AND PHTSIOLOGT OP THE TUKICATA. 323 



The two great branchial channels, the dorsal and ventral, com- 

 municate with each other, as we have already seen, by the nume- 

 rous transverse channels of the branchial sac ; they likewise in- 

 tercommunicate 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-ap- 

 paratus would seem at first sight to be complete ; and as the oppo- 

 site ends of the heart operate upon the two great branchial chan- 

 nels respectively, and as the blood oscillates first in one direction 

 and then in the other, we might look upon the mechanism as suf- 

 ficiently 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 tlie 

 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 mecha- 

 nism 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 inevitably take place in 

 the pallial plexus when the heart pulsates in the direction of the vis- 

 cera ; and when its action is reversed, exhaustion would ensue in 

 this portion of the system. 



IS'ow, though the branchial sac is attached to the walls of the 

 pallial chamber in front and behind and along by the dorsal mar- 

 gin, it is necessary that the lateral or reticulated portions of the 

 organ should be suspended, and in such a manner as to leave a 

 considerable space between the sac and the pallial walls. Conse- 

 quently a number of suspenders are provided, which, while they 

 retain the branchial sac in its proper position, allow the required 

 space. These suspenders are in the form of cylindrical bands or 

 ties, and are contractile ; they pass from the transverse branchial 

 channels and from the great ventral channel to the walls of the 

 pallial or respiratory chamber ; they are hollow or tubular, and 



