Sensory Discrimination: Vision 149 



is true also of the color-blind human eye, he argues that the 

 animals tested are totally color-blind. He holds, in fact, 

 that all invertebrate animals are totally color-blind, on 

 the same evidence. The feet of starfish belonging to the 

 Astropectinidae are, he says, very sensitive to light: red 

 light has Httle effect on them, blue and green light, even 

 when much darker than red to normal human vision, de- 

 cidedly more effect, 

 as they would have 

 for a totally color- 

 blind human being. 

 The same results 

 appear in the case 

 of the sea-urchin. 



A human being's Fig- 10. — Daphnia. at, antenna; all, antennule; 

 sensitiveness to '"•'^^- After Yerkes. 



light is increased when he remains in darkness for some 

 time. This efifect, called darkness adaptation, appears 

 according to Hess in the Astropectinidae (313). 



The problem as to whether light of different colors pro- 

 duces different sensations in the crustacean consciousness 

 was the subject of experiments a number of years ago, in 

 which the Preference Method was used. Lubbock (442, 

 443) arranged to have a sunUght spectrum thrown on a 

 long trough containing Daphnias, tiny crustaceans belong- 

 ing to the lowest subclass, that of the Entomostraca (Fig. 

 10). Daphnia is ordinarily positive in its response to light, 

 that is, it seeks light. At the end of ten minutes glass 

 partitions were slipped across the trough at the approxi- 

 mate dividing lines between the spectral colors. The num- 

 ber of animals in each compartment was then counted. 

 The experiment was repeatedjy performed, and the greatest 

 number was always found in the yellow-green region, Bert 



