Chap. III. GENUS BOLINA. 261 



motive flappers of the CtenophorfB, in whicli each fringe is a whole cell, do not 

 necessarily appear as a specific type of structure, but may constitute a natural link 

 between more complicated organs with distinct muscles and the simplest fringes 

 of structural cells. I entertain now so little doubt respecting such transitions, that 

 I have not hesitated, throughout this description, to consider the rows of vertical 

 locomotive fringes as true amlndacra; though there is as great a difference between 

 them and the ambulacra of Echinoderms, as there is between them and simple 

 vibi-atory cilia. We are, in fact, led to recognize, through the whole type of 

 Radiata, a natural gradation in the structure of the organs through which currents 

 of water are produced around the body, from tlie simplest combinations in Polypi 

 to the most complicated apparatus in Echinoderms. In Polypi we have only vibra- 

 tile cilia arising from structural cells over extensive surfaces of the whole body, 

 while in Beroid Medusae there are, in addition to such cilia, peculiar rows of fringes, 

 made up of special cells, which move by their own contraction, and in Echinoderms 

 each fringe, in the shape of an independent ambulacral tube, assumes as great a 

 structural complication as the whole system in Acaleph*. The ambulacral tubes 

 in Echinoderms generally, indeed, seem to me to bear the same relation to the 

 aquiferous system with its vesicles in Star-fishes, and to the true ambulacral gills 

 in Echini, as the fringes of the locomotive combs with their contractile base bear 

 to the ambulacral chymiferous tubes in Ctenophoraj. 



If, from this review of the superficial ramifications of the chymiferous tubes, 

 we proceed to an investigation of their connection with the internal stems and the 

 central cavity of the whole system, we find a very close resemblance in their 

 arrangement to what has already been noticed in the genus Pleurobrachia, — the 

 chief difference between the two genera consisting in the peculiar termination and 

 connections of these tuljes in the lobes of Bolina. The centre of the chymiferous 

 system constitutes in Bolina, as in Pleurobrachia, a vertical hollow axis, extending 

 from the centre of the abactinal area to the abactinal opening of the digestive 

 cavity, upon the sides of which it gives off two coeliac tubes extending as far as 

 the mouth. These tubes, however, are not so wide as in Pleurobrachia, while the 

 digestive cavity itself is larger, extending nearer the central black speck ; so that 

 the funnel, which Ijranches toward the circumscribed area, as in Pleurobrachia, is 

 shorter, the main cavity from which the main trunks to the ambulacra arise much 

 narrower, and the tubes extending toward the margin of the mouth along the 

 lateral walls of the digestive cavity in the same proportion longer. But the general 

 arrangement is identical. The differences exist only in the proportional development 

 of the different parts of the whole system, as also in the curve of the main trunks 

 of the ambulacral branches, which are more strongly bent upward, instead of 

 stretching horizontally across the body. Owing to the lesser development of the 



