LIGHT AND MOVEMENT 



angles to it ; if the light is placed horizontally the animal rotates 

 correspondingly, and if the light is placed below, it either rolls or 

 somersaults over to swim belly-upwards (Fig. 47). In the ventral 

 LIGHT REACTION an animal which normally swims belly-upwards 

 behaves analogously (Fig. 48). Occasionally, however, if the normal 

 direction of the incidence of the light is changed, the animal does not 



Fig. 47. — The Dorsal Lkjht Reaction. 



On the left half of the aquarium the Crustacean, Apiis, is illuminated 

 from its right side; on the right half of the aquarium, from its left side. As 

 it swims between the two, it orientates itself by rolling on its longitudinal 

 axis (after Seifert). 





® 



^"■"^^.^^^J 



Fig. 48. — The Ventral Li(;ht Keaction. 

 The change of orientation in the Crustacean, Artemia salitia, when the 

 light is changed from abo\p to below. Fig. 48«, by a rolling movement ; 

 Fig. 4S6, by a back somersault or pitching movement (in a photo-positive 

 individual) (after Seifert). 



act reflexly but becomes completely disorientated and swims aimlessly, 

 a reaction seen, for example, in the nemertine worm, Linens ruber, which 

 in normal circumstances is negatively phototactic (GoutcharofF, 1952). 

 The visual mechanism involved varies in different species. In 

 some larvae the response is mediated by the dermal light sense and 

 persists after total blinding (Schone, 1951) but as a rule the eyes are 



Linens ruber 



