.] THE FRESH-WATER POLYPES. 97 



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

 lay be found in other species. It becomes much larger 

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



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

 ucleated cell, is impregnated by the spermatozoa andl/>Jil' 

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

 ides into two; and so on, until the ovum is broken up into 



number of small embryo-cells. The mass of embryo-cells 

 hus formed becomes surrounded with a thick, usually tuber- 

 ulated or spinous, case ; and, detacbing itself from the body, 

 orms the ' egg,' from wbich a new Hydra is developed. 



Microscopic examination shews that the body of the 

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

 oembranes, an outer (ectoderm), and an inner {endoderm). 

 The tentacles are tubular processes of the sac, and therefore 

 .re formed externally by the ectoderm and lined internally 

 )y tbe endoderm. Both the endoderm and the ectoderm 

 tre made up of nucleated cells; the inner ends of those 

 )f the ectoderm being prolonged into delicate fibres, which 

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

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

 ■esults from the presence of chlorophyll grains imbedded 

 n the protoplasm of the cells. 



In both the ectoderm and the endoderm the protoplasm p^ 

 )f the cells contains very singular bodies, — the so-called 

 irticating capsules, thread-cells, or nematocysts — which are 

 )val bags, with thick and elastic walls, containing a spirally 

 ?oiled-up filament which is unrolled suddenly on the 

 slightest pressure and then presents the appearance of a long 

 ilament attached to the capsule, and often provided with 

 ;hree recurved spines near its base. As similar capsules of a 

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

 itin<T severely, just as nettles do when they are handled, there 

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



7 



