94 



and carotid arteries can be dissected ])y removing the 

 greater portions of both opercnla as indicated above, and 

 then cutting away the gills, having previously divided the 

 efferent vessels as far away from their attachments to the 

 epibranchial arteries as possible. The head is ihen placed 

 ventral side uppermost, and held in that position by hooks. 

 The ventral portion of the parasphenoid must be removed 

 in order to study the course of the internal carotid trunks. 



The first and 2nd eiferent branchial vessels 

 [Ef. Br. 1, Ef. Br. 2) open separately into the epi- 

 branchial trunk. The 3rd and 4th {Ef. Br. 3, Ef. Br. 4) 

 unite together to form a short trunk. On the left side 

 this opens into the left epibranchial, on the right it opens 

 into the coeliaco-mesenteric artery [A. cm.). It may 

 appear, however, that the coeliaco-mesenteric, instead of 

 taking origin from the epibranchial as represented in the 

 figure, springs from the common trunk of 3rd and 4th 

 efferent branchials. 



Immediately behind the union of <he epibranchial 

 trunks the subclavian arteries are given off from the dorsal 

 aorta. Each of these vessels [A. scl.) passes out trans- 

 versely, then bends down ventrally and runs along the 

 internal surface of the corresponding pectoral girdle, the 

 muscles of which it supplies. Behind these vessels trans- 

 verse arteries are given off from the dorsal aorta on either 

 side, one to each segment. These vessels are not repre- 

 sented in the figure. 



Several arteries leave the epibranchials to supply the 

 muscles of the gill arches with blood. The most impor- 

 tant of these is a paired vessel which leaves the epi- 

 branchial immediately anterior to the place of entrance of 

 the 2nd efferent vessel. It passes at first dorsally, then 

 backwards and downwards over the 3rd and beneath the 

 4th efferent trunks. Approaching the middle line it runs 



