94 
and carotid arteries can be dissected by removing the 
greater portions of both opercula 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 then 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 _ efferent branchial vessels 
(Ef. Br. 1, Hf. Br. 2) open separately into the epi- 
branchial trunk. The 3rd and 4th (Hf. Br. 3, Hf. 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 cceliaco-mesenteric artery (A. cm.). It may 
appear, however, that the cceliaco-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 the epibranchial 
trunks the subclavian arteries are given off from the dorsal 
aorta. Hach 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 iine it runs 
