254 



COMPARATIVE ANATOMY 



surface is covered by a number of warty projections beset with 

 minute stiff hair-like projections. Further examination shows 



54- 



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

 bd, basal disc ; en, endoderm ; ec, ectoderm ; gv, gastro-vascular 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 ; nig, 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 1 - 2 , C'^, stages in the development of a nematocyst. 

 Z>, 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. 



that each warty prominence is a lump of protoplasm, a single 

 ectoderm cell, in fact, in which a number of highly refringent 



