220 ANIMAL AGGREGATIONS 



have been the relative value of the protection and the toxicity of 

 such conditioned water. 



It is worth noting that all the members of the group exposed to- 

 gether were benefited by the fact that they were together. In such 

 circumstances it might have happened that certain animals at 

 the surface would take the brunt of the harmful rays. Their pres- 

 ence might serve to protect the other members, and the group might 

 thus be of value to the species if not to all the individuals composing 

 it. In one experiment, where the subsequent history was taken for 

 all the exposed animals, the interval before the first effects of ex- 

 posure was observed in the 30 animals exposed as a group ranged 

 from 27 minutes to 32.8 hours. Their final cytolysis ranged from 

 8.25 to 106 hours. The same conditions were observed for the 26 

 worms exposed in pairs in from i minute to 23.5 hours, and from 

 3.4 to 84 hours, respectively. Thirteen (50 per cent) of those ex- 

 posed as pairs were visibly affected before the end of the first hour 

 after exposure, while 6 (20 per cent) of the grouped animals were 

 similarly affected in the same time. Six of those radiated in pairs 

 were dead before the first of the group died. 



I am not prepared to generalize widely from these experiments 

 with the relatively large and highly pigmented Planaria doroto- 

 cephala that mass protection from ultra-violet radiation is always 

 due solely to some sort of physical interference rather than to the 

 possibly protective action of some exudate. With small organisms 

 such as sperm of sea-urchins or with Paramecia, which are more 

 translucent, the mass protection may be due to the latter factor, as 

 Hinrichs suggests. Her observations show, however, that even with 

 such minute organisms, the fact of physical interference is significant. 



Single planarians, and to a greater extent massed planarians, 

 would be more nearly analogous to sperm or protozoan aggregates 

 than to a suspension in which distribution is fairly equal in three 

 dimensions and each organism is surrounded by a medium of approx- 

 imately uniform consistency. In the latter case, it is conceivable 

 that a closer equilibrium between cells and medium must be main- 

 tained than with larger forms. This subject is one for experimenta- 

 tion rather than a priori discussion. 



