532 EVOLUTION OF ANIMALS Part V 



Characteristics of Ecology and Form of Nematodes 



Nematodes live everywhere that animals can exist. Great numbers of minute 

 free-living ones stir and enrich the soil. In both soil and water they constitute 

 links in the food chains that reach to higher animals. As parasites, large num- 

 bers of them are physiologically intimate with many species of plants and 

 animals. 



They are slender, cylindrical, and covered with a protective cuticle. They 

 have a functional body cavity containing organs, but not a true coelom. The 

 digestive tract is a canal with mouth and anal openings. The sexes are separate. 

 Ectoderm, mesoderm, and endoderm are present. 



The movements of nematodes are distinctive; swinging and thrashing due 

 almost completely to the use of longitudinal muscles. 



