COELENTERATA AND CTENOPHORA 



The Hydroids and Hydromedusae. With few exceptions, among which 

 the hydras are notable, the Hydrozoa are marine animals. They are attached 

 in at least one phase of the life cycle, and most species form colonies of 

 individuals variously specialized in correlation with feedin^^, protection, and 

 reproduction. The life cycle in some species, but not in the majority, includes 

 a free-swimming jellyfish stage or hydromedusa. The species Ohelia oenicu/ata 

 (Fig. 10.9) is representative of those that do present this feature. In its 

 hydroid ("hydra-like") phase or generation this species is a colony contain- 

 ing possibly thousands of individuals, or polyps, each individual comparable 

 with a single hydra and all united as the buds of a hydra would be if they did 

 not become detached. The obelia colony in fact begins as a single polyp, 

 which grows and buds repeatedly until it consists of many upright stems bear- 

 ing polyps and arising from the root-like hydrorhiza ("hydra root") growing 



Medusa 



Gonanth 

 Blastostyle 



Gonotheca 

 Hydrorhiza 



Fig. 10.9. Ohelia: structure and life cycle. ,4, portion of a colony of the hydroid genera- 

 tion. B, medusa, and sexual reproduction. C, growth of a young hydroid colony from the 

 attachment of a pianula larva; numbers indicate successive stages of growth. 



301 



