264 CTENOPIIOR^. Part IT. 



the impulse of the liquid pressed into the aniljulacral tubes is chiefly in one 

 direction, the Iji-anches from the main cavity meeting the ambulacra near their 

 upper termination, and not at about half their height, as in Pleurobrachia. So 

 that the chief, and, I may say, almost the only constant current, is from the 

 abactinal side of the body toward the actinal region, along the sides, following 

 the ambulacra and all the sinuosities of their tubes in their lower course through 

 the great lobes, as well as through the lateral auricles ; and a comparatively small 

 portion of fluid flows through the comparatively short abactinal end of the ambu- 

 lacra! lobes toward the circumscribed area. The ambulacral tubes therefore are 

 not the direct prolongation of the eight forks of the main branches of this system, 

 any more than in Pleuroljrachia, Ijut form an angle w ith these forks ; and there 

 is an aljactinal prolongation of the ambvdacral tubes, as well as a main actinal 

 branch, above and below the insertion of the fork from the main trunks. I there- 

 fore question the accuracy of those illustrations of the ambulacral tubes which 

 represent them as the direct prolongation of the forks arising from the main trunks.^ 

 The antagonism between the main currents is thus between the upper and the 

 lower side of the body, and by no means between the right and the left side, as iu 

 Pleurobrachia. Whether, however, the retrograde current runs exclusively backward 

 through the same tubes in which it has moved onward, or whether the winding 

 course of the narrow tubes in the lobes constitutes a kind of ca2)illary system, 

 thi'ough which the liquid may pass from one side of the ambulacral tubes into 

 the other, I am unable to decide. But I cannot help thinking that this long, 

 winding course of the ambulacral tubes upon the inner surflxcc of the large lobes 

 and along the margins of the auricles and of the mouth contributes to a more 

 extensive aeration of the chyme in circulation, than the straighter course in the 

 wider vessels of the whole system in Pleurobrachia. Perhaps the more active 

 alternate contractions in Pleurobrachia compensate, by their quicker movements, for 

 the absence of ramifications of the tubes which are so extensive in Bolina. 



The tentacular tubes, which run parallel with and upon the sides of the cceliac 

 tubes, enlarge near the middle of the lateral margins of the mouth into a small, 

 bulb-like dilatation, from which a bunch of tentacles may be jirojected or retracted. 

 But this bulb is by no means so complicated as the tentacular sac of Pleuro- 

 brachia, There is no flat disk at the base of the tentacles, no deep socket into 



^ Conip. Milne-Edwards, Ann. Sc. Nat. 2dc papers differ from those of tlie generii I have ex- 



st'r. vol. IG, PI. III. and 4e sc'r. vol. 7, PI. XIV.; amincd, in such a way as the figures suggest, this 



Will, Hora; tergestina-, PI. I. ; and GEGENnAii:, would constitute a remarkable, and to me unexpected, 



Archiv fur Naturg. vol. 22, PI. VII. Should the ditferenee between them ; but the letter-press gives 



ambulacral tubes of the genera described iu these no details upon this point. 



