THE HYDRAS, FRESH WATER JELLYFISHES 



be fertilized by a sperm cell from the same animal or from a 

 different one ; in any case fertilization occurs while the egg is 

 still attached to the parent's body. There the early develop- 

 ment of the young hydra begins under a kind of parental 

 protection. 



Hydras tide themselves over dangers of the cold by means 

 of winter eggs which are formed and fertilized in the autumn. 

 Encased in a protective shell, the egg drops from the parent's 

 body, lies dormant over the winter, but resumes its develop- 

 ment in the spring. This is a safety device similar to others 

 such as the winter eggs of small crustaceans like Daphnia, 

 and of planarians, or the gemmules of sponges. 



In the better known hydras like those just described there 

 is only one form of the body; but in certain fresh water jelly- 

 fishes to which these hydras are related the individuals of 

 one generation are very different from those of the next. In 

 this so-called "alternation of generations," the hydranths or 

 hydra-like animals of one generation differ greatly from' the 

 medusce or umbrella-like jellyfish of the next generation. 

 In the little known jellyfish, Craspedacusta, the hydra-like 

 stage is rarely seen, but the medusa form is occasionally found 

 in great numbers. In September, 191 6, millions of little 

 medusae of Craspedacusta were found floating in the waters of 

 Benson Creek about twenty-five miles from Lexington, PCen- 

 tucky, and again in September, 1924, though none had been 

 observed there between times. 



Habitat. — Hydras live in sunlit pools on plant stems and on 

 the under sides of leaves, abundant in early as well as late 

 summer. Dr. J. G. Needham has reported that the "brick- 

 red" hydras were so abundant in early -summer at Saranac 

 Inn, New York, that the water was "tinged with red" by 

 them; and Kofoid collected 5335 hydras from a cubic yard of 

 water taken from Quiver Lake in the spring of 1897. Hydras 

 have been reported growing on the walls of filter beds; at 

 Auburn, New York, such a growth of hydra was brilliant pink 



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