88 FRESH-WATER ANIMALS 



to their protective capsules they are enabled to 

 survive the cold, and other adverse conditions of 

 the winter months, which are often fatal to the 

 parent sponge; and in the spring the capsule 

 ruptures, and the contained cells crawling out 

 commence to develop into little sponges. Such 

 gemmules, or clusters of specially protected cells, 

 are unknown among marine sponges, and there can 

 be no doubt that their occurrence in Spongilla is to 

 be regarded as a special adaptation to the fresh- 

 water habits of this genus. 



Among Ccelenterates the production of small free 

 swimming ciliated embryos is almost universal, 

 and this is almost certainly the reason why so 

 very few fresh -water members of the group are 

 found. Of those that do occur, the best known is 

 the fresh-water Hydra, whose marvellous powers 

 of recovery from injury have been so admirably 

 investigated and described by Mr. Dunkerley.* As 

 regards its life-history Hydra exhibits several 

 peculiarities, which I believe, are to be associated 

 with its fresh-water habits. In the first place, un- 

 like its marine allies, it not only does not give 

 rise to free-swimming reproductive zooids or jelly- 

 fish, but does not even show the slightest tendency 

 to form such zooids. Secondly, only a single 

 ovum, and this of very large size, is found in the 

 ovary, a point in which Hydra is unique among 

 Ccelenterates, and a point the special advantage of 

 which, in fresh-water forms, has already been fully 



* " Hydra : its Anatomy and Development," by J. W. Dun- 

 kerley, F.R.M.S. 



