136 ZOOLOGY 



tions are said to have the power of slowly dissolving sand 

 and thus of turning it into soiL Darwin says, " It is a 

 marvellous reflection that the whole of the superficial mould 

 over any smooth expanse has passed, and will again pass, 

 every few years, through the bodies of worms. The plough 

 is one of the most ancient and most valuable of man's 

 inventions, but long before he existed the land was, in 

 fact, regularly ploughed, and still continues to be thus 

 ploughed, by earthworms." 



The group to which earthworms belong is closely related 

 to that which includes Nereis of the seashore. They are 

 both ringed worms or Annelids. 1 But whereas Nereis and 



O 



its allies have parapodia provided with numerous bristles, 

 the earthworm has 110 parapodia, and only a few bristles 

 on each segment. The group to which Nereis and its 

 allies belong is called (see Chapter X) Polychseta, and 

 the group to which the earthworm belongs is appropriately 

 named Oligocliceta. 



The aquatic Oligochaeta 2 are among the commonest 

 inhabitants of ponds and ditches, living sometimes in the 

 mud and sometimes at the surface of the water. 



Tubifex 3 is common in slow-running brooks, and lives 

 in the mud of the bottom, forming tubes in it. The 

 thread-like bodies of the worm are stretched up beyond 

 the surface of the mud and wave in the water in graceful 

 undulations. Often the worms are so numerous that their 

 reddish color gives a decided tinge to the bottom. They 

 thrive well in fresh- water aquaria. 



1 Annulus, a little ring. 



2 A key for the determination of the principal families of aquatic Oligo- 

 chseta is given in the Appendix to this Chapter, page 144. 



3 tubus, tube ; face re, to make. 



