RAIA. 317 



The heart lies in the pericardium. It has two chambers 



a thick-walled ventricle and a larger thin-walled auricle lying 



dorsal to it. The ventricle leads forwards 



' out of the pericardium as the cojius arteriosus 



containing valves, beyond which it is continued 



as the branchial artery.'^'' The auricle receives blood from 



a thin-walled triangular sinus venosus formed from the 



swollen termination of the main veins. 



The branchial artery gives off a pair of posterior in- 

 nominatesy which trifurcate into three afferent branchials 

 supplying the three posterior pairs of gills. The branchial 

 artery runs forward and terminates just behind the lower jaw, 

 near a small ductless thyroid gland. Here it diverges into two 

 anterior innominates^ each of which bifurcates into two afferent 

 branchials supplying the first two pairs of gills : this comprises 

 the afferent branchial system. On contraction of the ven- 

 tricle of the heart the blood passes forward to the gills, 

 hence the skate's heart is purely respiratory. If the coracoid 

 bar be now carefully removed, the sinus venosus will be seen 

 to run downwards and outwards on each side to the pre- 

 caval sinus, which communicates with a spacious hepatic 

 sinus in connection with the liver and receives a jugular 

 vein from the head, a lateral vein (formed of a pelvic from 

 the pelvic fin and a brachial from the pectoral fin) and 

 a cardinal vein from the posterior part of the body and 

 kidneys. A median caudal vein from the tail diverges into 

 a pair of renal portals to the kidneys, in which the veins 

 break up into capillaries. A skate has therefore a renal 

 portal system as well as a hepatic portal. 



If the ventral w^all of the pharynx and the skin of the roof 

 of the pharynx be removed, the efferent branchial system is 

 exposed (Plate II). It consists of five efferent branchials 

 leading from the gills towards the middle line and back- 

 wards. The two first unite into one, as also do the two 

 last ; hence three arteries are produced which then unite to 

 form the dorsal aorta. From the first efferent branchial on 

 each side runs forward a carotid, dividing into internal 

 carotid to the brain and external carotid to the head. The 

 dorsal aorta gives off paired subclavians to the pectoral fins, 

 and is continued along the dorsal line under the vertebral 



* Often termed the Ventral Aorta. 



