356 CONTRIBUTIONS TO THE NATURAL HISTORY 



The ambulacra are so closely connected with the appearance of the whole animal 

 and its movements, that we had better consider these first. As in all Beroid Medusse, 

 they constitute vertical rows of movable fringes, identical in structure in every respect, 

 as far as it has been traced, with those of Pleurobrachia, the difference consisting mainly 

 in their extent, the pairs which run along the anterior and posterior extremities of the 

 body, and extend upon the two large lobes, being by far the longest, and also somewhat 

 wider, their flapping combs tapering gradually towards the anal area, so that the ambu- 

 lacra terminate in points at some distance from the central black speck. This is equally 

 the case with the two pairs of lateral ambulacra, which, however, extend somewhat farther 

 inwards, the tip of the eight ambulacra encircling an oblong area, the longer axis of 

 which is in the same plane as the circumscribed anal area, which extends, however, far 

 beyond forwards and backwards, between the rows of combs of the anterior and posterior 

 pairs of ambulacra. Another peculiarity which distinguishes Bolina from Pleurobrachia, 

 in the arrangement of this extremity of the body, is the circumstance that the body here 

 is not simply rounded, but somewhat depressed along the longitudinal axis ; so much so, 

 that the two sides bulge sensibly above the level of the central speck, while the anterior 

 and posterior extremities are on a level with it. The consequence of this form of the 

 main gelatinous mass of the body is, that the upper extremity of the anterior and posterior 

 ambulacra runs almost straight, while in the lateral ambulacra it is arched over the two 

 rounded parallel ridges which inclose the circumscribed anal area. It is easily as- 

 certained, that eight small tubes, similar to those observed in Pleurobrachia, extend be- 

 yond the upper extremity of the ambulacra towards the central black speck, or rather 

 towards the bulb under it, and that they are the prolongation of the vertical ambulacra! 

 tubes of the chymiferous system. Below the main bulk of the body, along its sides, 

 the ambulacra gradually taper also towards their lower extremity, and as soon as they 

 reach the height of the dilatation of the lobes, the locomotive combs disappear, and the 

 vascular tubes which accompany them can alone be traced farther. In the lateral am- 

 bulacra, however, these combs are reduced much sooner. The rows themselves taper 

 also sooner, and terminate at the base of the small lateral lobes near their inner margin, 

 for a considerable length above the lower extremity of the ambulacra of the large lobes. 

 In the small lobes we trace, also, a narrow prolongation of the chymiferous tubes, 

 which extend beyond the locomotive fringes. The course of these narrow tubes upon 

 the lobes is very difficult to trace, and their connection with each other and with the 

 central chymiferous cavity has been entirely overlooked by former observers, though 

 there are, in the figures of Bolina elegans published by Mertens, indications that he 

 noticed the outline of their convolutions. 1 shall first trace the course of these tubes 



