312 "Journal of Comparative Neurology and Psychology. 



orientation because of differences of locomotor mechanism and other factors, they 

 have in common, that they ahgn with the Ught more or less accurately as a result 

 of its differential effect upon the opposite sides of a bilateral symmetrical body. 

 There is thus a response to localized stimulus. The available evidence goes to 

 show that animals responding to localized light stimulus have in general this same 

 character. Even the bell-shaped jelly-fish and a spherical form such as Volvox 

 come within the category inasmuch as radially symmetrical animals must be also 

 bilaterally symmetrical. In the further use of the term phototaxis we shall there- 

 fore imply alignment by the differential effect of light upon the sides of bilaterally 

 symmetrical organisms. 



Jennings attaches little value to this view of phototaxis because it does not pre- 

 tend to seek a full explanation of things as did the old mechanical theory. He says, 

 " In order to retain any of its value for explaining movements of organisms, it would 

 have to hold at least that the connections between the sense organs and the motor 

 organs are of a perfectly definite character so that when a certain sense organ is 

 stimulated a certain motor organ moves in a certain way. " It is to be granted that 

 there is little of an explanatory character in phototaxis as defined above. Never- 

 theless it has the value which attaches to all categories. It represents a certain stage 

 in the classification of facts, and is a unit of behavior which will simplify further 

 attempts at analysis in the same direction. 



LoEB and Radl rightly claim that there is a graded series between such a loco- 

 motor response as we have just defined and the training of the two eyes of a verte- 

 brate upon any object. The comparative anatomy of various types of eyes, as well 

 as those experiments upon light response which bear upon the subject, strongly 

 indicate that there is also a graded series between orientations to a single source and 

 those to a varied field. The question of the practicability of applying the term 

 phototaxis, which originally referred to locomotor responses of lower animals alone, 

 to a series including the orientation of eyeless animals on the one hand and of the 

 vertebrate eyes upon the other, is simply one of convenience in terminology. In 

 this paper it will be used in the wider sense. 



Perhaps no contribution has appeared which shows more clearly the relation 

 between phototaxis and the general nervous activity of an animal than does the 

 study by Holmes ('05a, '07) of the reactions of the insect Ranatra to light. The 

 behavior of the animal is dominated to a surprising degree by photic stimuli. It 

 is marked not only by phototaxis of the body but its eyes and breathing tubes sway 

 towards an alignment with the light even when the animal is not engaged in loco- 

 motion. If various parts of the eyes be blackened there results the phototactic 

 response which we would expect if the part of the environment dark to the animal 

 were really devoid of light. Holmes points out that this slavish and mechanical 

 response is probably due to simple reflexes. 



But the behavior of Ranatra also reveals more complex nervous processes exist- 

 ing side by side with phototaxis. Hemisection of the brain destroys light response 

 almost completely. Therefore it is probable that the crossing optic fibers in the 

 brain are part of the reflex arc. A number of stereotyped procedures such as hunt- 

 ing food and cleaning the body may inhibit phototaxis. Of especial interest is the 

 result of blackening all but a small posterior portion of one eye. There is a marked 

 disturbance of orientation as one would expect. In spite of this fact, the animal 

 in time learns to move towards the light quite accurately. Holmes argues that no 

 simple reflex can explain orientation under these conditions. 



