ACALEPH.E. 



75 



alimentary canal passes straight through the globular or barrel- 

 shaped body, commencing at one extremity by two prominent and 

 sensitive lips. No apparatus of prehension is here needful ; for, as 

 these animals swim along by the action of their cilia, the water 

 passes freely through this capacious channel, and brings into the 

 stomach materials proper for food. From both extremities of the 

 digestive cavity arise vascular canals which empty themselves into 

 two circular vessels, one surrounding the oral, and the other the 

 anal portions of the body : from these two rings eight double vessels 

 arise, which run longitudinally from one pole to the other of 

 the creature beneath each of the cartilaginous ribs upon which 

 the cilia are placed ; and from these, others more minute arise, 

 which are distributed in a delicate network through the sub- 

 stance of the animal. In the Beroe, therefore, we must regard 

 the vessels which convey the nutritive juices beneath the ciliated 

 arches, not merely as arteries, but as organs of respiration ; for, 

 thus placed close beneath the outer surface of the body, the water, 

 which is perpetually made to rush over them by the ciliary move- 

 ments, will serve to aerate the fluid contained within. 



The Cestum Veneris (fig. 23) is nearly allied to the Beroe in 

 the arrangement of its nutritive apparatus, notwithstanding the 

 difference of form observable in these Ciliograde medusae. In Ces- 

 tum the digestive cavity, which is exceedingly short in comparison 

 with the length of the animal, passes transversely across the body 

 in a straight line from one side to the other, as represented in the 

 engraving (fig. 23) ; but the details of its 

 structure, and the nature of the vessels aris- 

 ing from it, will be best understood by a 

 reference to the enlarged diagram of these 

 parts given in the annexed figure (fig. 30). 

 The mouth (i) is a rhomboidal depression 

 seen near the centre of the body, between 

 the two lateral rows of locomotive cilia 

 which extend from one end of the animal to 

 the other. From the mouth arise two 

 tubes, (/,y,) which terminate in a globular 

 cavity common to both ; these would seem 

 to constitute the digestive apparatus : and 

 a straight and narrow tube (o), prolonged 

 to the margin of the body opposite to that 

 which the mouth occupies, may be regarded 



Fig. 30. 



