90 FRESH-WATER ANIMALS 



years, and there is no doubt that their geographical 

 distribution, and their persistence as fresh-water 

 forms is largely dependent on this power, for while 

 in the desiccated condition they can withstand the 

 action of cold or drought, and can also be blown 

 by the wind for very considerable distances from 

 pond to pond, or stream to stream. 



Polyzoa again, of which some of the most beauti- 

 ful and interesting forms are fresh -water, exhibit 

 special modifications in accordance with their 

 habitat. Almost all the fresh-water Polyzoa give 

 rise to specially protected buds or " statoblasts," 

 which are collections of cells very similar to the 

 gemmules of Spongilla,* and like these, enclosed in 

 hard protective capsules. These statoblasts survive 

 the winter, which is usually fatal to the parent, 

 and in the spring a young fully formed Polyzoon 

 emerges from each, and at once adopts the mode 

 of life of the parent. 



Among the Entomostraca, such as Daphnia 

 and Cyclops, we meet with what seem at first 

 sight to be striking exceptions to the rules we have 

 laid down concerning fresh- water animals ; for 

 while some, such as Daphnia, produce large and 

 specially protected eggs which hatch in the full 

 form of the parent, yet a very large number of 

 others, such as Cyclops and Cypris, produce very 

 small eggs, from which small free-swimming 

 embryos arise. We may note however that in 



* These have since been shown (Ephydatia Mullen) to retain 

 the power of giving rise to free embryos after two years desicca- 

 tion. ED. 



