Chap. HI. GENUS PLEUROBRACHIA. 229 



I never investigated a more difficult subject, never had to devote so much time 

 to the same point, and never taxed my patience to such an extent as during 

 these investigations. I insist upon these details, and state them at full length, 

 because I know that I have now cleared up this subject, and may perhaps induce 

 some other student to go through the long description I am about to give of it, 

 since he may expect to have the matter settled for him. Let us proceed in this 

 description as we should with a minute description of the ramifications of the 

 bloodvessels of some highly organized animal. The difference which exists between 

 the digestive cavity and the main cavity of the chymiferous system will first engage 

 our attention. 



In a front view (PI. IP. Fig. 23), when the two tentacles appear right and 

 left, and the plane which passes through the longitudinal fissure of the mouth 

 divides the body into halves, we have before us, on our right, one of those halves 

 of the body, which alternates in its contractions with the other half on the left. 

 It is according to this diameter that the antagonism between the two sides is 

 introduced. Seen in this view, the digestive cavity appears throughout like a nar- 

 row fissure {h c) ; but as it is much wider in another direction, its outline, as seen 

 in Fig. 22, is very broad. The fact is, this cavity is a flattened sac, flat as 

 long as it is not full of food, and the two surfaces of the flattened bag are pressed 

 upon each other : so that when seen in profile, that is to say, facing the diacceliac 

 diameter of the body, as in Fig. 2.3, it appears like a mere double skin, or a slit 

 lined with a membrane ; but when seen from its broadside, that is to say, facing 

 the right or left side of the body, as in Fig. 22, it appears like a wide sac. 

 During the process of digestion, when filled with food, it is swollen into a more 

 rounded sac or cylinder. The abactinal extremity of this sac opens into the main 

 cavity of the chymiferous system, terminating there in an oblong fissure, which, at the 

 will of the animal, can be shut or opened ; so that, like the stomach of Actinia, 

 the digestive cavity of Pleurobrachia communicates with the chymiferous cavity, or 

 may be shut by itself. The difference between the two types, however, consists 

 in the limitation of the cavity of the body, which is circumscribed within the centre 

 of the animal in Pleurobrachia, and sends off large trunks and tubes, branching 

 diversely into its mass and along its surface ; while in Actinia the whole body is 

 hoUow, and the stomach empties into that one large cavity. 



The central chymiferous cavity has two main stems, one extending into the 

 right, and the other into the left, half of the annual {Fig. 23 e c). It would seem, 

 from Fig. 23, as if the digestive sac [h c) were hanging loosely into the chymiferous 

 cavity : this is, however, not the case, for the spaces (/• r) which communicate with 

 the main chymiferous cavity right and left of the digestive sac do not form a 

 continuous cavity encircling the whole digestive sac, but are only two simple but 



