112 Records oi the Indian Museum. [Vol. XVI, 



and is stated by natives of these places (who are well acquainted 

 with it under the names " water-wheels" or " water-flowers") to 

 be absent at all other times of the year. In the pools it feeds 

 on minute Crustacea, which are probably alwa3''3 to be found in 

 abundance at the bottom, near which the medusa spends most 

 of its time. All attempts either to breed the animal in captivity 

 or to discover what becomes of the species when the medusa is not 

 present have hitherto failed. Investigation of the latter point is 

 rendered difficult at Medha by the inequalities of the rocky basin 

 of the pool. I found that a heavy dredge invariably caught 

 in projections of the rock, but succeeded by careful manipulation 

 in dragging a D-net over the bottom and in bringing up pebbles 

 and vegetable debris from it. I could detect no organism of a 

 coelenterate nature in this matter ; nor could I find any trace 

 of a parasitic or quasiparasitic stage ou or in any of the fish, 

 molluscs or crustaceans of the pool. The structure of the stream 

 and of the sir rounding country renders it almost impossible that 

 the medusa could be introduced periodically ; at other localities 

 it appears in bodies of water completely isolated at the time of its 

 appearance. We know that it is killed off annually by the red mud 

 brought down from higher up stream in the first summer freshets. 

 We knov.' also that sexual reproduction is active immediately 

 before this occurs, and evidence that asexual budding ever occurs 

 is altogether lacking. I am sure no ordinary hydroid, if at all 

 abundant, could have escaped my notice in the investigations I 

 undertook at Medha, while Mr. Agharkar's experiments at the 

 same place had equally negative results. The most probable 

 explanation seems to me to be that the hydrozoon remains for the 

 greater part of the year in an encysted condition and that the 

 medusa becomes fully developed in favourable conditions of 

 temperature, as soon as the hot season is fully established. The 

 union of ova and spermatozoa probably gives rise to planulae 

 of the type usual in hydroids. Probably those planulae which are 

 produced in the earlier part of the brief season of activity 

 develop directly into medusae, bu': there may be a short-lived 

 hydroid generation. Those planulae, however, which have not 

 undergone further development at the time when the water is 

 rendered unfit for the activities of the medusa by the freshets, 

 perhaps become encysted and lie on the bottom until conditions 

 favourable for active life return Their cysts may be very minute 

 and have no definite characters by which they could be recognized. 

 I know of no case' in the Hydrozoa precisely parallel, but that of 

 the encysted embryo of Hydra is similar and encystment is a 

 phenomenon of such common occurrence in many diverse groups 

 of freshwater invertebrates that it would not be at all surprising 

 to find it in Lintnocnida. 



1 In Annulella, an anomalous hydroid from the brackish water of the 

 Gangetic delta, an encysted resting stage has been described.. See Ritchie, 

 Rec. Ind. Mtis., XI, p. 552, pi. xxxa, fig. 9 (1915). 



