238 



C(ELENTERATA. 



develops mouth and tentacles, and, by budding, produces a. 

 hydroid colony. This hydroid colony lacks sexual organs. It 

 produces by budding the sexual individuals, the medusae, which 

 separate and swim freely. Since polyp and medusas are morpho- 

 logically comparable, there is a time before the escape of the 

 medusae when the colony is polymorphic, consisting of asexual 

 individuals (hydranths) which reproduce only asexually and of 

 others which have taken over the sexual reproduction (medusas). 

 Hence we conclude that the alternation of generations here has 

 arisen from a division of labor or polymorphism of individuals 

 originally of equivalent value, in which some individuals (the 

 sexual) have separated and acquired a peculiar structure. 



While alternation of generation has arisen from polymorphism, 

 it can again produce it. This occurs when the medusae, instead 

 of separating, remain permanently attached to the colony. They 

 then degenerate into ' sporosacs/ which always lack mouth, tentacles, 

 and velum (fig. 182), often also radial and ring canals, so that at last 



FIG. 182. Comparison of a medusa and a sporosac (orig.). A, fully developed medusa; 

 B, medusa with the manubrium closed, still attached to the blastostyle ; C, 



', last stage, eggs being pro- 



medusa reduced to a simple manubrium (sporosac) ; D 

 duced in the body wall (Hydra). 



there remains only the manubrium ('spadix') and the sexual 

 organs, the latter enveloped by the rudiments of the umbrella. 

 Since medusae and sporosac replace each other in closely allied 

 species, a common name, gonopliore, has been applied to both. 



This developmental history may be modified in two ways: 

 either the polypoid or the medusan generation may be suppressed: 

 In the first case we have polyps which reproduce both sexually and 

 asexually, in the other medusae whose eggs develop directly into 

 other medusae. (A few medusae may produce new medusae by 

 budding.) Thus we can have four conditions: (1) Polyps which 

 produce sometimes asexually, sometimes sexually, but always 



