66 



ACALEPH^. 



(94.) Pulmonigrada. The most ordinary examples of the 

 aealephse found in our climate, when examined in their native 

 element, are seen to be composed of a large mushroom-shaped 

 gelatinous disc, from the inferior surface of which various pro- 

 cesses are pendent, some serving as tentacula, others for the pre- 

 hension of food. In Rhizostoma (fig. 21) the central pedicle 

 resembles in structure 

 and function the root of 

 a plant, being destined 

 to absorb nourishment 

 from the water in which 

 the creature lives. The 

 body of one of these 

 medusae is specifically 

 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 sur- 

 face, and in some mea- 

 sure to row it from place 

 to place, is the um- 

 brella-shaped expansion 



or disc, which is seen continually to perform movements of con- 

 traction and dilatation, repeated at regular intervals about fifteen 

 times in a minute, having 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 

 disc, the medusa can strike the water with sufficient force to 

 insure its progression in a certain direction when swimming in 

 smooth water, but of course utterly inefficient in stemming the 

 course of the waves, at the mercy of which these animals float. 

 The tentacula, 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 disc, when cut 

 into, seems perfectly homogeneous in its texture, nor is any 

 fibrous appearance recognisable to which its movements could be 

 attributed ; but in the larger species its inferior surface appears 



