106 ITYDEOZOA. 



cient in stemming the course of the waves, at the mercy of which these 

 animals float. The tentacular appendages, situated around the margin 

 of the disk in such species as are provided with these organs, are likewise 

 capable of contractile efforts, and may in some slight degree assist as 

 agents of impulsion, although they are destined to the exercise of other 

 functions. The locomotive disk, when cut into, seems perfectly homoge- 

 neous in its texture, nor is any fibrous appearance easily recognizable, to 

 which its movements could be attributed ; nevertheless in the larger 

 species its inferior surface appears corrugated, as it were, into minute 

 radiating plicee, which seem to contract more energetically than the other 

 portions, and resemble a rudimentary development of muscular fibre. 



(271.) In the Acalepha?, indeed, the substance of the body is gene- 

 rally entirely soft and gelatinous, emulating, in the delicacy of its tex- 

 ture and perfect translucency, the structure of the vitreous humour of 

 the eye, its entire organization apparently consisting of a transparent 

 aqueous fluid contained in innumerable polyhedral hyaline cells. In 

 some species, however, certain parts of the animal are of semicartilagi- 

 nous tissue ; and in a few instances cartilaginous or calcareous lamellsa 

 are found imbedded in their substance, which may be compared to a 

 rudimentary polypary or internal skeleton. 



(272.) Interesting as these creatures may justly be considered when 

 we contemplate the singular beauty of their external configuration and 

 the wonderful design conspicuous in their locomotive organs, a more in- 

 timate acquaintance with their habits and economy will be found to 

 disclose many facts not less curious in themselves than important in a 

 physiological point of view. In the higher animals, we are accustomed 

 to find the nutritive apparatus composed of several distinct systems 

 one set of organs being destined to the prehension of food, another to 

 digestion, a third to the absorption of the nutritious parts of the aliment, 

 a fourth provided for its distribution to every part of the body, and a 

 fifth destined to ensure a constant exposure of the circulating fluid to 

 atmospherical influence ; these vital operations 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. 



(273.) In the Pulmonigrade AcalepTice we have the best illustration of 

 this arrangement : in these, the stomach or digestive cavity is excavated 

 in the centre of the disk, and is supplied with food by a mechanism that 

 differs in different species. In Rhizostoma (fig. 49), 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 

 * pi%a, a root ; trro/ia, n mouth. 



