186 FRESHWATER FAUNA 



are regularly laid on to form a second coat around the 

 first (Fig. 57, C). The finished product is a little round 

 ball, containing living sponge cells in a state of hiberna- 

 tion, with a protecting double envelope, which is con- 

 tinuous save for one small hole. The rest of the sponge 

 now disappears by death and decay, leaving only its bare 

 skeleton and a multitude of the little seed-like balls or 

 gemmules behind. Each gemmule is capable of giving 

 rise to a young sponge, a fact that may be easily proved 

 by any one who cares to make the experiment ; the 

 gemmules are not hard to find, and when obtained they 

 should be dried and kept dry till spring : then if placed in 



FIG. 58. 1. Egg of Hydra viridis enclosed 

 in a horny shell. 2. Egg of Hydra 

 grisea, with the horny envelope pro- 

 duced into spines. Both magnified. 

 From Bronn's " Thierreichs." 



fresh water a young sponge will soon make its appearance, 

 creeping out of the small hole left for its exit. 



A simple but important distinction is to be observed 

 among the gemmules. Thus some are firmly attached to 

 the skeleton of the dead sponge ; these are the inheritors of 

 the estate, and the young sponges to which they give rise 

 clothe the skeleton of their parent with new flesh ; but a 

 host of others are not so attached, and these are free to 

 float away with the stream, with a good chance of being 

 washed ashore along its banks. They may then be dried 

 by the wind and carried away by it for long distances, 

 perhaps to other river systems. A vast number, no doubt, 



