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 sim- 
ple worms such as the land-planarians, which 
had begun the profitable habit of moving with 
one end of the body always in front. In ma- 
rine 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 
