114 



PHYLUM CCELENTERATA 



cell which needs them. Possibly also the current helps in taking 

 oxygen to the cells, for many jelly-fish are of large size (C^/anea 

 atmta may be six feet in diameter) and considerable thickness^ 

 The bulk of the body is. however, mesogloea. The eggs are liberated 

 mto the enteron, are fertilised there, and develop into planute. 

 These escape, and settle on a rock or weed, and each develops 

 into a polyp known as a scyphistoma. This may divide by budding 

 into more polyps like itself, but its chief function is to produce 



.1 «• H. 



Fig. 78. — The life-history of Aurelia. 

 A, Planula ; B-H, stages in the development of the scyphistoma ; L, ephyra. 



medusa by a series of horizontal fissions, the whole process being 

 called strobilisation (Fig. 78). The structure looks much Hke a 

 pile of saucers, of which each in turn, known as an ephyra, as 

 it becomes the top one floats off, turns over, and grows to be a 

 new jellyfish. Strobilisation occurs only in winter and when the 

 animal is well fed. There is much variation in the development 

 of the species of this class, and even in individuals of a species. 

 Some genera, including Haliclystus and Lucernaria of British 

 coasts, have only one stage in the life-history, which is a sessile 

 polyp. 



