HYDRA 



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number of coarse transverse wrinkles. When fully extended 

 it is attenuated and filiform, the wrinkles disappear, and the 



Fig. 48. 



A , a longitudinal section of Hydra viridis, semi-diagrammatic ; in, mouth ; 

 bd, basal disc ; en, endoderm ; ec, ectoderm ; gv, gastrovascular cavity. 

 The mesogloea is represented by a black line. B, portion of a longitudinal 

 section through an extended tentacle, highly magnified, to show, the 

 batteries of nematocysts ; mg t mesogloea ; n, nucleus of the ectoderm 

 cell containing a battery ; en, endoderm ; en, cnidocil. C, a nematocyst 

 of the large oval kind enclosed in a cnidoblast ; the thread partly everted. 

 C 1 , a large oval nematocyst with the thread coiled up, enclosed in a 

 cnidoblast ; en, cnidocil. C*, C 3 , stages in the development of a nematocyst. 

 D, an elongate oval nematocyst, showing the short relatively thick thread 

 everted. E, a small oval nematocyst with everted coiled up thread ; the 

 cnidoblast is produced below into a long contractile process. 



surface is covered by a number of warty projections beset with 



