X.] THE FRESH-WATER POLYPES. 97 



polype may be single, as in Hydra viridis, or as many as eight 

 may be found in other species. It becomes much larger 

 than the testis and is the ovary. Within it is developed 

 a single large egg, or ovum. This ovum, which is a huge 

 nucleated cell, is impregnated by the spermatozoa and 

 undergoes division into two parts. Each of these again di- 

 vides 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 becomes surrounded with a thick, usiially tuber- 

 culated 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, an outer (ectoderm), and an 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 those 

 of the ectoderm being prolonged into delicate fibres, which 

 run parallel with the long axis of the body on the inner 

 face of the ectoderm. The green colour of the Hydra viridis 

 results from the presence of chlorophyll grains imbedded 

 in the protoplasm of the cells. 



In both the ectoderm and the endoderm the protoplasm 

 of the cells contains very singular bodies, the so-called 

 urticating capsules, thread-cells, or nematocysts which are 

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

 coiled-up filament which is unrolled suddenly on the 

 slightest pressure and then presents the appearance of a long 

 filament attached to the capsule, and often provided with 

 three recurved spines near its base. As similar capsules of a 

 larger size are the agents by which many of the jelly fishes 

 sting severely, just as nettles do when they are handled, there 

 is every reason to believe that the thread-cells of the Hydra 

 M. 7 



