Sensory Discrimination : Vision 1 35 



two look alike to our eyes, was shown by Lubbock (251, pp. 

 215 ff.). An effect of physiological condition suggesting the 

 law of general adaptation in human vision is evidenced by the 

 fact that individual Daphnias which have been a long time 

 in darkness will respond to a lower intensity than those which 

 have been long exposed to illumination (94). Many curious 

 results of physiological condition upon orientation to light 

 in Crustacea will be discussed later. 



Experiments on the reactions of the crayfish, which is 

 moderately negative in its phototropism, to light coming 

 through colored glasses indicate that the animal seeks red 

 when the light falls vertically, but shows no marked prefer- 

 ence when light is passed horizontally through the glass. 

 The tendency to seek red is characteristic of negatively photo- 

 tropic animals, but in this case it seemed to be stronger 

 even than the tendency to seek black. No definite proof 

 of a specific color reaction is, however, offered (21). The 

 positive reactions to light of Pycnogonids, or sea spiders, 

 a curious group of animals whose classification is uncertain, 

 have been found to depend on the presence of a visual organ 



(79). 



48. Vision in Spiders 



Spiders do not have the compound eye, but a number of 

 ocelli, or simple eyes; the typical fully developed inverte- 

 brate eye with cornea, lens, vitreous humor, rod layer, and 

 pigmented layer in the retina^the latter lying in front of the 

 nerve fibres supplying the retina, instead of behind them as 

 in the vertebrate eye. Experiments have been made on color 

 discrimination in spiders; some by the Preference Method, 

 where the spiders showed an inclination for red when offered 

 a choice of compartments illuminated through red, green, 

 blue, and yellow glass (320) ; others by attempting to form an 

 association between paper of a certain color and the spider's 



