus, including the 

 gill filaments borne on the posterior side of the hyoid, there 

 are four and a half gills. The absence of an operculum or 

 gill cover is obvious. The gills are outgrowths from the 

 wall of the gut, and therefore endodermic. 



Circulatory System. 



The impure blood from the body enters the heart by a 

 bow-shaped sinus venosus, which leads into a large thin- 

 walled auricle. Thence through a bivalved aperture the 



Fig. 



i5i. — Heart and adjacent vessels of Skate, 

 after Monro.) 



(In part 



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

 v.a.^ ventral aorta; a.i.^ anterior innominate; Th., thyroid; in., 

 mouth ; a., auricle ; s.t., sinus venosus ; s.c, precaval sinus or sinus 

 of Cuvler ; Jl.s.. hepatic sinus; j.. jugular; br., brachials; cd., car- 

 dinal ; f/jf. , epigastric. 



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 



