COELENTERATA 49 



The simplest form of Hydromedusa is represented by Hydra, 

 which exists only in the hydroid form. Its reproductive organs 

 show no trace of a medusoid nature ; and although the medusae 

 can be traced in other and more specialised species through 

 stages of degeneration till they become Little more than pro- 

 tuberances on the body-wall full of sexual cells, still there is 

 nothing in the ovary and testis of Hydra to warrant the view 

 that they are n(^t simply sexual organs. 



In some Hydromedusae a distinct alternation of generations 

 is present ; that is, the hydroid person produces asexually, by 

 budding, a medusoid person which produces sexually, by means 

 of ova and spermatozoa, the hydroid person again. Thus asexual 

 and sexual modes of reproduction alternate in the life-history 

 of these animals, and each mode is associated with a distinct 

 kind of animal : the asexual with the hydroid, usually a fixed 

 form; the sexual with the medusoid, a free-swimming form. 

 This kind of alternation of generation — budding alternating with 

 the sexual method — has been termed metagenesis ; it occurs in 

 many of the lower animals. 



Although the fixed hydroid differs a good deal in appear- 

 ance from the free-swimming medusa, they can both be reduced 

 to a common type. In both forms of person very considerable 

 complexity of form is often combined with great simplicity of 

 ultimate structure. Many forms are colonial, and the in- 

 dividuals composing the colonies are commonly modified to 

 subserve various functions, and thus may become degraded to 

 the level of organs. 



