BEGINNINGS OF CO-OPERATION 63 



materials, and second against the ill effects of expo- 

 sure to lethal ultra-violet rays. To complete the pic- 

 ture I have now to describe the results of exposing 

 animals to harmful conditions in which the difficulty 

 is caused by the absence of elements normally pres- 

 ent in their natural environment. The experiment 

 has been made on aquatic animals in a number of 

 ways, for example, by putting fresh-water animals 

 into distilled water; but it is easier to demonstrate 

 when marine animals are placed in fresh water. 



Again I select one experimental case from several 

 available. Near Woods Hole, on Cape Cod, a small 

 flatworm Procerodes (Figure 5) lives in certain re- 

 stricted areas in large numbers. They are most abun- 

 dant along a stony stretch at about the low tidemark 

 or a little beyond it. (5) There, if one finds the proper 

 location, one may take from ten to fifty flatworms 

 from the lower surface of a single stone. Usually 

 they are more or less clumped together. They are 

 not easy to see since each is only a few millimeters 

 long and all are of a dull gray color. Once seen, 

 they are hard to detach, for the posterior end has a 

 muscular sucker, by means of which the animal can 

 cling pretty securely even to smooth stones. When 

 these worms are put into fresh water, pond water for 

 example, they swell greatly and soon begin to dis- 



integrate. 



