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GENERAL ZOOLOGY 



a free-swimming colonial form, the individual, highly poly- 

 morphic members of which are in communication with one 

 another by means of the common gastrovascular cavity. The 

 colony is attached to a float (pneumatophore) containing gas 

 that can be released through a pore, and later regenerated, 

 enabling the animal to drop below the surface and rise again. 

 There is an alternation of generations (Fig. 223). 



Fig. 225. — Dissection of Metridium marginatum, a sea anemone, showing 

 the internal structure. 1, 2, 3, and 4, primary, secondary, tertiary, and quater- 

 nary mesenteries extending inward from the body wall, only the primary reaching 

 the gullet, g. The gullet opens into a common basal gastrovascular cavity, 

 o, ostia, pores through which water passes from one chamber to the other through 

 the primary mesenteries, r, reproductive organs; s, scyphonoglyphe, a ciliated 

 groove in either side of the gullet; t, tentacles. (Modified from Lineville and 

 Kelly, Textbook in General Zoology, Ginn and Company.) 



CLASS II. SCYPHOZOA. Jellyfishes. Radially symmet- 

 rical, usually with an alternation of generations, although the 

 medusoid or the hydroid generation alone may be present in 

 some. In general, the medusoid stage is more prominent in 

 the group than the polyp. The latter, known as the scy- 

 phistoma, differs from Hydra in the following points: (1) the 

 attachment of its aboral end in a cup; (2) the presence of four 

 endodermal mesenteries projecting into the gastrovascular 

 cavity ; (3) the possession of an ectodermal gullet. The medusae 

 are acraspedote (lacking a velum), and are produced from the 

 scyphistoma by terminal budding (strobilation) . 



