SURFACE -SWIMMING FAUNA (INVERTEBRATES). 93 



pile of saucers. These discs break away from the 

 base and from the parent stock to grow into the 

 form and size of the adult Jelly-fish. 



We have here an example in the life-history 

 of the common Jelly-fish, of what is known as 

 " alternation of generations." The eggs give 

 rise to sessile Polyps, and these produce a 

 number of buds which, when fully grown, give 

 rise in their turn to the eggs ; or, in other 

 words, the egg-producing generation of large 

 surface-swimming Jelly-fish regularly alternates 

 with the small sedentary bud-producing genera- 

 tion. Now as the bud - producing or Polyp 

 generation of the common Jelly-fish referred to 

 is fixed to the bottom, the proximity to a coast, 

 or at any rate to a shallow water area, is a 

 necessity for the continuation of the species. 

 Many of the Jelly-fish are undoubtedly drifted 

 out into the open ocean by the tides, but the 

 larvae they produce, after swimming about in 

 search of something solid to which they can 

 attach themselves, must at last perish. It is 

 only those larvae which are hatched near enough 

 to the shore to be able to reach the bottom 

 during the tenure of their lives, that can con- 

 tinue the generation of these Jelly-fishes. 



But even in the open ocean far away from 

 shallow water or a coast line, Jelly-fish, belonging 

 of course to different species from those of the 

 coasts, are found. What is their natural history ? 

 How is their life different from that of the 

 Jelly-fish of the shore ? Some of them produce 

 larvae very similar to those described above but 

 they seek, instead of the rocks or sea- weed, other 



