CCELEXTERATAHYDROZOA. 141 



skin, as many a bather knows to his cost. The principal 

 mass of the body consists of a bell-shaped disc, or um- 

 brella, from the lower part of which a single polypite is 

 suspended. The mouth of this polypite is surrounded 

 by four long processes, or arms. A number of canals 

 (generally eight) proceed from the extremity of the poly- 

 pite to the margin of the umbrella. Near this margin 

 they are broken up into a great number of tubes, and 

 form a kind of network. All these communicate with a 

 circular canal which runs round the margin of the disc. 

 This margin is furnished with a fringe of tentacles, which 

 are supplied with branches from the circular canal. A 

 number of coloured spots, connected with little sacs filled 

 with fluid, and containing stony particles, are also placed 

 round the margin. These specks are concealed by an 

 expansion of the ectoderm, shaped like a hood. On this 

 account, the name, hidden-eyed medusas, has been ap- 

 plied to this group of jelly-fishes. 



205. The following are the principal points of difference 

 between the hidden-eyed and naked-eyed medusa : 



In the naked-eyed medusae, the cavity of the disc is 

 furnished with a veil ; the radiating canals are usually 

 four in number ; these canals never form a network, and 

 the eyes are always uncovered. 



The hidden-eyed medusas have usually eight canals, 

 which form a network; the eyes are covered, and tha 

 mouth of the disc is not protected by a veil. 



206. The reproductive organs "form a conspicuous 

 cross, shining through the thickness of the disc." After 

 swimming about for a time, they develop fertile ova, and 

 then die. The ova, however, instead of producing gigantic 

 sea blubbers, result in minute hydra-tubes, which form 

 colonies as before. Thus we are furnished with another 

 instance of the " alternation of generations." 



