204 ANIMAL AGGREGATIONS 



"Bref, si chez diverses especes que nous avons examinees jusqu 'id 

 (Paramecies, Colpodes, Stylonichies, Stentors, Hydres, Convoluta, 

 Glossiphonies, tetrads de Grenouille) tout se passe comme s'il y 

 avait emission rapide d'une substance ou de substances assurant une 

 defense; chez Polycelis c'est le contraire:' on assiste non pas a une 

 auto-protection mais a une auto-destruction.''^ 



They do not believe that the greater resistance of the group is 

 due to a more rapid using-up of the toxic substance, and cite two 

 types of experiments to support their contention (1921). In the 

 first type they expose some hundred of Convoluta in one suspension 

 and 2 each in the accompanying dishes. Even if the former receive 

 10 drops of colloidal silver to 25 cc. of water and the isolated animals 

 have only 2 drops for the same volume, they find that the large 

 group survives after the others are disintegrated. This is a fairer 

 test than if the same amount of colloidal silver were added in both 

 cases, but still it must be remarked that the ratio of colloidal silver 

 per individual is about ten times greater in the case of the 2 individ- 

 uals, when compared with the larger group. 



We have made some exact tests to cover this point, and I present 

 the summary of our results in Table XIX. The tests reported 

 show clearly that if a group of 25 Planar ia are placed in the 

 same volume and the same concentration of colloidal silver as 

 are isolated individuals, the former survive the experience in 

 good condition while the majority of the latter succumb, and 

 almost all the others are severely affected. Further, if similar pla- 

 narians 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 per 

 cent of the isolated individuals alive, even if most of them were 

 strongly affected. And finally, if the groups are placed in propor- 

 tionate volume and proportionate concentration of colloidal silver, 

 their condition approaches that of the isolated individuals. This 

 evidence shows that the greater protection which the mass furnished 

 when exposed to colloidal silver was due to the smaller amount of the 



' Against KCl (W. C. A.). 



