FEEDING OF MARINE ANIMALS 215 



needs a more varied diet than that supphed 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 fiatworm dies of 

 starvation, though not before it has laid large numbers 

 of eggs for the maintenance of the race. This may 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 algae in return 

 for surplus food, have finally become entirely dependent, 

 essentially parasitic, upon them, and cannot even develop 

 if they are absent. 



In the Giant Clams and their allies (Tridacnidcs) the 

 animals farm zooxanthellae in the greatly thickened tissues 

 which project between the open shell valves. In this case 

 the animals retain the power of feeding on minute plankton, 

 like all bivalve molluscs, and, although the contained 

 algae form a valuable subsidiary source of food, the animal 

 never becomes completely dependent on the plants, and, 

 in consequence, never suffers premature death like Con- 

 voluta. In the corals the zooxanthellae, as already noted 

 in Chapter VII, are never digested but they certainly act 

 as automatic organs of excretion. The possible significance 

 of this has already been discussed. 



