494 



CCELENTERA TES. 



water itself. Indeed, the common blue medusa (Aurelia) is slightly lighter than 

 water, whereas most of these jelly-fish are somewhat heavier than that element, 

 and sink during the pauses in their contraction. 



Discomedusae are also found in the deep sea, one form of a delicate violet, with 

 darker tentacles, being the so-called Periphylia mirabilis, figured below, which 

 was dredged from a depth of over six thousand feet during the Challenger 

 expedition off the coast of New Zealand. 



These beautiful quiet creatures, themselves apparently so harmless, are not 



Periphylia (f nat. size 



exempted from the struggle for existence. Not infrequently, small Crustaceans 

 belonging to the orders Isopoda and Amphipoda, related to the wood-lice and 

 sandhoppers, become parasitic upon them, and many genera are attacked by a 

 small species of fish. These fish collect in small companies under the umbrella of 

 their prey, eating its arms, and especially their stinging-capsules, which do not 

 appear to injure them. Although some of these splendid forms develop directty 

 as jelly-fish from the egg, the great majority commence life as attached polyps, so 

 that we have here again another instance of alternation of generations. The 

 sexes are usually separate, and from the egg arises a ciliated larva, which is oval, 



