OF THE ACALEPH^ OF NORTH AMERICA. 357 



upon the larger lobes. As long as the tubes follow a straight course in the prolongation 

 of the anterior and posterior ambulacra, they remain at the surface of the gelatinous mass, 

 covered only by the epidermis beyond the ambulacra themselves. But as soon as they 

 diverge towards the sides, and approach the lower margin, where they bend to take again 

 an inward course, they penetrate deeper and deeper into the substance, across the whole 

 thickness of the lobe itself, till they reappear upon its inner surface, where they are nearest 

 to each other, then rise, diverging again, and following almost exactly the outline of the 

 lateral margins of the lobes, along which they ascend towards their bases, rising higher than 

 the lower termination of the ambulacral combs, as high as the bases of the short ambulacra, 

 then converge again, bend downwards, and in a sinuous, winding course descend again 

 towards the lower margin of the lobe, remaining nevertheless above the first lower comb, 

 and converging from the two sides immediately in the medial line ; so that there is a direct 

 communication between the right and left ambulacral tube of the anterior and posterior 

 pairs, passing in their course from the margin of the outer surface to the middle of the in- 

 ner surface, first descending, then rising, then descending again in undulating lines, until 

 they meet to form a central continuous channel. Such a connection between any of the 

 tubes on the oral side of the ambulacral tubes has not been seen in Pleurobrachia ; but, as 

 I mentioned when describing the structure of that genus, I should not be at all surprised 

 if it should be finally found that there is such a connection around the mouth, for there 

 are vascular tubes following the walls of the digestive cavity which reach the margin of 

 the mouth, along the folds of which I have repeatedly thought I saw something like 

 tubes and a current ; but though I could never satisfy myself completely upon this 

 point, and though I have always been unable to trace the ambulacral tubes in Pleuro- 

 brachia beyond the oral termination of the locomotive combs themselves, the existence 

 of such minute tubes in the substance about the mouth of Pleurobrachia is rendered 

 very probable, since it is seen here that extensive, and, indeed, even large vessels 

 wind their course in the lobes, and we shall presently see that there is a further com- 

 munication between those of the larger lobes and those of the smaller lobes, as well as 

 between them and those around the mouth, in its margin and in the tentacular appa- 

 ratus, so that the circulation of fluid from one summit towards the other, and the re- 

 curring movement in the opposite direction, are fairly established by a direct course of 

 tubes in Bolina, though the movement of fluid in these tubes takes place in a backward 

 and forward direction, which therefore does not imply the possibility of a complete want 

 of such communication in Pleurobrachia, where the circulation would be reduced to a 

 forward and backward movement in the main trunk of the system. The ambulacra of 

 the sides are reduced to a simple chymiferous tube as soon as they reach the base of the 



