THE COMMON HYDRA OF BENGAL. 1353 



one " tank. ' ' This pool has at least three attractions : — (i) it is deep in the centre and has 

 shallow edges ; (2) it supports a luxuriant growth of the water-plant I y imnanthemum 

 and, (3) it is densely shaded at one side. The formation of the pool is beneficial to the 

 polyp for the following reasons : — the shallow edges afford abundant food during the 

 cold weather, while the deeper centre offers a comparatively cool retreat during summer. 

 The leaves of Limnanthemum, which float on the surface and are attached to a long 

 stalk growing up from the bottom, are the favourite station of the polyp during 

 winter and early spring. Habitually it remains attached to their lower surface, with 

 its body hanging down in the water ; but if the sunlight grows too strong, it 

 makes its way down the stems of the plant and takes shelter among the young leaves 

 which have not yet reached the surface, or the roots which grow out from their base. 

 Should this position become uncomfortable in the unshaded part of the ' ' tank, ' ' the 

 animal moves to those parts which are shadowed by trees and rushes. I have pointed 

 out elsewhere ' that snails of the genus Paludina play an important part in such migra- 

 tions. If the temperature rises suddenly, Hydra will be found not among the floating 

 leaves of Limnanthemum, but on the submerged portion of the plant. After a 

 high temperature which has remained steady or risen for several weeks, the polyp 

 forsakes the open part of the pool altogether ; while during summer it is found very 

 rarely in shallow water. On one occasion, however, towards the end of May, during a 

 few days' comparatively cool weather, I found large numbers of individuals at the 

 shaded edge of the pond. 



In captivity three circumstances are most inimical : firstly, a sudden rise in the tem- 

 perature, which may either kill the polyp directly or cause it to hasten its decease 

 by becoming sexually mature ; secondly, the lack of a free current of air on the surface 

 of the aquarium ; and, thirdly, the growth of a bacterium, which forms a scum on 

 the top of the water and clogs up the interstices between the leaves and stems of the 

 water-plants, soon killing them. If adult polyps are kept even in a shallow opaque 

 vessel which is shut up in a room with closed shutters they generally die in a single 

 night ; indeed, they rarely survive for more than a few days unless the vessel is placed 

 in such a position that air is moving almost continuously over its surface. The 

 bacterium to which I allude practically seals up the aquarium, especially in March and 

 April, when its growth is very rapid. Strands of slime produced by it often surround 

 the polyp and even enter its mouth. In this event the polyp retracts its tentacles until 

 they become mere prominences on its disk, and shrinks greatly in size. The pigment 

 in its body becomes broken up into irregular patches owing to degeneracy of the 

 endoderm cells, and it dies within a few hours. (As I have dealt elsewhere 2 with the 

 enemies and the prey of Hydra orientalis, I need only refer here to the Chironomid 

 larva which feeds on it in Calcutta.) 



Adaptability to unfavourable circumstances is more marked in young than in 

 older individuals. When adults die in an aquarium owing to a rise in temperature or 



J Journ. Asiat. Soc. Bengal, III, 1906, p. ill. 2 Ibid., p. 112. 



