174 



Aquatic Organisms 



FIG. 82. Dero, in its case made of floating seeds. 



Aquatic earthworms, more like the well-known 

 terrestrial species, burrow deeply into the mud of the 

 pond bottom. 



Other worms occur in the water in great variety; we 

 have mentioned only a few of the commonest, and 

 those most frequently seen. There are many parasitic 

 worms that appear in the water for only a brief period 

 of their lives: hair-worms (Gordius, etc.), which are 

 freed from the bodies of insects and other animals in 

 which they have developed; these often appear in 

 watering troughs and were once widely believed to 

 have generated from horse-hairs fallen into the water. 

 There are larval stages (Cercaria) of Cestodes and 

 others, found living in the water for only a brief interval 



of passage from one 

 host animal to ano- 

 ther. There are 

 smaller groups also 

 like the Nemertine 

 worms, sparingly 

 represented in fresh- 

 water ; for informa- 

 tion concerning 

 these the reader is 

 referred to the 

 larger textbooks of 

 zoology. 



FIG. 83. Tubifex in the bottom mud. 



