STUDIES IN ANIMAL AGGREGATIONS. 305 



III. 71 planarians in three lots each in 10 cc. of watei plus two drops of colloidal 

 silver. 



Normal, crawling when stimulated <>i 



Normal, not crawling when stimulated i 1.4 



Head started to disintegrate 7 '.* 



Head completely disintegrated " 0.0 



IV. 75 planarians in three lots each in 10 cc. of water plus 50 drops of colloidal 



silver. 

 All wholly disintegrated. 



The tests reported show clearly that if a group of 25 Planaria 

 are placed in the same volume and the same concentration of col- 

 loidal silver as are isolated individuals, that the former survive the 

 experience in good condition while the majority of the latter suc- 

 cumb, and almost all the others are severely affected. Further, 

 if similar planarians are placed at the same time in the same volume 

 of water, to which has been added sufficient colloidal silver to make 

 the concentration per individual equal to that given the isolated 

 worms, the bunched animals all succumb to a treatment that left 

 37-5 P er cent- of the isolated individuals alive, even if most of 

 them were strongly affected. And finally, if we place the groups 

 in proportionate volume and proportionate concentration of col- 

 loidal silver, their condition approaches that of the isolated indi- 

 viduals. We conclude from this evidence that the greater protec- 

 tion which the mass furnished when exposed to colloidal silver is 

 due to the smaller amount of the toxic substance which each in- 

 dividual has to remove from suspension in order to lower the 

 strength below the threshold of immediate toxicity. The mechan- 

 ism for this removal will be discussed later. 



Another type of experiment was run with Ophioderma to de- 

 termine whether or not the protective effect of the bunch was due 

 to the fact that the bunched animals were exposed to a smaller 

 concentration of colloidal silver than were the isolated animals. 

 Since the area of the surface would affect the aeration and so af- 

 fect survival values, the larger numbers were placed in crystal- 

 lization dishes with a diameter of 24 cm., while the smaller bunches 

 and the isolated animals were in the usual finger bowls with a 

 diameter of 10 cm. The essentials of the set-up were: 



One bunch of ten animals with 25 cc. sea water and 1.5 drops of 

 colloidal silver each, and with a surface exposure of 45 sq. cm. 

 per animal. 



