PROTECTION FROM TOXIC REAGENTS 219 



die more quickly. The difference in the survival periods is usually 

 small, and the periods themselves are highly variable for different 

 lots of worms. Eleven such group comparisons are possible with the 

 data at hand; and in these, nine cases definitely favor the well-water, 

 one favors the conditioned water, and the other is recorded as a tie 

 but vo.th more careful observation it would probably have been 

 placed definitely in the majority column. The main inquiry was 

 concerned with the possibility of there being a protective value in 

 the conditioned water, and there is no doubt of the negative an- 

 swer given by the experiments. Rather, the converse is indicated 

 though the evidence is not complete concerning the possibility 

 that with different water and with smaller proportions of the con- 

 ditioning matter, such conditioned water may prove advantageous 

 to the worms placed in it. 



With regard to the definite protection furnished when many 

 worms were exposed to ultra-violet radiation in a limited amount of 

 water and with a limited surface area, the experimental evidence 

 indicates that this protection is due to some sort of interference with 

 the penetration of injurious rays or to some other biophysical effect 

 of numbers, rather than to the presence of some exudate or exudates 

 released as a result of the radiation. As has been stated above, water 

 containing such exudates produced harmful rather than beneficial 

 results. 



In the experiments where the worms were more densely crowded 

 (60 to I cc. of water), the protection was obviously connected with 

 the "shading" which Hinrichs mentions in her studies on the radia- 

 tion of sperm suspensions. In the less dense groups (15 to i cc), 

 "shading" in the usual sense is less obvious; but exposure at this 

 density also resulted in definite protection to the group-exposed 

 worms, which suggests that some other factor or factors may have 

 been operating. Protection furnished by the presence of so few 

 worms may be an illustration of the phenomenon called by Drzewina 

 and Bohn "catalysis by contact." Similar shading would undoubt- 

 edly have resulted from the exposure of isolated worms in water 

 conditioned by the pressure of products of cytolysis. The experi- 

 ment was not tried, since the only information to be gained would 



