THE CONQUEST OF VHE 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 
