36 BRITISH TUNICATA. 



little distance along the intestinal tube, and then, leaving- it, 

 penetrates the mantle in the dorsal region, and goes associated 

 with the third branch from the dorsal extremity of the heart, 

 already described, to ramify in the test. Thus originates the 

 double vessel that carries the nourishing fluid to and from 

 that envelope or tunic. The stem that goes towards the 

 oesophagus passes along by the side of the lower extremity of 

 the intestine, and, just before reaching the anus, turns aside 

 to join a large vessel that extends along the ventral margin 

 from one end to the other of the branchial sac (PI. Ill, figs. 

 2 and 3). This, which is the great ventral branchial channel, 

 is the dorsal sinus of Milne-Edwards. It communicates with 

 the ventral extremities of the transverse branchial channels ; 

 and its lower extremity bifurcates, a branch passing on each 

 side of the mantle. 



The two great branchial channels, the dorsal and ventral, 

 communicate with each other, as we have already seen, by 

 the numerous transverse channels of the branchial sac ; they 

 likewise intercommunicate above through a sufficiently obvious 

 channel that encircles the entrance of the sac, immediately 

 above the vascular network, and just below the anterior cord, 

 afterwards described ; the lower extremities also appear to 

 communicate with each other by a much-constricted channel. 

 Thus the circle of the blood-apparatus would seem at first 

 sight to be complete; and as the opposite ends of the heart 

 operate upon the two great branchial channels respectively, 

 and as the blood oscillates first in one direction and then in 

 the other, we might look upon the mechanism as sufficiently 

 perfect for all the purposes of the circulation. 



But something is still wanting, as is evident when we refer 

 to the fact that the influence of the heart is chiefly confined 

 to the branchial organ, the visceral plexus, and the vascular 

 system of the test. The pallial plexus of the right side is 

 certainly in connexion, as we have seen, with the visceral 

 plexus of that side; but, so far as our examination extends, 

 the plexus of the left side of the mantle is connected with the 

 general system through the minute network of the pallial 

 plexus only. It is obvious that the blood-current would be 

 feeble in these parts, if the whole of the mechanism is now 

 before us. And, moreover, it would be most languid in the 

 left pallial plexus in that very portion of the mantle, in 

 fact, that is most amply supplied with muscular fibres, and 

 which, being comparatively free, has undoubtedly the greatest 

 mobility. Indeed, unless some additional means exist to aid 

 the circulation, engorgement of the blood-channels must in- 



