360 CONTRIBUTIONS TO THE NATURAL HISTORY 



proceed to an investigation of their connection with the interior 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 their peculiar termination and connections in the peripheric 

 lobes. The centre of the chymiferous system constitutes in Bolina (Plate VIII, Fig. 2 

 and 9), as well as in Pleurobrachia, a vertical hollow axis, extending from the centre of 

 the anal area down to the sides of the digestive cavity, being, however, not so spa- 

 cious as in Pleurobrachia, while the digestive cavity itself is larger, extending nearer the 

 central black speck, so that the funnel which branches towards the circumscribed area 

 below the tubercle of the black speck is shorter, the main cavity from which the main 

 trunk to the ambulacra arises being much narrower, and the tubes extending towards 

 the margin of the mouth, along the lateral walls of the digestive cavity, being 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 ambulacral branches, which are more strongly bent upwards, 

 instead of stretching horizontally across the gelatinous mass. Owing to the lesser de- 

 velopment of the central cavity of this system, and the difficulty of preserving alive 

 these animals after injecting colored liquid into the chymiferous sac, I have not succeeded 

 in discovering a regular contraction between the right and left sides of the system. It 

 may be, also, that the transverse diameter, being so much shorter in this genus than in 

 Pleurobrachia, and the means of establishing a retrograde current from the periphery much 

 more extensive, the circulation takes place through alternate dilatations and contractions 

 of the whole body, causing an injection of the fluid in all directions, rather than by an 

 alternate passage from one side into another ; and for various reasons of analogy I incline 

 rather to this view. In the Discoid Medusae, we have an absolutely radiating circu- 

 lation, and a movement simply to and fro from the centre to the periphery, and back 

 throughout the whole system. In Pleurobrachia there is an alternation between right 

 and left, with a prominent circulation to and fro. In Bolina, there is also a bilateral 

 symmetry, but the radiating circulation seems to be recurring in itself through a com- 

 plete circle, which arrangement would already approximate the Beroid Medusae of the 

 genus Bolina to the type of Echinoderms, though in a lower condition of the circu- 

 lating system. 



Whatever may be the value of these suggestions, so much is plain, — that the digestive 

 cavity constitutes a capacious sac with a longitudinal mouth, the fissure of which opens 

 in the same plane with the anal area, precisely as in Pleurobrachia, in a gelatinous, oblong 

 disk, extending with its longer diameter flat between the anterior and posterior lobes 

 (Plate VIII. Fig. 6). This disk is entirely surrounded by the large lobes when they are 



