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ESSENTIALS OF ZOOLOGY 



of multiceUular 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. 



Olassificatioii 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 group of corals, some smaller 

 jellyfish es, and the fresh-water polyps. 



Fig. 104. — 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, The C. V. Mosby Company.) 



Order Leptolina (may be divided into Anthomedusae and Leptome- 

 dusae) — a group which has a sedentary or sessile polyp stage. Such 

 examples as Hydra, Ohelia, Gonionemus, Campanularia, Tuhularia, 

 and Craspedacusta are well-known forms. 



Gonionemus is a small jellyfish form, measuring about a centi- 

 meter across, and is found in the pelagic waters along our eastern 

 shores. Its shape reminds one somewhat of an umbrella with a 

 fancy fringe but with practically no handle and made of clear cello- 

 phane. The exumbrella is the convex upper, or aboral side while the 

 subumbrella is the concave, lower, oral side. A short stalklike part, 

 the manubrium, hangs down from the center of the subumbrella. At 



