174 



THE SPINY DOGFISH -THE SKATE 



remaining three pairs of efferent vessels are similar and form loops round the 

 second, third and fourth gill-clefts. There is a half-loop or single vessel on the 

 anterior side only of the fifth gill-cleft, as this cleft has no gill on its posterior side. 

 This vessel is joined at its middle to the loop round the fourth gill-cleft. The loops 

 are all connected with each other at about the middle of their lengths by short hori- 

 zontal vessels passing across the gill-arches. From the inner anterior end of the 

 first loop of each side a common carotid artery arises. These carotids curve for- 

 wards and inwards along the inner surface of the fioor of the skull, each dividing 

 into an external and an internal carotid artery. The former runs forwards to 

 supply the upper jaw and snout. The latter passes inwards to the mid-ventral line, 

 where the two internal carotids cross each other, unite with the hyoidean artery, and 

 supply the brain. The dorsal aorta is prolonged anteriorly in the middle line ; 

 and it bifurcates to form the pair of posterior carotid arteries, each of which joins a 

 common carotid artery just where that vessel divides into external and internal 

 carotids. From the middle of the anterior half of the first loop, and in line with the 

 short horizontal vessels, the hyoidean artery arises : it runs forwards dorsal to the 

 hyomandibular cartilage, passes to the spiracle to supply the pseudobranch, and 

 enters the cranium by a small foramen in the inner wall of the orbit, andjoiks the 

 internal carotid.] 



[In the Skate (Raia) the efferent vessels form loops round the first four gill-clefts, 

 just as in Scyllium. After leaving the clefts, the first and second efferent branchial 

 vessels unite into one, and thus only three pairs of main efferent vessels join to form 

 the dorsal aorta. The fifth gill-cleft has a small vessel on its anterior side, this 

 vessel being connected by a short horizontal vessel with the loop round the fourth 

 gill-cleft. The first efferent vessel gives off a ■vertebral artery, similar to that in 

 Acanthias, passing to the brain and spinal cord. There is only a small hyoidean 

 artery, which passes to the side of the hyomandibular cartilage and does not take a 

 course as in Scyllium. The dorsal aorta is not produced anteriorly as in Scyllium, 

 but resembles that of Acanthias. The common carotid artery resembles that of 

 Acanthias and Scyllium. The dorsal aorta gives rise to a pair of subclavian 

 arteries to the pectoral fins.] 



Subclavian 

 "Dorsal aorta 



External carotid 

 -■Hyoidean 

 ^-Spiracle 

 XvommoD carotid 

 ' hyomandibular 

 cartilage 



;Gills 



Subclavian 

 Dorsal aorta 



Fiij. 59.— The Efferent Branchial or Dorsal Arterial System (A.) of the Skate 

 (Raia) ; and (B.) of the Dogfish (Scyllium). 



