192 THE HAUNTS OF LIFE 



the great seaweeds might be gradually trans- 

 formed into terrestrial plants. Who knows ? 



THE THREE GREAT INVASIONS OF THE 

 DRY LAND 



In the conquest of the dry land we can dis- 

 tinguish three great invasions or colonisations. 

 The first was the Worm- Invasion, led by simple 

 worms such as the land-planarians, which had 

 begun the profitable habit of moving with one 

 end of the body always in front. In marine 

 animals of comparatively low degree, such as 

 jelly-fishes and swimming-bells, sea-anemones 

 and corals, the symmetry of the body is more 

 or less radial, that is to say, there is no right 

 or left, no head- or tail-end. The body can be 

 cut into two almost identical halves along many 

 different planes. Radial symmetry may be 

 illustrated by an orange and by the circular 

 plate it rests on. It is well suited for easy- 

 going life, for drifting in the sea, or for waiting 

 for food to drop into the mouth. But certain 

 worms acquired bilateral symmetry, moving 

 with one end of the body always in front. 

 This was better suited for quick and definite 

 movements, such as are needed in the pursuit 



