Sensory Discrimination: Vision 141 



of some degree of development, these very species have 

 been shown to be devoid of sensitiveness to light (836, 837). 



"Skioptic" reactions, or reactions to shadows, appear 

 among various echinoderms. The sea-urchin Centroste- 

 phanus longispinus, for instance, which lacks even a rudi- 

 mentary eye spot, will when a sudden shadow falls upon 

 it direct its spines towards the shaded side. The reaction 

 time involved is decidedly longer than that to mechanical 

 stimulation, and moreover, although pieces of the animal 

 will react to the latter, responses to shadows depend on keep^ 

 ing the system of radial nerves intact. (This observation, 

 according to Cowles (155), does not hold for the sea-urchin 

 Toxopneustes.) Hence Von Uexkiill, who made the above 

 observations, concluded that a special set of nerve fibres 

 is concerned in photic reactions (735). 



Dubois had suggested from studies on the mollusc 

 Pholas dactylus, that in such cases the pigment changes 

 which occur, under the influence of light, over the surface 

 of the body, furnish the stimulus (195), but Von Uexkiill 

 thinks this impossible, as the light reactions occur before 

 the pigment changes do. This migratory pigment, he be- 

 lieves, acts merely as a screen ; the source of excitation for 

 the optic fibres may lie in another pigment which he has 

 extracted and found very sensitive to light (735). Cen- 

 trostephanus, according to Hess (313), shows pigment 

 changes when the light is decreased by a very slight amount, 

 just enough to be perceptible to the human eye. 



Starfish have pigment or eye spots on the arm-tips. 

 As a rule, they seek light : Romanes (641) and Tiedemann 

 (710) report that the light reactions are abolished if the eye 

 spots are removed. MacCurdy (453) finds, however, that 

 in Asterias forbesii the light reactions are independent of 

 eye spots: Cowles (156) has shown that Echinaster will 



