72 ACALEPH.E. 



are carried on in vessels specially appropriated to each ; but, in the 

 class of animals of which we are now speaking, we find but a 

 single ramified cavity appropriated to the performance of all these 

 functions, and exhibiting in the greatest possible simplicity a 

 rough outline, as it were, of systems afterwards to be more fully 

 developed. 



In the Pulmonigrade acalepha we have the best illustration of 

 this arrangement : in these the stomach or digestive cavity is ex- 

 cavated in the centre of the disc, and is supplied with food by a 

 mechanism which differs in different species. In Rhizostoma, 

 (fig. 21), which receives its name from the nature of the 

 communication between the stomach and the exterior of the 

 body,* the organ destined to take in nourishment consists of a 

 thick pedicle, composed of eight foliated divisions, which hang 

 from the centre of the disc. Each of these appendages is found 

 to contain ramifying canals, opening at one extremity by nu- 

 merous minute apertures upon the external surface, whilst at the 

 opposite they are collected into four large trunks communicating 

 with the stomach ; as the Rhizostoma therefore floats upon the 

 waves, its pendent and root-like pedicle absorbs, by the numerous 

 oscules upon its exterior, such food as may be adapted to its 

 nutrition, finding most probably an ample provision in the mi- 

 croscopic creatures which so abundantly people the waters of the 

 ocean. The materials so absorbed are conveyed through the canals 

 in the interior of the arms into the stomachal cavity, where their 

 solution is effected. 



But it is not upon this humble prey that some of the medusae 

 feed ; many are enabled, in spite of their apparent helplessness, to 

 seize and devour animals which might seem to be far too strong and 

 active to fall victims to such assailants : Crustacea, worms, mollusca, 

 and even small fishes are not unfrequently destroyed by them. In- 

 credible as this may seem when we reflect upon the structure of these 

 feeble beings, observation proves that they are fully competent to 

 such enterprises. The long tentacula or filaments, with which 

 some are provided, form fishing-lines scarcely less formidable in 

 arresting and entangling prey than those of the Hydra ; and, in all 

 probability, the stinging secretion which exudes from the bodies of 

 these medusae speedily paralyzes and kills the animals which fall 

 in their way. The mouth of these acalephse is a simple aperture 

 leading into the gastric cavity, and sometimes surrounded with 



* 'Pi&, a root j frozen, a mouth. 



