THE JELLYFISH AND THEIR ALLIES 



3469 



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

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

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

 sand hoppers, 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 directly 

 as jellyfish 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, 

 hollow, "and somewhat flattened, recalling the shape of a locket. This is the 

 so-called planula, which for a time swims about, then attaches itself firmly by 

 the end of its body and becomes pear shaped, the stalk of the pear being 

 represented by the attached end; a horny envelope is then secreted over the whole 

 surface, the mouth breaks through the free end of the central cavity, four tentacles 

 appear, and we have a four-armed polyp or scyphistoma. The tentacles increase in 

 number, and the scyphistoma can produce at its base a number of young polyps 

 which again can multiply by division. At a certain period, this method of 

 multiplication by budding of the polyps from the base ceases, and each scyphis- 

 toma divides up in quite a different fashion. The polyp becomes horizontally 

 constricted in several places, until it appears like a 

 number of cups placed one inside the other; this is 

 called a strobila (pine-cone). When ready, the top 

 cup breaks away, turns over, and swims as a young 

 form of medusa, called an ephyra, which gradu- 

 ally acquires the shape of the perfect discomedusa. 

 We thus have here an alternation of generations 

 in which a sexual medusa generation is succeeded 

 by an asexually-reproducing polyp generation, 

 this again being followed by another medusa 

 generation. 



In relation to these, and constituting a kind 

 of transition form connecting the Discomedusse 

 and the polyps, are the Calycozoa, or cup-shaped 

 medusae, which either swim about freely or are 

 attached by their apices, where the firm gelatinous 

 disc attains its greatest thickness. At the margin 

 of the disc, these forms carry eight to sixteen 

 arm-like processes. In the attached forms 

 (Lucernaria') the ends of these processes are 

 provided with short tentacles, occasionally broad- 

 ened into discs and used for attachment, and 

 also with stinging capsules. The Calycozoa may 

 leave their place of attachment and swim about Tessera. 



for a time, with a rotatory motion, and then again (20 times natural size). 



