122 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



provided with tentacles and the eight mesenteric 

 folds, are other far simpler individuals : the zooides, 

 the function of which is respiratory and of which 

 the greater part of the organs have degenerated ; 

 the generative organs are lacking, the tentacles are 

 very small, and the mesenteric folds only number 

 two instead of eight. Degeneration, then, is 

 exhibited side by side with specialization. 



Ajnong the Hydromedusce similar examples 

 abound. It is known that the polyp-like or 

 medusa-like forms of this group which may live 

 independently, frequently associate themselves 

 together to form colonies, sometimes predominantly 

 polypoid, sometimes completely medusoid, and occa- 

 sionally a mixture of the two. 



In these cases a marked polymorphism is often 

 apparent. The different individuals become adapted 

 to definite functions, and the corresponding organs 

 undergo special development ; the other parts of 

 the body having become either unnecessary or 

 merely accessory, begin to degenerate and finally 

 disappear. Thus we see in Hydroid colonies, not 

 only the hydra-like members, nutritive, fixed and 

 sterile, and the medusa-like members which are 

 reproductive and become free from the colony, but 

 also certain individuals which are termed gono- 

 phores. These gonophores are really medusa-like 

 members which have lost their independent move- 

 ment, and have consequently more or less lost 

 both their tentacles arid their unibrella-like discs, 



