596 BIOLOGICAL EFFECTS OF RADIATION 



DAPHNIA 



Daphnia is a small crustacean with one compound median eye and two 

 locomotor antennae, one attached to either side at the anterior end. 



In diffuse constant illumination, it orients with the long axis vertical 

 and the anterior end up, and swims about very slowly and irregularly, 

 now sinking a little and then rising again, with but little change in loca- 

 tion. In a horizontal beam of light, it behaves in the same way but it 

 orients so that the back faces the light. If the light intensity is rapidly 

 decreased, it stops, sinks a little, and then either retains its orienta- 

 tion and swims slowly toward the light backward or turns through 

 180° and swims rapidly toward it forward. If the intensity is rapidly 

 increased, it stops, sinks a little and swims from the light forward. 

 If the intensity remains constant after it has become positive or nega- 

 tive, it soon becomes neutral, i.e., it rapidly becomes adapted to the 

 changed illumination. If the intensity is slowly changed these responses 

 do not occur. They are, therefore, dependent upon rate of change in 

 intensity, and have the essential characteristics of shock-reactions. 

 Ewald (60) calls them "Bewegungsreflexe." 



If Daphnia is kept in darkness 12 to 24 hr., it becomes inactive and 

 sinks to the bottom; and if it is then illuminated, it gradually becomes 

 active and swims upward. This response occurs whether the light is 

 slowly or rapidly increased; it is, therefore, not specifically dependent 

 upon the rate of change in intensity (Koehler, 115). 



There are at least two systems involved in response to light in 

 Daphnia; one which is primarily dependent upon rate of change in 

 intensity, and one which is not. 



The relation between the rate of change in intensity and the mag- 

 nitude of change in activity should be more thoroughly investigated. 



Clarke (37) studied in some detail the Bewegungsreflexe. He was 

 unable to make accurate measurements concerning the different phases 

 of the response, owing to the gradual change in the activity of the 

 organisms ("the gradual onset of the response")- This doubtless 

 accounts in large measure for the marked variability in the results 

 recorded. He came to the following conclusions: 



The "threshold amount of reduction" in light varies inversely 

 with the rate of reduction and the "duration of previous exposure to 

 light" and directly with "duration of previous sojourn in dark." The 

 latent period varies inversely with the amount of reduction and the 

 temperature. The "speed of response" varies directly with the amount 

 of reduction and the temperature. The "magnitude of response" varies 

 directly with the amount of reduction and the "duration of previous 

 exposure to light" and inversely with the "duration of previous sojourn 

 in dark." The "duration of response" varies directly with the amount 



