FEEDING OF MARINE ANIMALS 215 



plants cannot form their starch, it has to expose itself to 

 the full glare of the sun for as long as possible, that is, 

 be exposed on the sand for the whole time the tide is out ; 

 when the tide returns it has to burrow or it would be washed 

 away. 



But, owing probably to the fact that the Convoluta 

 needs a more varied diet than that supplied by starch, it 

 begins after a time to feed upon the algae — to kill the geese 

 which laid the golden eggs — so that they gradually disappear, 

 their owner at this stage presenting the strange appearance 

 of an animal with a green head and a white tail. Finally, 

 having killed its best friends, the little flatworm dies of 

 starvation, though not before it has laid large numbers 

 of eggs for the maintenance of the race. This ma)^ be 

 considered a case of symbiosis which has gone too far, for, 

 although the algae can live freely in the sea and they are 

 by no means dependent upon the flatworms, the latter, 

 after originally no doubt sheltering the algse in return 

 for surplus food, have finally become entirely dependent, 

 essentially parasitic, upon them, and cannot even develop 

 if they are absent. 



