THE CONQUEST OF THE DRY LAND 193 



of prey or the avoidance of enemies. And as 

 the acquisition of bilateral symmetry was associ- 

 ated with the acquisition of head-brains, we may 

 say that it was the beginning of our knowing 

 our right hand from our left. In any case, 

 radial symmetry is out of the question on dry 

 land, and the first colonisation was attempted 

 by simple bilateral worms. 



The most important members of the "worm- 

 invasion " were the earthworms, which probably 

 evolved from a fresh-water stock. This is sug- 

 gested by earthworms like Alma and Dero, 

 which have gills, and there are many not very 

 distant relatives of earthworms now at home in 

 fresh water, such as Nais and Tubifex, common 

 in streams. The importance of earthworms in 

 the conquest of the dry land is well known, for 

 they have made the fertile soil of the globe. 

 Their successful possession of the subterranean 

 world at an early date implies the previous 

 establishment of some terrestrial vegetation, for 

 earthworms depend for food on the plant 

 remains in the earth, which they swallow, and 

 on such fragments as they are able to capture 

 on the surface. 



We know that earthworms have been land 

 animals from very early times, for, though no 

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