CROWDING AND INCREASED DEATH-RATE 141 



reacts similarly. These investigators believe that the planarians give 

 off a substance which causes autodestruction, and that, if this be 

 true, such destruction is hastened by increasing the mass of indi- 

 viduals in proportion to the amount of liquid. 



Their interpretation is supported by the observation that if fresh 

 Planaria are introduced into a solution of KCl which has already 

 contained others, their death is hastened. If, after some time in such 

 a solution, a part are removed to a new solution of similar strength 

 of KCl, these die more slowly than do their fellows left in the original 

 solution, which contained not only KCl but also some substance or 

 substances given off by the worms themselves. 



Later (1928) they observed that around cytolyzing flatworms the 

 H-ion concentration (acidity) of the solution is greatly increased, and 

 they considered this to be the factor which causes the increased 

 mortahty of the groups. The larger the number of cytolyzing in- 

 dividuals, the more rapid and the greater the increase of the H-ion 

 concentration, and consequently the more rapid and pronounced are 

 the lethal elTects involved. Such a process accelerates itself in the 

 presence of many individuals, or, on the other hand, is not effective 

 in the case of a few scattered animals. If the latter die, they do so 

 because of the lethal effect of the KCl alone. 



Fowler (1927, 193 1) undertook to test the effect of a large number 

 of electrolytes upon the rate of survival in certain crustaceans, using 

 mainly a species of Daphnia. His results show that there is a distinct 

 correlation between the survival value of the group and the degree 

 of toxicity of the salt solutions employed. Tests upon the rate of 

 oxygen consumption have shown that crowded Daphnia, in the con- 

 centrations tested, use less oxygen per individual than do isolated 

 Daphnia under similar conditions. When the toxicity is sufficiently 

 great so that death occurs within a relatively short time, this group 

 depression tends to favor group survival. On the other hand, when 

 the concentration is low and the effect of the chemicals is de- 

 ferred, the isolated individuals live longer than the group. Such re- 

 sults are in accord with Child's (191 5) differential susceptibility find- 

 ings when planarians are subjected to relatively strong or relatively 

 weak concentrations of various toxic agents, particularly KCN. 



