REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 45 



the organism is at first not oriented to lines of influence coming from a 

 certain direction, as in Fig. 17, x-y, and then becomes oriented, as at 

 Fig. 17, y-z, there are clearly more ways than one by which the orienta- 

 tion can be produced. The essential question for deciding as to the 

 nature of the reaction is not whether orientation occurs, but how the 

 orientation is brought about. This consideration has been too often 

 lost sight of in discussions of the behavior of the lower organisms. 



According to the theory of tropisms, as defined by Verworn, Loeb, 

 and Holt & Lee, the orientation should be brought about by the differ- 

 ential action of the external agent on the different sides of the organism ; 

 the organism should turn directly into the line of action of the external 

 agent, and the direction of turning should be determined by an external 

 factor, the direction of the infalling rays, or the side on which they 

 strike the organism. Now this is a matter which can be settled by 

 direct observation. Direct observation shows us in Stentor that orien- 

 tation is not brought about in the manner demanded by the theory. 

 The direction of turning is determined by internal factors. The reac- 

 tion which produces orientation is identical with the typical reaction 

 to a mechanical shock, to chemicals, to heat and cold. The difference 

 between what is demanded by the theory of tropisms and what is 

 actually observed may be made quickly evident to the eye by Fig. 17. 

 According to the theory of tropisms the orientation of a negatively 

 phototactic organism should take place by turning in the direction of 

 the arrow a-b ; in a Stentor in the position shown (x-y), orientation 

 actually occurs by turning in the opposite direction, as shown by the 

 arrow c-d. 



The further question then arises as to why the organism remains 

 oriented. All the facts point, in the case of Stentor, to the conclusion 

 that the reaction to a constant light is due to the intense illumination 

 on the sensitive anterior end. As soon, therefore, as the anterior end is 

 turned away from the light, as is the case in the position y-z, Fig. 17, 

 there is no further cause for reaction ; the animal therefore remains 

 with its anterior end directed away from the light ; that is, it remains 

 oriented. If, as a result of reaction to some other stimulus, or in any 

 accidental manner, the animal comes into a position such that it is no 

 longer oriented, the 4 * motor reaction "is repeated until the animal 

 comes again into the position of orientation in which it is no longer 

 stimulated. 



How does the method of reaction to light here described for Stentor 

 agree with what we know of light reactions in other ciliates? As 

 noted in the introductory paragraphs, comparatively little is known as 

 to light reactions in this group of organisms. The observations of 



