CLASSIFICATION OF ARTICULATA. 83 



and the nervous system being very indistinct, never presenting 

 the double ventral cord in a well-developed condition: these 

 have very frequently no distinct head. 



VIII. ROTIFERA, or Wheel- Animalcules, a group of minute 

 animals, in which the Articulated structure is often very indis- 

 tinct, and which are especially characterised by the possession of 

 cilia, or little hair-like filaments, arranged in rows in the neigh- 

 bourhood of the mouth, and serving by their movements, both 

 for the acquirement of food, and for the aeration of the fluids of 

 the body. 



