IV 



PHYLUM CCELENTERATA 



99 



The Obelia-colony is sexless, having no gonads, and develop- 

 ing only by the asexual process of budding ; but certain of 

 its buds the medusae develop gonads, and from their 

 impregnated eggs new Obelia-colonies arise. We thus have 

 an alternation of an asexual generation the Obelia-colony 

 with a sexual generation , the medusa. 



FIG. 44. Stages in the development of two zoophytes (A-H, Laomedea. I-M, 

 Eudendrium) allied to Obelia; A-F, stages in segmentation; G, the planula 

 enclosed in the maternal tissues; H, the free-swimming planula; I-M, fixation 

 of the planula and development of the hydrula. (From Parker's Biology, after 

 Allman.) 



The majority of the Hydrozoa resemble Obelia in form- 

 ing fixed colonies ; but there are a few exceptional cases in 

 which the animal remains simple. One of these is Hydra, 

 the Fresh-water Polype. In Hydra the entire organism (Fig. 

 45) consists of a simple cylindrical body with a conical hypos- 



