MEDUSA. 193 



of a mantle, extending loosely downwards from 

 the circumference ; together with a central pe- 

 dicle descending from the lower surface, like the 

 stalk of a mushroom, and terminating below in 

 several fringed laminae, or processes, which have 

 sometimes been denominated tentacula. 



The whole substance of the body of these 

 medusae is semi-transparent and gelatinous, with- 

 out any distinct fibrous structure ; yet it has 

 considerable elasticity, and possesses also some 

 degree of contractile power. The animal is 

 seen alternately to raise and depress the mar- 

 gin of its hemispherical body, and to flap with 

 the fringed membrane or mantle, which descends 

 from it, in a manner somewhat similar to the 

 opening and shutting of a parasol. This pulsa- 

 tory movement is performed about fifteen times 

 in every minute, with great regularity : and by 

 the reaction of the water, the animal is sus- 

 tained at the surface ; or by striking the water 

 obliquely, it may even perform a slow lateral 

 movement. They descend in the water by 

 simply contracting their dimensions in every 

 direction. Sometimes, in order to sink more 

 quickly, they turn themselves over, so that their 

 convex part is undermost. 



Medusae are met with of very various sizes ; 

 the larger abound in the seas around our coast ; 

 but immense numbers of the more minute and 

 often microscopic species occur in every part of 



VOL. I. o 



