THE CONQUEST OF THE DRY LAND 193 



needed in the pursuit of prey or the avoidance 

 of enemies. And as the acquisition of bilateral 

 symmetry was associated with the acquisition 

 of head-brains, we may say that it was the be- 

 ginning 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 colonisa- 

 tion was attempted by simple bilateral worms. 



The most important members of the " worm- 

 invasion" were the earthworms, which prob- 

 ably evolved from a fresh-water stock. This 

 is suggested 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 impor- 

 tance 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 frag- 

 ments as they are able to capture on the surface. 



We know that earthworms have been land 

 animals from very early times, for, though no 



