172 THE LIFE OF CRUSTACEA 



serve the same purpose as the resting eggs of other 

 species. 



Numerous species of Ostracods, belonging to the 

 genus Cypris (see Fig. 13, B, p. 38), and other closely 

 related genera, occur in fresh water. Like the 

 Cladocera, they reproduce largely by partheno- 

 genesis, and the males of many species are rarely 

 found, while in some species they have not j'et been 

 discovered. In Professor Weismann's laboratory 

 at Freiburg a colony of Cypris was kept in an 

 aquarium for eight years, and during the whole of 

 that time no males made their appearance, the 

 colony reproducing exclusively by parthenogenesis. 

 Probably in all species the eggs survive drying. 



The common '* Freshwater Shrimp " {Gammarus 

 pulex), which has already been described, may be 

 taken as a type of a large number of Amphipoda, for 

 the most part closely allied, which are widely dis- 

 tributed in most regions of the world, with the 

 exception of the tropics. G. pulex itself ranges from 

 the British Islands to Mongolia. As the eggs 

 are carried, till they hatch, in the brood-pouch of 

 the parent, and are not known to survive drying, it 

 is difficult to understand in what way Gammarus 

 and its allies contrive to spread from one locality 

 to another. 



The little fresh-water Isopod Aselhis aquaticus 

 (Fig. 60) is common in ponds and canals in this 

 country. It may be recognized by its general resem- 



