312 W. C. ALLEE AND J. F. SCHUKTT. 



A still more severe test of the specificity of the protection was 

 made by placing recently killed Asellus in glass dishes containing 

 10 cc. of water plus two drops of colloidal silver, and introducing 

 with these dead isopods one Planaria dorotocephala. At the end 

 of 7 hours exposure, when 20 planarians isolated into a similar 

 volume of the same concentration showed, 6, one third disinte- 

 grated; 13, one half disintegrated, and one wholly so, the five 

 worms isolated into suspensions containing dead isopods showed 

 one intact though bloated ; 3, heads only, disintegrated ; and one 

 with both head and posterior end disintegrated. Thus the pres- 

 ence of the recently killed isopods decidedly protected the other- 

 wise isolated worms from the toxic action of the colloidal silver. 

 This, taken with all the other evidence at hand, furnishes con- 

 vincing proof that the protective action of the mass against col- 

 loidal silver, extends beyond the species limits. 



DISCUSSION. 



We have shown in the preceding pages that, within limits and 

 other conditions being equal, there is greater protection the greater 

 the mass of the animals present, when exposed to the same amount 

 of colloidal silver in the same volume of water. Further, the 

 protection is largely and perhaps completely, furnished by the fixa- 

 tion of the toxic substance by the mass of animals, so that each 

 escapes receiving a lethal dose ; while, with the same concentra- 

 tion, isolated individuals receive a stronger dose of the toxic sub- 

 stance. The colloidal silver may be differently fixed in different 

 animals, but with those that secrete slime, like planarians, the col- 

 loidal silver is adsorbed on the slime. With other animals ob- 

 served, it may be removed by adsorption on the surface of the 

 animals themselves. Finally, as would be expected from this 

 mechanism, we have demonstrated 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 favorite hypothesis of 

 Drzewina and Bohn that group protection among these aquatic 

 animals is furnished by the rapid production in the presence of a 

 toxic agent, such as colloidal silver, of a more or less mysterious 

 auto-protective secretion. It is true that the production of slime 



