THE VISION OF INVERTEBRATES 



based on different sensory experiences is, however, doubtful. It has 

 been generally accepted in the case of the worm which emerges in 

 twihght and hides again in daylight ; but does it equally apply to the 

 protozoan which shows the same response ? We do not know the 

 answer to this riddle.^ 



A more analytical method is the application of training techniques 

 which, incidentally, are more susceptible to scientific control. Thereby 

 an animal is trained to respond to or reject one stimulus to the exclusion 

 of all others by an appropriate reward or punishment, the stimulus 



569 



Ur12in.-J 



Fig. 729. — Ground Plan for Discrimination Box. 



L, light box. F, food ; Z)i, hinged door ; D^, hinged door with 3 X 3 in. 

 opal glass panel ; G, glass partition ; R, restraining chamber. The box is 

 13 in. high (R. Gunter, J. Physiol, 1951). 



being more minutely differentiated from related stimuli as the process 

 of training proceeds. The disadvantage of the method is the limitations 

 of its applicability since it requires more intelligence, consistency in 

 behaviour and amenability than most animals possess ; moreover, an 

 experiment of this type must excite the animal's interest so that the 

 technique would be expected to break down if the sensation in question 

 were not of importance in its life. 



A simple and typical experimental set-up for such a training experiment is 

 seen in Fig. 729. In its essentials it is a Y-shaped " discrimination box " or 

 maze wherein the animal is first retained in an outer chamber and then, entering 



1 p. 102. 



