344 ELEMENTARY BIOLOGY. [CHAP. 



body, but they are not unfrequently restricted to the free 

 end, at or near the bases of the tentacles. Within them 

 (testes) great numbers of minute spermatozoa, each moved 

 by a vibratile cilium, are developed and eventually set free. 



The former are large globular bodies, from one to eight 

 in number, usually formed near the attached end of the 

 polype. Each becomes much larger than the testis, and is 

 the ovary. Within it is developed a single large egg-cell, or 

 ovum. This ovum, which is a huge nucleated cell, is im- 

 pregnated by the spermatozoa and undergoes division into 

 two parts. Each of these again divides into two; and so 

 on, until the ovum is broken up into a number of small 

 embryo-cells. The mass of embryo-cells thus formed be- 

 comes surrounded with a thick, usually tuberculated or 

 spinous, case; and, detaching itself from the body, forms 

 the 'egg,' from which a new Hydra is developed. 



Microscopic examination shews that the body of the 

 Hydra is a sac, the wall of which is composed of two 

 membranes, a transparent outer (ectoderm), and a coloured 

 inner (endoderm). The tentacles are tubular processes of 

 the sac, and therefore are formed externally by the ectoderm 

 and lined internally by the endoderm. Both the endoderm 

 and the ectoderm are made up of nucleated cells; the inner 

 ends of certain of these are prolonged into delicate fibres, 

 those of the ectoderm, which are most marked, running 

 parallel with the long axis of the body. The green colour 

 of the Hydra viridis results from the presence of chloro- 

 phyll grains imbedded in the protoplasm of the endoderm 

 cells. 



The cells of the ectoderm, and especially that of the 

 tentacles, contain very singular bodies, the so-called 

 Urticating capsules, thread cells, or nematocysts. These are 

 oval bags, with thick and elastic walls, containing a spirally 



