PHYLUM COELENTERATA 



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readily available, easily collected and handled, and is representative 

 of multicellular animals of simple formation. The study of Hydra 

 as a simple metazoan will go far in giving insight into the much 

 more complex make-up of the body and life of man. 



Classification of the Phylum 



The phylum is divided into three classes, each with three or four 

 orders. 



Class Hydrozoa. — These are typical polyp forms, many of which 

 produce medusae forms by budding. The group includes marine, 

 colonial polyps, or hydroids, floating colonial hydroids, such as 

 Portuguese man-of-war, one special gro"up of corals, some smaller 

 jellyfishes, and the fresh-water polyps. 



Fig. 57. — Structure of Gonionemus. ad. Adhesive pad ; g, gonads ; li, lithocyst ; 

 m, mouth ; mn, manubrium ; n, nematocyst ; ra, radial canal ; re, ring canal ; st, 

 stomach; t, tentacle; ve, velum. (From "White, General Biology.) 



Order Leptolina — a group which has a sedentary or sessile polyp 

 stage. Such examples as Hydra, Ohelia, Gonionemus, Canipanularia, 

 Tubularia, and Craspedacusta are well-known forms. The first one 

 is a fresh-water polyp form and is the best known of the group. 

 The last one listed is a fresh-water form with a small polyp stage 

 lacking tentacles but with a disclike medusa possessing many 

 tentacles. Hydra, of this order, will be discussed as a general repre- 

 sentative of the phylum, but since Gonionemus and Ohelia are com- 

 mon marine forms, a brief description of them may be included 

 here. 



