Sensory Discrimination : Vision 133 



mosaic formed of these spots. Into its characteristics, how- 

 ever, we need not enter. In the present chapter we are 

 concerned only with the evidence that light stimulation in 

 general, and light of different wave lengths in particular, 

 produces specific sensations. 



That the visual reactions of Crustacea are accompanied by 

 a special visual sensation, if we suppose these animals to be 

 conscious, is sufficiently evidenced by their dependence on 

 the eyes. To movements and shadows the responses are for 

 the most part given. Bateson, watching shrimps and prawns, 

 noted that they apparently could not see their food when it 

 had been taken from them and lay near at hand, but quickly 

 raised their antennae when an object was passed between 

 them and the light (i i). The little fairy shrimp, Branchipus, 

 will stop swimming as soon as the edge of a shadow falls 

 upon it. "Skioptic" reactions in the family of Cirripedia, 

 to which the barnacles belong, were noted by Pouchet and 

 Joubert in 1875, as well as the fact that those individuals 

 which were attached to rocks, where a sudden shadow 

 might mean danger, reacted, while those attached to floating 

 objects, and therefore exposed normally to light fluctua- 

 tions, did not (348). The problem as to whether light 

 of different colors produces different sensations in the 

 crustacean consciousness was the subject of experiments 

 a number of years ago, in which the Preference Method 

 was used. Sir John Lubbock arranged to have a sunlight 

 spectrum thrown on a long trough containing Daphnias, 

 tiny crustaceans belonging to the lowest sub-class, that 

 of the Entomostraca (Fig. n). Daphnia is decidedly 

 positive in its phototropism. At the end of ten minutes 

 glass partitions were slipped across the trough at the approxi- 

 mate dividing lines of the spectral colors. The number of 

 animals in each compartment was then counted. The ex- 



