JELLY-FISH 169 



Medusa do not develop into Medusae, but into hydroids, 

 so that there is an alternation of generations. 



The geographical distribution of these littoral 

 Medusae depends upon the habitat and distribution of 

 their hydroids. Some have a very wide range ; others 

 are limited to definite localities. It is the bathy- 

 metrical limit of these hydroids, which seldom extend 

 to a greater depth than the loo-fathom line, that con- 

 fines this group of Medusae to the littoral waters. 



The oceanic Hydromedusae have not a fixed hydroid 

 stage, and the eggs develop directly into Medusae. 

 Their natural habitat is the ocean, but occasionally 

 they drift into littoral waters. The species generally 

 have a very wide geographical range, and some are 

 found at great depths. 



The large jelly-fishes, which are commonly seen from 

 the deck of a ship slowly swimming at the surface 

 of the sea, or found stranded on the shore by the 

 receding tide, are representatives of the Scyphomedusae 

 (Figs. 93, 95). A few have the power of stinging like 

 a nettle, but most of them may be handled with 

 impunity. One (Fig. 95) is noted for its brilliant 

 phosphorescence, and at night has the appearance of 

 a globe of fire gliding through the water. 



Some of the Scyphomedusae have an alternation of 

 generations. The eggs from an adult Medusa develop 

 into tiny anemone-like animals, which are attached to 

 stones, shells, etc., at the bottom of the sea. These 

 fixed forms finally give off a number of little free- 

 swimming rayed discs, called Ephyrae, which develop 

 into adult Medusae. Others omit the fixed stage in 

 the course of development, and the eggs develop directly 

 into Ephyrae (Fig. 94). 



The Scyphomedusae are found all over the world ; 

 some are oceanic, and others are more at home near the 



