CoNGDON, Reactions to Light. 315 



shows a slight falling oft" of direct reactions from 8 candlemeters up to 192 candle- 

 meters instead of the uniform increase seen in Perichaeta. Of course it cannot be 

 expected that the dift^erent genera of worms used by the two experimenters should 

 agree in the details of their reactions. 



The trial method as described by Holmes with its production and checking of 

 varied movements is confirmed by Harper as far as Perichaeta is concerned, and 

 it is hinted at by Smith. The observations of Parker and Arkin do not invali- 

 date its occurrence, because we do not know that they attempted to record a check- 

 ing of exploring movements. There is, therefore, little doubt that there are both 

 phototactic and trial phases in the behavior of worms, as well as that dependent 

 upon the relation of the stimuli anterio-posteriorly along the animal. 



Only those parts of Jennings' study ('00, '04, '05, '06, '06a) of Protozoa need 

 be considered here which refer to the method of orientation to light and to his con- 

 ception of the trial reaction. He found that alignment takes place by a swinging 

 of the anterior end of the animal away from a structurally defined side due to an 

 unfavorable change of intensity. This he terms an avoiding reaction. In case 

 it is initiated by an abrupt entry into a field of perpendicular light of unfavor- 

 able intensity there is usually a quick return to the ordinary spiral course. 

 The turn often serves to take the animal out of the unfavorable field. If it 

 does not accomplish that end the process will be repeated until it gets out or 

 becomes acclimated to the new conditions. If at any time it blunders into a field 

 of favorable intensity it is evident that it will be held there as in a trap. A second 

 variety of the reaction usually occurs if the animal be moving at an angle with hori- 

 zontal light. The beat of the cilia which produced the swing is then likely to con- 

 tinue longer and the anterior end move around a larger circumference than usual. 

 If forward motion be entirely stopped it may describe the surface of a cone or disk 

 by whirling on its posterior end. Some part of the curve which is traversed by the 

 anterior end of necessity leads into increasingly favorable light intensity and the 

 stimulus for the swinging, which was an unfavorable change of illumination, is 

 thus removed. The ordinary spiral course is resumed but the direction is now 

 more nearly in line with the light. By a series of such turns, often very close together 

 the protozoan soon becomes directed as nearly toward the light as its spiral motion 

 will permit. 



The following definition of trial is given by Jennings ('06) which he applies 

 to the protozoan methods and to that of the earthworm as well. "The organism 

 performs varied movements, some features of which are not determined by the local- 

 ization of the stimulus but by other factors; it then continues those movements which 

 bring it into or toward a certain condition; this condition usually being a greater 

 or less action of the stimulating agent as the case may be." 



This statement of trial differs from that of Holmes in two of its features. Jen- 

 nings does not confine varied movements to such as are produced by an unfavor- 

 able change of illumination. A comparison of earthworm and protozoan varied 

 movements will show whether the latter may be considered due to a change of light 

 intensity. The exploring movements of the earthworm constitute its varied move- 

 ments. They may be clearly distinguished from the movements which carry the 

 animal along because they are confined to the anterior end of the body. The avoid- 

 ing reaction of a protozoan, upon the contrary, may consist of ordinary locomotor 

 movements modified by a swing from the structurally defined side due to an unfav- 



