Some of the Lower Metazoans 391 



Other Hydrozoans 



Obelia. — The members of the genus Ohelia demonstrate the typical 

 alternation of generations. They are found in the coastal waters of 

 this country from Long Island to Labrador and along the Pacific coast. 

 In the hydroid stage, Ohelia exists as a colonial animal. An individual 

 colony contains two types of individuals and has considerable morpholo- 

 gical variation. 



The entire colony is firmly fastened to the substrate by a series of 

 interlacing rhizomes or stolons collectively known as the hydrorhisa. 

 From the individual stolon arises the stem of the colony, the hydrocaulus. 

 Hundreds of polyps may be present in an individual colony. 



There are two types of polyps : the feeding polyp or hydranth, 

 and the reproductive polyp or gonangimn. The feeding polyp has many 

 tentacles surrounding the mouth and hypostome. Surrounding this 

 polyp is a vaselike protective covering, the hydro fheca. The reproduc- 

 tive polyp is protected by a similar protective covering, the gonotheca, 

 and has a central axis, the hlastostyle, which produces lateral buds. 

 These buds develop asexually into the medusoid or jellyfish stage. The 

 entire branching stem of the colony is covered with the noncellular 

 perisarc which is continuous with the theca of the polyps. The hollow 

 portion of the stalk is continuous with the gastrovascular cavity of the 

 vegetative polyps. The inner, living stalk is known as the coenosarc. 



There are actually two types of asexual reproduction in Ohelia. 

 In the one type, reproduction occurs simply by the growth of new 

 polyps on a single colony or by the production of new colonies from the 

 stolons. In the other method of asexual reproduction, the medusae cr 

 jellyfish are produced by the development of the lateral buds of the 

 stalk of the gonangium. These small medusae break loose and swim 

 out as free-swimming sexual stages in the life cycle. 



In the medusae of most of the hydrozoans, the eggs and sperm are 

 produced by separate individuals in epidermal folds. The eggs and 

 sperm are released directly into the water. After fertilization, the zygote 

 develops into a planula larva. This is a minute, flattened, ciliated form 

 which swims about for a brief period of time. It soon settles down and 

 by asexual budding produces a new colony. 



Unfortunately the medusa of Ohelia is very small and difficult to 

 study. But another hydrozoan, Gonionemus, has a conspicuous medusoid 

 stage and clearly shows the typical structures. 



