THE FISHES, MOLLUSCA, AND OTHER OBJECTS. 57 



He noticed that, in winter, many specimens of Hydra 

 vulgaris were studded with little round, white lumps. 

 These burst, and extruded spermatozoa, as seen in 



Fig. 53. 



Hydra attacking Water-flea, magnified 



Pig. 50, fc. The sperm cells and the ovi-sacs are found 

 in the same polyp, and the ovum sinks to the bottom 

 of the water and lies hidden in the mud. It is from 



