2o6 ANIMAL AGGREGATIONS 



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

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

 animal. 



One bunch of 5 animals with 5 cc. of sea-water and 0.3 drops of 

 colloidal silver each, and with a surface exposure of 15.6 sq. cm. 

 per animal. 



Five isolated animals with 25 cc. of sea-water and 1.5 drops of 

 colloidal silver each, and with 78 sq. cm. surface exposure. 



A second set was exactly similar, except that the animals were 

 exposed to 2, 0.4, and 2 drops of colloidal silver per individual. 



The exposure in a typical experiment lasted 15.5 hours, after 

 which the animals were washed as usual and transferred to fresh 

 sea-water for observation for the following 24 hours. At intervals 

 records of numbers that righted after having been turned over, and 

 other evidences of activity, such as spontaneous motion of tube feet, 

 were recorded. 



All of the animals bunched in the 10 cm. finger bowls recovered 

 sufficiently to right themselves within 24 hours, despite the fact 

 that 4 of the 10 were much corroded by the action of silver that had 

 settled to the bottom of one of the finger bowls. In both experi- 

 ments, the isolated animals fared next best, although they made a 

 much poorer showing than did the bunched individuals that were 

 exposed to a much lower amount of colloidal silver per animal; 

 only 6 of this set of 10 righted. The bunched animals, exposed to 

 the same volume and amount of colloidal silver as those isolated, 

 ranked a poor third, which may be due to the decreased surface 

 exposure or to some of the better-known ill effects of crowding. 



These experiments indicate either that the decreased amount of 

 colloidal silver or the reduced volume of sea-water present per 

 individual, or both, markedly favored the survival of the animals 

 bunched in the small amount of water present in the finger bowls. 

 Drzewina and Bohn emphasize the latter factor, and accordingly 

 experiments were run to test out this point. Contrary to the expe- 

 rience of Drzewina and Bohn (1921a), the toxicity with Ophioderma 

 was found to depend on the concentration of the colloidal silver 

 rather than on the volume to which the animals are exposed. 



