CHAPTER XV 



PHYLUM CCELENTERATA; PHYLUM CTE- 

 NOPHORA 



The coelenterates are all modified gastrulas which are 

 radially symmetrical, and possess peculiar " stinging cells," 

 or nematocysts. They are all aquatic, and most of them 

 are marine, only three species occurring in fresh water. 

 Well-known examples of the phylum are polyps, such as 

 corals and hydras, and the jellyfishes, or medusae. In many 

 species there is an alternation of generations a polyp 

 stage giving rise to a medusa, and it in turn to a polyp 

 again. There are three classes: Hydrozoa, Scyphozoa, 

 and Anthozoa. 



CLASS 1. HYDROZOA 



On account of the bush-like forms often assumed by the 

 colonies of polyps when growing alongshore, the coelen- 

 terates in this class were long known as " zoophytes" 

 or ' ' animal plants." A ' ' bush " may bear hundreds or even 

 thousands of little polyps at the tips of the branches, and 

 all are connected through the twigs with the main stalk of 

 the colony which is attached to some solid object. Though 

 polyps are the most prevalent types of animals in this 

 class, minute medusae are usually produced at intervals. 

 All medusae are shaped more or less like an umbrella and 

 those of the Hydrozoa are easily distinguished from others 

 by the presence of a velum a shelf-like structure pro- 

 jecting inward around the entire margin of the umbrella 

 (Fig. 68, v). Along the Atlantic coast of the United States 

 a little hydrozoan grows very commonly on docks, piles, 

 and seaweeds. This is Obelia geniculata (Linnaeus), a 

 Colonial Hydroid, which may well serve as an example of 



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