202 ANIMAL AGGREGATIONS 



based largely upon our own work (Allee and Schuett, 1927), it is only 

 because I am more familiar with this, and by no means because it is 

 more important than work in this field done by Drzewina and Bohn 

 in following out their original discovery. 



It is easy to demonstrate, as Drzewina and Bohn state, that 

 within limits, the greater the mass of animals, the better the pro- 

 tection. Thus with Planaria dorotocephala, twelve sets of 2 each, 

 exposed at room temperature to 10 cc. of water containing 10 drops 

 of the standard suspension of colloidal silver, showed the beginning 

 of head disintegration in from 4.5 to 10 hours; while ten similar lots, 

 each containing from 10 to 75 worms, all lasted over 36 hours in the 

 same volume and concentration. A species of Cladocera and the 

 isopod Asellus communis showed similar, but less marked, group 

 protection. 



Similar tests, in which the animals were exposed to the action of 

 dilute suspensions of colloidal silver for considerable time and were 

 then washed and transferred to water to which they were accus- 

 tomed, yielded such results as are listed in Table XVIII. With the 

 worms, the results are summarized in terms of survival after ex- 

 posure, with added information concerning the number of worms 

 that were able to crawl either with or without stimulation with a 

 camel's hair brush. Other data at hand show that the protective 

 effect of the group was greater than is indicated by this summary, 

 but the summarized account is sufficiently conclusive. With the 

 brittle starfish, Ophioderma, tests were made concerning the righting 

 ability of the animals after exposure to colloidal silver. In these 

 tests, after exposure, the washed animals were transferred to sepa- 

 rate finger bowls each containing 250 cc. of well-aerated sea-water. 

 It will be noted that the mean righting time of the isolated animals 

 is given in minutes while that of the animals tested in groups of 

 5 is given in seconds. 



More cases with different concentrations could be given, but 

 the result is the same in all. This agrees entirely with the experience 

 of Drzewina and Bohn, who used a wide range of animals; and with 

 that of Bresslau, using different toxic reagents with ciliates. Yet the 

 possible menace of crowding, so frequently given as the only result 



