588 BIOLOGICAL EFFECTS OF RADIATION 



process of rotation one eye comes to be directed toward and the other 

 from the Hght, the muscles on the illuminated side contract vio- 

 lently, turning the head toward the Ught. This occurs twice during 

 each rotation and soon results in orientation, after which, if the light is 

 from a single source, the two eyes are continuously equally illuminated 

 and muscular contraction ceases. 



Mast (140) contends that these muscular contractions are reflexes 

 dependent upon the rate of change of luminous intensity in the eyes, and 

 that the orienting stimulus consequently ceases after the organism is 

 oriented. Garrey (75) holds that they are the result of difference of 

 tonus in the muscles on opposite sides, due to difference in the amount 

 of light received by the two eyes ; that the tonus of the muscles on either 

 side is continuously proportional to the amount of light received by the 

 eye on that or the opposite side; and that the orienting stimulus conse- 

 quently continues after the organism is oriented, being equal when the 

 two eyes are equally illuminated, i.e., when the organisms swim directly 

 toward the light. If the larvae are held so that they cannot rotate, and 

 the light in one eye is increased or that in the other is decreased, the 

 muscle on the more highly illuminated side contracts, but it is unfor- 

 tunately not known how long this position is held. The results in hand 

 consequently show that contraction of the muscles is correlated with 

 difference in the intensity of the illumination of the two eyes, but they 

 do not show whether it is due to difference in tonus in accord with 

 Garrey's contention or change of intensity in accord with Mast's. It 

 may, however, be said that the response occurs so rapidly that there 

 does not appear to be time to induce what is ordinarily called tonus. 



The distribution of stimulating efficiency in the spectrum for larvae 

 of Arenicola is essentially the same as it is for Euglena viridis. The 

 maximum is approximately at 4850 A, from which it decreases rapidly 

 in either direction (Mast, 144). 



EARTHWORMS 



The response to light is essentially the same in the different species 

 of earthworms investigated. The following statements refer to Lum- 

 hricus terrestris and Eisenia foetida. These species respond very vigor- 

 ously to a flash of light by violent contraction or by raising the anterior 

 end and swinging it from side to side. If repeatedly stimulated in fairly 

 close succession these responses become less and less definite and finally 

 disappear (Hoffmeister, 108). If the intensity is slowly increased, they 

 do not occur. They are, therefore, shock-reactions, i.e., responses 

 dependent upon the rate of change in intensity. 



If specimens are kept in darkness or in constant illumination they 

 become quiet, and if the illumination is now increased, they very slowly 

 become active. This response is not directly dependent upon the rate 



