1 68 Journal of Comparative Neurology and Psychology. 



creased, swerve more rapidly toward the dorsal side and swim less 

 rapidly forward, and then proceed as before but in a new direction. 

 In course of this the anterior end of the body describes a circle while 

 the posterior end moves but little and is virtually a pivot for the freer 

 end. "The reaction which occurs when the illumination is changed 

 is really an accentuation of a certain feature of the usual movements." 

 Now if the change in illumination has been slight, the animal will 

 slowly resume its forward movement swerving less noticeably toward 

 the dorsal side, so that presently it will be moving along its former 

 spiral course. But if the modified illumination is now too great, the 

 animal while describing the pivotal movement will find its anterior 

 (and more sensitive) end turned toward the light at some points of its 

 circular course and away from it at others ; since now the light is 

 supraoptimal, the animal will commence the forward movement while 

 its anterior end is turned from the light, so that its new spiral course 

 as it is gradually resumed will be directed at least somewhat away 

 from the source of light. If this change in course is not sufficient to 

 make the animal recede from the source of light, the animal will 

 either tack again in the same way as before, or else change its orienta- 

 tion gradually but in a way very comparable with the previous. That 

 is, the organism will swerve farther than normally toward the dorsal 

 side and this will cause its anterior end to point successively in many 

 different directions. In some of these positions the anterior end is 

 directed more nearly toward the source of light, in other positions far- 

 ther away from it. In the latter case the swinging toward the dorsal 

 side becomes less marked ; hence the succeeding phase of the swing, 

 which carries the anterior end toward the light, is less pronounced; 

 The anterior end therefore does not swing so far in the direction toward 

 the light as in the preceding phase it swung away from the light. 

 But as a result of such swerving as does occur the anterior end is now 

 directed more away from the source of light than before. Thus this 

 gradual change of orientation is effected by means of the same prin- 

 ciples as the former, more sudden change. In either case the direc- 

 tion of the rays of light is not a determining feature, and the orienta- 

 tion is effected by a definite sort of "motor response." 



This description applies to the movements of orientation of cili- 

 ates and flagellates in general. The ciliate studied was Sfentor oreru- 

 leus: the flagellates were Eugkna viridis, Cryptomonas ovata (sometimes 

 called Chilomonas ovatd), and a species of Chlamydomonas . 



Third Paper. Reactions to Stimuli in Certain Rotifera. 

 The author studies the Rotatoria, a group of metazoa much re- 



