allee] simple experiments IN ANIMAL BEHAVIOR 177 



with black paper in such a way that no light is admitted and yet 

 the cover can be easily removed to read the position of the animals. 

 Cover the other in the same way except for a narrow slit at one 

 end to admit light and set these pans side by side. Leave undis- 

 turbed for perhaps five minutes at the start. Remove the covers 

 first from one then from the other and note the position of the 

 animals. Replace the cover as quickly as possible. Record the 

 position of the animals as follows : 



Experiment Control 



Light third Middle third Dark third Light third Middle third Dark third 



235433 



Repeat as many times as desired. Ten is often a very fair number 

 of trials. 



Results. — From the table compiled as above one can determine 

 the response of the animals to the light used by comparing the 

 reaction given by the animals in the experimental pan with those 

 in the control. The common shrimp (Palaemonetes) and the 

 fairy shrimp (Eubranchipus) will collect near the opening in 

 moderate light. Isopods and amphipods will be found in the 

 darker parts of the pan, although they will come nearer the opening 

 if only a narrow slit is left open in faint light. 



Discussion. — In order to test the reaction to light of different 

 intensities the width of the slit may be varied, or the pans may be 

 placed in more or less direct light. Care must be taken to have 



* 



the light enter the pans from as near a vertical position as possible, 

 otherwise the end nearest the light will cast a shadow and the 

 region under the opening will be less strongly illuminated than 

 that some distance away. Artificial light, especially electrical 

 light, may be used to advantage in a darkened room. When this 

 is used it should be placed directly above the open slit and a shal- 

 low dish of clear glass containing filtered water (distilled if possible) 

 should be placed between the light and the opening. This acts as 

 a heat screen and avoids complications from a thermotactic 

 response. 



This experiment does not give evidence as to whether animals 

 react to light because of the direction of the rays or because of the 

 difference of intensity on different parts of the organism. It 

 simply shows the amount of light in which the given animals 

 will come to rest when most other stimuli are removed. 



