PROTECTION FROM TOXIC REAGENTS 211 



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 2 drops of colloidal silver, and introducing 

 with these dead isopods one Planar ia dorotocephala. At the end of 

 7 hours' exposure, 20 planarians isolated into similar volumes of 

 the same concentration showed, 6, one-third disintegrated; 13, one- 

 half disintegrated; and one, wholly so. The 5 worms isolated into 

 suspensions containing dead isopods showed i intact though bloated; 

 3, heads disintegrated; and i with both head and posterior end 



TABLE XXIII 



Showing Community Protection against Colloidal Silver 

 Two drops of colloidal silver in 5 cc. pond water; exposure 13 hours, then washed and 

 placed in pond water. Temperature 21° C. 



disintegrated. Thus the presence of the recently killed isopods de- 

 cidedly protected the otherwise isolated worms from the toxic ac- 

 tion of the colloidal silver. This, taken with all the other evidence 

 at hand, furnishes convincing proof that the protective action of 

 the mass against colloidal silver extends beyond the species limits. 

 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 protec- 

 tion is largely, and perhaps completely, furnished by the fixation 

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

 receiving a lethal dose; while, with the same concentration, isolated 

 individuals receive a stronger dose of the toxic substance. The 

 colloidal silver may be differently fixed in different animals; but 

 with those that secrete slime, like planarians, the colloidal silver is 



