DIVISION OF LABOR IN THE C0ELENTE1\ATES 



185 



Hydroids 



Hydra vulgaris is a fresh water form, but many more representatives 

 of the Coelenterate group are found in salt water, the most famiUar 

 being the hydroids found attached to the piles of wharfs and other 

 submerged objects. Among the most common hydroids are members 



hydranth. 



-gbnobVzsca 



^9 ^onacC 



medusa /;<^^^»v^^",'^^ 

 ^^^ (^..fertiTe 



asexual *~-^-Viyctrorhi3a. - y 



Stage / 



^..blastula 



^T-planula 



Life cycle of Obelia — showinff alternation of generations.' Compare with text 

 pages 18.5-186 for explanation of diagram. 



of the genus Obelia. These animals form colonies, in which the indi- 

 viduals, called polyps, or zooids, are attached to each other by means 

 of hollow stalks, covered with a chitinous, cellophanelike perisarc. 

 At the tip of each branch, the covering expands into a cuplike hydro- 

 theca, which surrounds the living polyp. As in Hydra, each individual 

 polyp of Obelia is hollow and two layered, with a circle of tentacles 

 about the raised hypostome, in. which the mouth is located. The 

 tentacles are provided with nematocysts that act in the same manner 

 as in the Hydra. The food cavity, however, extends down each stalk- 

 like branch or individual and is continuous with that of the other 

 polyps, thus forming a common gastrovascular cavity in which food 



