64 



THE SOCIAL LIFE OF ANIMALS 



If these flatworms are washed thoroughly to re- 

 move sea water from their surfaces, and then placed 

 in fresh water, a certain proportion of the grouped 







Fig. 5. The small marine flatworm Procerodes. 



animals survive decidedly longer than isolated worms. 

 The first worms to die in the group do so almost 

 as soon as the first isolated worms. As the dead worm 

 disintegrates it changes the surrounding water; we 

 say it conditions it; and as a result of this condition- 

 ing the remaining worms of the group have a bet- 

 ter chance of life. 



