274 Journal of Comparative Neurology and Psychology. 



In addition to the facts regarding the effect of direct lighting 

 upon body-orientation, which have been presented in the form 

 of these nine cases, several other conditions might be mentioned: 



1. If the longitudinal axis of the larva was parallel to the direc- 

 tion of the incident light rays, and the head away from the light, 

 then the introduction of light produced no change in the body- 

 orientation, but it might cause a positive or a negative progressive 

 orientation. 



2. In order that the unmodified forward or backward rotation 

 might occur, it was learned that the light rays must strike both 

 eyes with equal intensity, and consequently in a direction exactly 

 perpendicular to any transverse body-axis of the larva. 



3. In case the incident rays came from a direction that was 

 not exactly perpendicular to the transverse axis of the larva, be 

 the angle of difference ever so slight, the perfect backward and 

 forward rotation would not occur, but would be greatly modified 

 by swingings of, and revolutions on, the longitudinal axis of the 

 body. 



4. This type of behavior could not be observed unless the con- 

 ditions of light were reduced to a single directive influence, and 

 this factor handled with very great precision. 



The effect of blocking the illumination — In the previous section 

 we have examined the reactions which were brought about bv 

 suddenly introducing rays of light in directions which maintained 

 a certain definite and specified relation to the longitudinal or trans- 

 verse axis of the larval lobsters. In the present instance, however, 

 we are to consider the nature of the reactions which are produced 

 as a result of suddenly excluding or blocking the principal source 

 of light by which the larvae have just previously been stimulated. 

 The "cut-off" was made by closing the window through which 

 the light came, and thus leaving the larvae in the subdued and 

 diffuse light which entered the dark box from the room. Since 

 the body-orientation of the larvae to the directive influence of the 

 light is always the same, obviously there could not be many differ- 

 ent varieties of orientation caused by the change in the conditions 

 of light. Such as were possible, however, may be described as 

 follows : 



Case 10. Larva oriented with the back above and the longitudinal 

 body-axis exactly parallel to the incident rays — In case the larva 

 was oriented as described above, when the light was shut off 



