STUDIES IX A XI. MAT. A< ;<;KK< lATlOX S. 3 I 3 



by planarians actually serves as an auto-protective agent, not only 

 in fixing colloidal silver in these experiments, but very probably in 

 protecting the planarians from sudden changes in culture or habi- 

 tat water. But slime production cannot be regarded as a specific 

 auto-protective secretion, either in the sense that it is used for no 

 other purposes, for obviously it plays many other roles in the 

 economy of slime producing organisms, or in the sense that slime 

 is limited in its protecting power to the one species producing it. 

 since the protection furnished by aggregations of mixed species 

 is easily demonstrated. 



It is obvious that our experiments do not demonstrate that even 

 in these cases there is no production of a more subtle sort of auto- 

 protective secretion. Conclusions on this point must remain ten- 

 tative pending the completion of appropriate quantitative chemical 

 determinations. These experiments do make it unnecessary to as- 

 sume the production of such a substance in order to explain group 

 protection in all cases. Even the experiments with dead isopods 

 are not conclusive on this point. In these experiments, the pres- 

 ence of freshly killed Ascllus, in numbers, furnished marked pro- 

 tection for the solitary planarians. The supposition can be ad- 

 vanced that this protection was due to the diff vision of some chemi- 

 cal from the dead bodies. These experiments do demonstrate 

 that the presence even of other living organisms is unnecessary for 

 protection from the toxic effect of colloidal silver. So far as that 

 is concerned, any other inert substance capable of fixing the col- 

 loidal silver by adsorption or otherwise should have the same 

 effect. 



This general phenomenon of the uptake and fixing of dyes and 

 other possibly toxic reagents is well known, particularly by plant 

 physiologists, who have developed a considerable literature con- 

 cerning the method of fixation of such substances by living plants 

 (cf. Scarth). The same phenomenon has been described for 

 animals, and the protective value it furnished to a mass of animals 

 has been recognized particularly by Bresslau. 



In their work upon the relation between mass of animals present 

 and toxic reagents. Drzewina and Bohn found that in the presence 

 of certain chemicals, principally KG, members of the group died 

 more rapidly than did isolated animals. They explain such results 



