76 



THE EYE IN EVOLUTION 



A pus 



implicated. The water-flea, Daphnia, orientates itself in the typical 

 manner by means of a single dorsal median eye ; while the fresh-water 

 crustacean, Apus, has two compound eyes and a median eye on the 

 dorsal surface. Other crustaceans have two eyes ; when one is 

 removed or painted over, rolling and circling movements occur towards 

 the seeing side, and if both are thrown out of action the light reaction 

 disappears (the brine-shrimp, Artemia, Seifert, 1930-32). 



The relation between the statocyst and the eyes in those animals 

 which possess the dual mechanism was prettily shown by von Hoist 



(a) 



(h) 



\ \ 



\ t 



t t 



Fig. 49. — The Dor.sal Light Reaction. 



In the fish, Crenilahrus rostratus. 



Upper two fish. The Hght comes from above ; (a) in the intact animal, 

 (b) in the labyrinthectomized animal. Orientation is normal. 



Lower two fish. The light comes from below ; (a) the norinal posture 

 is retained owing to the influence of the labyrinth ; (b) the labyrinthec- 

 tomized animal swims in an upside-down posture (after von Hoist). 



(1935) in the fish, Crenilahrus rostratus. Normally the balance is 

 maintained essentially by the static reactions of the labyrinth which 

 are supplemented by the light reaction. If, however, a light is placed 

 horizontally, a compromise orientation is assumed with the body 

 slightly tilted towards the light, the inclination varying directly with 

 the strength of the illumination ; when the light is placed underneath, 

 the static reactions control the animal and the light is without effect 

 (Fig. 49). When, however, the labyrinths are put out of action, the 

 optical reaction functions in the pure form, the movements of the 

 trunk, the fins and the tail, hitherto controlled by the labyrinth, now 

 being entirely coordinated by the eyes so that with a transverse light 

 the fish swims on its side ; with a light below, upside-down ; finally, 

 when one eye is put out of action, the fish rolls towards the seeing side 

 for a time until an adaptive reaction asserts itself. 



This reaction, of course, is often combined with other types of phototaxLs. 

 Thus the water-flea, Daphnia, is usually positively tropotactic and also exhibits 

 a compa'is reaction (von Frisch and Kupelwieser, 1913 ; Eckert, 1938), the 

 brine-shrimp, Artemia, may be positively or negatively phototactic, and so on. 



