EHIZOSTOMA CUVIEKI. 



105 



formed all the solid framework of the body, and which, in an animal 

 weighing five or six pounds, will scarcely amount to as many grains ; 

 and even if the water which has escaped from this cellulosity be col- 

 lected and examined, it will be found to differ in no sensible degree from 

 the element in which the creature lived. The conclusion therefore at 

 which he naturally arrives is, that, in the Acalephse, the sea- water col- 

 lected and deposited in the delicate cells of an almost imperceptible film 

 becomes, in some inscrutable manner, instrumental to the exercise of 

 the extraordinary functions with which these creatures are endowed. 



(269.) The ACALEPBJE have been divided by zoologists into groups 

 distinguished by the nature of their means of progression : in describing, 

 therefore, the organs of locomotion, with which we commence their his- 

 tory, the reader will be made acquainted with the principal modifications 

 of outward form exhibited by various races of these interesting beings. 



(270.) PTJLHONIGRADA. The most ordinary examples of the Acalephae 

 found in our climate, when examined in their native element, are seen 

 to be composed of a large ^ 49 



mushroom-shaped gelati- 

 nous disk, from the in- 

 ferior surface of which 

 various processes are pen- 

 dent, some serving as 

 tentacula, others for the 

 prehension of food. In 

 Rhizostoma (fig. 49) the 

 central pedicle resembles 

 in structure and function 

 the root of a plant, being 

 destined to absorb nou- 

 rishment from the water 

 in which the creature 

 lives. The body of one of 

 these Medusae is specifi- 

 cally heavier than the 

 water of the ocean, and 

 would consequently sink but for some effort on the part of the animal. 

 The agent employed to sustain it at the surface, and in some measure 

 to row it from place to place, is the umbrella-shaped expansion or disk, 

 which is seen continually to perform movements of contraction and dila- 

 tation, repeated, at regular intervals, about fifteen times in a minute, hav- 

 ing some resemblance to the motions of the lungs in respiration, whence 

 the name of the order (pulmo, the lung; gradior, I advance). By 

 these constant movements of the disk, the Medusa can strike the water 

 with sufficient force to ensure its progression in a certain direction when 

 swimming in smooth water; but of course such efforts are utterly ineffi- 



Khizostoma Cuvieri. 



