BEGINNINGS OF CO-OPERATION 65 



For more careful experimentation, a sort of worm 

 soup was prepared by killing a number of well- 

 washed worms and allowing them to remain in the 

 water in which they had died and so condition it. 

 Freshly collected Procerodes lived longer in such 

 conditioned water than their fellows which were 

 isolated into uncontaminated, clean pond water. The 

 difference between the two waters was only that 

 caused by the fact that in one the worms had died 

 and disintegrated, while the other was clean. This 

 difference in survival persisted even when, to make 

 the test more revealing, the total amount of salt in 

 the two waters was made identical by adding some 

 dilute sea water to the clean pond water. Results 

 from these experiments are shown in Figure 6. In 

 this chart, distance above the base line gives the 

 percentage of survival, and distance to the right 

 shows time of exposure. It will be noted that the 

 worms lived decidedly longer in the conditioned 

 water than they did in dilute sea water of the same 

 strength of salts. 



The mechanism of this superficially mysterious 

 group protection is now known. (86) The dead and 

 disintegrating worms, or more slowly, the living 

 worms, give off calcium into the surrounding water, 

 and calcium has a protective action for marine ani- 

 mals placed in fresh water or for fresh-water animals 



