THE COMMON HYDRA OF BENGAL. 359 



coloured individuals lies in the granules which are rejected by polyps in unfavourable 

 circumstances, there can, I think, be little doubt. My statement of the facts is founded 

 on a large series of observations. It would appear, therefore, that the cells of the 

 endoderm are unable to ingest solid particles except when the whole organism is in a 

 vigorous state, and that if it is not vigorous they can only absorb the liquid constituents 

 of the food. Probably temperature plays an important part in this physiological 

 change. 



*s* 



Summary. 



Hydra orientalis is related to H. gvisea, which it resembles in the structure of its 

 normal egg. The number of tentacles differs at different seasons, in different genera- 

 tions. The limits of the geographical distribution of the Bengal species are still 

 unknown. 



Comparatively few bads are produced. During winter they appear in a definite or- 

 der, which ceases to be constant as the temperature rises in spring. Vertical fission oc- 

 curs occasionally. The species is dioecious ; but sexual reproduction appears not to play 

 an important part in its biology. A rise in temperature induces a proportion of the 

 individuals present in an aquarium or a pond to develop testes ; if considerable, it may 

 induce a few of those that remain to produce eggs. As a result of exhaustion eggs are 

 sometimes produced which do not secrete a horny outer shell. Individuals are short- 

 lived and perish after sexual reproduction, several generations being completed in a 

 year. 



The organism is more delicate than in the case of European species. The animal 

 does not adapt itself readily to a change of conditions ; but young individuals do so 

 more readily than older ones. Heat is most inimical to its life. 



As regards the capture and ingestion of food H. orientalis resembles European 

 members of the genus. Mucus secreted in the alimentary canal plays an important part 

 in the process. The more solid parts of the food do not descend to the base of the alimen- 

 tary canal but are retained in the upper part. 



H. orientalis, unlike northern species, moves away from light, probably because it 

 is repelled by heat. It progresses chiefly by crawling, the body not being looped. 

 Young individuals are distributed more readily than older ones, because their specific 

 gravity, when they are fully expanded, closely resembles that of water, and because, 

 perhaps consequently, they are more active in their habits. 



Colour is not due to a symbiotic organism but to the presence in the endoderm cells 

 of solid particles derived from the food. Under unfavourable conditions these cells be- 

 come incapable of ingesting solid particles and consequently the polyp loses its colour. 



