LIGHT AND MOVEMENT 53 



tudinally so that on lateral illumination each eye is alternately illuminated and 

 shaded. As each eye becomes exposed to the light, the muscles of the illumi- 

 nated side contract violently turning the head towards the light (Fig. 24). Since 

 this occurs twice during each rotation, the larva is rapidly orientated towards 

 the light until the two eyes are equally illuminated all the time, whereupon 

 further muscvilar contraction and orientation cease. 



A very similar and typical reaction is seen in the rotifer, Branchionus 

 (Viaud, 1948), and in the photo-negative larvae of the flour-moth, Ephestia, 

 which are provided on either side of the head with an aggregate eye composed of 

 six ocelU (Brandt, 1934) (Fig. 25). 



A further evolutionary step is seen in earthworms. As is the 

 general rule, impulses originating in the photoreceptors on one side of 

 the body determine orientation by inducing a simple reflex contraction 

 of the muscles on the opjDOsite side, but it is obvious that if these 

 impulses can be modified and integrated in the central nervous system, 

 a more effective response is obtained. 



Such responses have been fully studied in the earthworm, Lumbricus 

 terrestris, and Eisenia foetida.^ In these animals the existence of photoreceptor 

 organs associated with a subepidermal nerve-net was demonstrated by Richard 

 Hesse (1896) and confirmed by W. N. Hess (1925)^ ; they are most numerous 

 and receptive near the anterior extremity of the animal. The response to light 

 is somewhat complicated and has given rise to some difference of opinion ; but 

 it would seem most likely that if the worm is sluggish and is exposed to dim 

 light, it slowly extends, turns its anterior end away from the light, and continues 

 to move thus. If, however, the worm is active when it is illuminated from the 

 side, the anterior end is quickly raised and turned in the direction opposite to 

 that in which it happens to be, whether it is directed to the light or not, and 

 thereafter swung from side to side, a position and direction being eventually 

 adopted in which the anterior end is least exposed to the light. 



If now the cerebral ganglion is removed or destroyed or if it is inhibited by 

 a reduction of temperature or the injection of depressant drugs such as cocaine 

 or alcohol, the opposite reaction of a positive phototaxis results ; in these 

 circumstances lateral illumination of the more posterior photoreceptors produces 

 a contraction of the muscles of the same side which causes the worm to turn 

 towards the light, a reaction due to reflexes mediated through the ventral cord 

 (Hess, 1924 ; Prosser, 1934). It would seem that normally this weak positive 

 ipsilateral response mediated through the cord is overshadowed by the stronger 

 negative contralateral response derived from the receptors in the highly sensitive 

 anterior end and mediated by the cerebral ganglion, and that the final response 

 of the animal is the resultant of the two antagonistic tendencies after integration 

 and coordination in the central nervous system. 



It is obvious that the bilateral balance of the tropotactic response 

 will be upset if one eye is blinded, either by painting it over or by its 

 removal, so that with lateral illumination the animal will tend con- 



1 Loeb (1894), R. Hesse (1896), Parker and Arkin (1901), Smith (1902), Adams 

 (1903), Hoknes (1905), Harper (1905), Mast (1911), W. N. Hess (1924), Nomura (1926- 

 27), Prosser (1934), and others. 



2 pp. 131, 518. 



