362 CONTRIBUTIONS TO THE NATURAL HISTORY 



respoiiding posterior ones, owing, no doubt, to the lateral compression of the bodj. And 

 from the wider space between the two main branches of one side arise again the ver- 

 tical tubes, which descend along the digestive cavity, towards the base of the tentacles. 



Again, the four main branches of ambulacral tubes, instead of stretching horizontally 

 towards the ambulacra, are bent upwards, and then divide each into two branches, to 

 provide the eight ambulacra with as many vertical ambulacral tubes. The consequence 

 of this arrangement is, that the impulse of the liquid pressed into the ambulacral tubes 

 is chiefly in one direction, the branches 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 current, and, I may say, almost the only constant current, is downwards 

 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 very small portion of fluid flows towards the upper centre, through- the 

 very thin tubes extending from the upper summit of the ambulacra towards the anal 

 area. The main antagonism between the currents, therefore, is between the upper 

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

 and vice versa. Whether, however, the retrograde movement takes place upwards, 

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

 of the narrow tubes in the lobes constitutes a kind of capillary system, through which 

 the liquid passes 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 surface of the large lobes, and along the margins of the auricles 

 and 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. But per- 

 haps the more active alternate contractions in Pleurobrachia compensate by their quicker 

 movements for the deficient ramification of the tubes themselves, which is so extensive 

 in Bolina. 



Of the narrow tubes about the anal area, I shall have to speak again presently. I 

 shall only add here, that the vertical tubes upon the sides of the digestive cavity 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 issued or retracted. But this bulb is by no 

 means so complicated as the tentacular sac of Pleurobrachia. There is no flat disk 

 with elastic springs, but simply two narrow tubes arising from the main cavity, a little 

 outside of the tubes of the digestive cavity, and following its course to the tentacular 

 bulb. As, on account of the lateral pressure, the tubes of the digestive cavity and 

 those of the tentacular bull) are brought into close contact, they appear at first sight 



