2i8 ANIMAL AGGREGATIONS 



of greater numbers, to the conditioning of the water by some sort 

 of autoprotective exudate. Robertson brings his observations into 

 Hne with his general theory of the extrusion of an autocatalytic 

 substance, while the others regard mass protection from high tem- 

 perature as an excellent example of the effect of an autoprotective 

 secretion. 



PROTECTION FROM ULTRA-VIOLET RADIATION 



Another t>pe of experimentation which should yield critical tests 

 of the efhcacy of group protection and of its mechanism should be 

 found in treatment with radiant energy applied as ultra-violet 

 radiation. Hinrichs (1927) has reported that Arbacia sperm are less 

 affected in concentrated than in dilute suspensions (obviously a 

 mass protection) ; and Petersen has obtained similar results in this 

 laboratory in radiating Paramecia (unpublished). Accordingly, tests 

 (Allee, 1928) were made concerning various aspects of survival 

 value of groups as compared with isolated planarians, when exposed 

 to the full spectrum of a quartz mercury- vapor arc, with temperature 

 controlled during the exposure. The results collected demonstrated 

 two facts: 



In the first place, the presence of products of cytolysis produced 

 by exposure to ultra-violet radiation are more harmful than bene- 

 ficial to those worms that have been exposed to the action of ultra- 

 violet radiation or to those that have not been exposed. Similarly, 

 w^ater containing products of metabolism or exudates given off by 

 the animal, either in its usual laboratory culture or when exposed 

 to ultra-violet radiation, shortens rather than lengthens the survival 

 of other worms isolated into it. 



In the second place, the exposure of a number of worms in a 

 limited amount of water with limited exposed area gives much 

 greater protection for the individuals than if they had been exposed 

 singly or in pairs. Some of the implications of these results deserve 

 consideration and will be discussed in the order just given. 



When the survival of worms in various sorts of worm-conditioned 

 water is compared with that of similarly treated planarians in 

 aerated well-water, the worms in the conditioned water are found to 



