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. 
Discomedusse 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 (J 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 J)rey, eating its arms, and especially their stinging-capsules, which do not 
appear to injure them. Although some of these splendid forms develop directly 
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, 
