316 'Journal of Comparative Neurology and Psychology. 



orable change of illumination. It is the avoiding reaction which constitutes the 

 varied movement of the protozoan. All components of its motion are not evoked 

 by change of light intensity. Then why refer in a definition of trial to the method 

 by which the varied movement is produced } Harper ('07) in a recent paper 

 gives a reason. There are a great number of irregular movements, especially among 

 lower animals, which by carrying their possessor into a large number of regions 

 help them better to test the surroundings. Such, among others, are the spiral 

 movements of the protozoan not involved in the avoiding reaction, and certain 

 writhings of insect larvae. These do not aid in orientation to light and in most 

 cases do not result from unfavorable change of illumination. The trial reaction is 

 therefore to be considered as resulting from varied movements produced in v^hole 

 or in part by change of light intensity. 



Jennings does not make the checking of some varied movements an essential 

 part of trial. He says merely: "Movements are continued which bring the animal 

 intoor toward the favorable condition." Atnoment's consideration of the method 

 of the trial procedure in Protozoa makes the reason for his attitude clear. There 

 is first an increase of the ciliary stroke producing the movement from a structurally 

 defined side. When the anterior end of the animal in pursuing the enlarged spiral 

 is brought into more favorable light intensity the increased vigor of stroke dis- 

 appears. The process is not a checking of any movement by unfavorable illumina- 

 tion. 



The orientation of protozoan and earthworm plainly have some diflFerences. 

 Yet the two have sufficient in common to warrant their inclusion within a single 

 category. Orientation by trial then consists in the production of varied move- 

 ments which are at least in part produced by an unfavorable change of illumination, 

 and the following up of those leading towards favorable illumination. 



Jennings' account of protozoan behavior to light was soon followed by a paper 

 from Mast ('06) upon the protozoan, Stentor. As the latter says: "Jennings 

 laid particular stress on the detailed movements of the individuals while I directed 

 most careful attention to the regulation of the stimulus." 



One carefully planned device which he employed gave him a graded field of ver- 

 tical light. When subjected to it, Stentor, which is negatively phototropic, becomes 

 directed in some path which does not lead into greater intensity. That is to say, 

 it becomes oriented to such an extent that its head points within 90° to one side or 

 other of the line which would carry it most directly toward the dark. 



Mast raises a doubt as to whether the avoiding reaction of Protozoa is a trial 

 response at all. His examination of the threshold of light stimulation for different 

 parts of a Stentor shows that the peristomal region is probably much more sensitive 

 than the rest of the surface. Therefore light stimulus is most likely to be received in 

 this region. Inasmuch as the animal turns from the peristomal side in an avoiding 

 reaction it is giving a definite response to a localized stimulus just as truly as is an 

 insect which upon one eye being blinded turns away from the remaining one. The 

 localization of light sensitiveness and the turning from the localized area cannot 

 as yet be considered as an established fact. 



Does the protozoan show an alignment of a bilaterally symmetrical body to the 

 light ? It does so only imperfectly because of its spiral movement. Inasmuch 

 as many phototactic animals only roughly approximate a straight course because 

 of peculiarities in their locomotor mechanism, such a condition would not prevent 



