﻿331 CONTRIBUTIONS TO THE NATURAL HISTORY 



diameter that the antagonism between the two sides is introduced. Seen in this view, 

 the digestive cavity appears throughout like a narrow fissure, c ; but as it is wider in an- 

 other direction, its outline, as seen in Fig. 1, is very broad. The fact is, that this cavity 

 is a flattened sac, flat as long as it is not full of food, and the two surfaces of the flat- 

 tened bag are pressed upon each other ; so that when seen in profile, that is to say, facing 

 the longitudinal diameter of the body, as in Fig. 2, 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. 1, it appears like a wide sac, and only during 

 the process of digestion is it swollen into a more rounded sac or cylinder. The lower 

 extremity of this sac is projected into the main cavity of the body, terminating there in a 

 large opening, which, at the will of the animal, can be shut or opened ; so that, like the 

 stomach of Actinia, the central digestive cavity of Pleurobrachia communicates with the 

 cavity below, or is shut up by itself. The difference between the two genera, however, 

 consists in the limitation of the cavity of the body, which, as such, is circumscribed with- 

 in 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 hollow, and the stomach empties into that one large cavity. 



The central cavity has two main stems, one extending into the right, and the other 

 into the left half of the animal, as is seen in Plate III. Fig. 2, 5, and 4. It would seem 

 from Fig. 2 as if the largest sac were hanging loosely in the central cavity ; this is not the 

 case, however, for the spaces communicating with the main cavity right and left of the 

 digestive sac in this figure do not form a continuous cavity encircling the whole digestive 

 sac, but are only two tubes, which arise from the main trunks of the central cavity, and 

 follow the middle of the lateral surface of the compressed digestive sac, in an ascending 

 course, up to the margin of the mouth, being simple narrow tubes, as Fig. 1 shows. 

 Downwards, however, the main cavity extends in the form of a funnel, terminating with 

 two holes near the centre of the area below. This funnel descends in the centre of 

 the animal vertically, and lies, therefore, in its central axis. It assumes nearly the same 

 appearance in whatever position it is seen, excepting, however, its termination below, 

 which is furcate when seen from the side, as in Fig. 1, and simple when seen in front, 

 as in Fig. 2. This part of the cavity and the main lateral trunks being, as it were, the 

 centre of the circulation, we may view it as an axis which branches right and left, and 

 which rises in the centre in two parallel forks up to the mouth ; so that, when seen 

 from the side, the double upper fork is seen as one, but the lower fork, which is at 

 right angles with the former, is distinctly seen, and vice versa; the main lateral stems 

 and their ramifications present in the first position their broadside, and appear fore- 

 shortened in the other. 



