142 The Animal Mind 



react to light without eye spots, although some evidence of 

 dependence on the sense organ is indicated by the fact 

 that the response is slower ; Plessner (607) holds that skin 

 sensitiveness is responsible for reactions to light intensity, 

 and that the eye spots enable the animal to respond to the 

 direction of the light; Cowles (155), again, observes that 

 when pieces of starfish and sea urchins are cut off, their 

 tentacles and suckers still move in response to the casting 

 of a shadow. 



Among Crustacea, which are provided with a peculiar 

 visual organ, the compound eye, to be described laterj^^the 

 chief function of the eye seems to be that of responding to 

 shadows and movements. 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 (24). 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 normally 

 exposed to Hght fluctuations, did not (615). 



When we come to animals with well-developed eyes, 

 the specialized response to changes in light intensity gives 

 place to reactions involving the use of a more or less ade- 

 quate image of the stimulus object. But as we have seen, 

 the most primitive type of reaction to a sudden change of 

 light intensity is the checking or inhibiting of the animal's 

 movements. This type of reaction is cafled by Loeb 

 ^'sensibility to difference." In many cases its relative 



