212 ANIMAL AGGREGATIONS 



adsorbed on the slime. With other animals observed, it may be 

 removed by adsorption on the surfaces of the animals themselves. 

 Finally, as would be expected from this mechanism, we have dem- 

 onstrated that the protection furnished by the mass, is, at least to a 

 considerable extent, independent of the species present. 



Our experiments do not support the hypothesis that group pro- 

 tection among these aquatic animals is furnished by the rapid pro- 

 duction in the presence of a toxic agent, such as colloidal silver, of a 

 more or less mysterious autoprotective secretion. It is true that 

 the production of slime by planarians actually serves as an auto- 

 protective agent, not only in fixing colloidal silver in these experi- 

 ments, but very probably in protecting the planarians from sudden 

 changes in culture or habitat water. But slime production cannot 

 be regarded as a specific autoprotective 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. 



OTHER CASES OF GROUP PROTECTION 



Bresslau (1924) found that Protozoa give off a substance which 

 he calls "tektin," a mucin-like body, which is given off in greater 

 abundance when the animals are stimulated by heat, pressure, meth- 

 ylene blue, iodine, etc. The tektin, when given off, takes up water 

 rapidly and exhibits strong surface activity, adsorbing foreign par- 

 ticles readily. Bresslau tells of putting 2 cc. of liquid from a culture 

 of Infusoria (either Colpidium or Paramecium) containing many in- 

 dividuals in one dish and a similar amount from the same culture, 

 but with few individuals, into another. Into each he introduced i 

 cc. of I per cent solution of methylene blue. Both produced the 

 tektin and adsorbed this poison : the culture with the many animals 

 so much more completely that the possibility of surviving the toxic 

 action of the poisonous material was greatly increased. 



Carpenter (1927) approached this problem from a wholly different 

 angle. Without referring to the work of Drzewina and Bohn, she 



