262 CTENOPIIORiE, Part II. 



central cavity of this system, and the difficnlty of preserving these animals alive 

 after injecting colored liquid into the chyniiierous sac, I have not succeeded in 

 discovering a regular alternation hetween the contractions of the right and left 

 sides of the s^'stem. It may 1)e also, that, the transverse diameter being so mi;ch 

 shorter in this genus than in Pleiu'oljrachia and the means of establishing a retro- 

 grade current from the periphery very 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 to 

 another ; and, for various reasons based upon analogy, I incline to this view. In the 

 Discoid Medusa) we have an absolutely radiating circulation, 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 sym- 

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

 plete circle in the loljes and around the mouth, which arrangement would already 

 approximate the Beroid Medusa) of the genus Bolina to the type of Echinoderms, 

 though in a lower condition of the circulatory 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 fi.ssure 

 of which opens in the same plane with the circumscribed area precisely as in 

 Pleurobrachia, in an oblong disk, extending with its longer diameter flat between 

 the anterior and posterior lobes {Fig. 91). This disk is entirely surrounded by the 

 large loljes when they are shut, but it forms the lower outline of the body when 

 the lobes are entirely open and fully spread. In this attitude the mouth is shut, 

 but the lobes are wide open, to inclose any food that may come within reach; 

 and whilst dropping fragments of oysters upon them, as they are generally turned 

 mouth upward, in this extreme state of dilatation, I have sometimes seen the lobes 

 close upon such morsels to secure them, and afterward the mouth expand and 

 open witliin to swallow the food, the tentacles being alternately drawn out and 

 retracted. 



The visible outline of the digestive cavity changes most remarkaljly in these 

 various operations. When the mouth is shut and the digestive cavity is empty, 

 the digestive sac is completely flattened and compressed in the direction of the 

 longer diameter, rising like a tapering funnel toward the central chymiferous cavity; 

 that is to say, the folds of the digestive sac which are stretched between the 

 anterior and the posterior angles of the mouth converge towards the abactinal 

 extremity of the body, and the flattened walls are pressed upon each other. In 

 this position the coeliac chymiferous tubes run in a straight course toward the 

 actinal pole along the middle of the outer surfxce of the digestive cavity, and reach, 



