FRESH WATER ANIMALS . 209 



in the sea. This consists of creatures the reverse of those 

 just mentioned which, wholly aquatic when adult, in their 

 younger stages or as eggs or dormant bits of tissue can be 

 carried overland or through the air. 



The gemmules of fresh water sponges and the statoblasts of 

 polyzoans are formed within their bodies; on their death they 

 float away and by their barbed spines become attached to 

 birds and other creatures which carry them from pond to 

 pond. Fresh water clams have curious young like Httle veil 

 pins which attach themselves to fish or legs of birds. By this 

 means the clams are able to ascend the swiftest streams and to 

 pass overland from one stream to another. Many fish, like 

 perch, have sticky eggs, and these are sometimes carried on 

 the feet of birds. 



We have already mentioned fairy-shrimps and how they get 

 about; many other of the small crustaceans and very many of 

 the rotifers resemble these in this respect, while very many of 

 the protozoans are similarly transported in a dormant resting 



stage. 



Thanks to this means of transportation pond hfe is every- 

 where about us; our gardens, lawns and trees are dusted with 

 aquatic animals in the egg or resting stages, and they are in 

 our houses almost everywhere. This sounds improbable, I 

 know; but take some withered grass or hay or, better still, 

 bark from the fire-wood in the cellar, and put it in a glass of 

 water. In a week or so the water will be brown and foul with 

 a scum of algae on the surface. Look at a drop under a mi- 

 croscope; it swarms with life, mostly with protozoans and a 

 few kinds of rotifers, but you sometimes find other things. 

 From every different sort of culture you get a different com- 

 plex of this water life. The richest cultures that I raised last 

 winter were from some leaf-mould from a damp ravine and 

 from the moss used to pack some rose plants sent me. This 

 last yielded gastrotrichas in abundance and copepods and 

 nematodes and other things. Bark dust swept from the cellar 

 floor was very rich, and the finest place I know for vorticellas 



