320 Journal of Comparative Neurology and Psychology. 



A large number of observations upon the effects of other tropisms, physiological 

 states and various internal factors are recorded. For example, some individuals 

 were found to change from the usual response of the species for a time and then to 

 return to it. 



By a certain arrangement of conditions it is shown that negative photokinesis is 

 overcome by phototaxis. The tendency to wander may result in many excursions 

 contrary to the phototactic influence. This increases the effectiveness of negative 

 photokinesis by bringing the animals into dark regions to which they would not 

 come through phototaxis. 



CRUSTACEA. 



The photic reactions of crustaceans have received much attention. Their pho- 

 totaxis is characterized by quickness of alignment with the light and straightness 

 of course. Photokinesis is often strikingly marked. One is especially impressed 

 in looking over the papers upon the group by the variety of ways in which a reversal 

 of phototaxis has been produced. 



TowLE ('oo) found that the positive Cypridopsis could be readily made negative 

 by squirting it through a pipette. Negative animals could less readily be turned 

 to positive by the same means. 



In a series of papers upon entomostracan light reactions by Yerkes ('oo, '03) 

 a similar condition of things is described for Cypris and Daphnia. Cypris is made 

 positive in this way, and Daphnia faintly negative. In the latter animal it could 

 not be determined whether the opposite effect could be produced, for the negative 

 condition was at the best very weak and transitory. Yerkes believed that in gen- 

 eral the reversal most readily effected is from the less to the more common reaction 

 for each species. There is some probability that the stimulus producing the change 

 is thigmotactic inasmuch as according to Parker's observation, the crustacean 

 Labidocera though affected in like manner to these others when squirted through 

 a pipette, is not influenced as to its light response by vigorous shaking. 



Yerkes attempted to find whether increase of intensity calls forth greater accu- 

 racy of phototactic response. The question was answered by observing the duration 

 of trips of constant length made by Daphnia and Cypris under various conditions 

 of illumination. There occurred a marked shortening of the period occupied by a 

 trip if the illumination was increased. Yerkes believes this partly due to a straight- 

 ening of the course and therefore to more accurate orientation. The difference 

 between its phototaxis and that of Corixa, therefore, consists only to the degree of 

 accuracy of the orientation. Daphnia was found to recoil and turn back into the 

 light if its head came into a shadow somew hat as in the avoiding reaction of Protozoa. 



Yerkes has been able to obtain a physically perfect graded field of vertical light 

 by means of a lens consisting of the segment of a cylinder. All light which passes 

 through the bottom of the aquarium is deflected away by a mirror, thus avoiding 

 reflected light. A Daphnia placed in the apparatus goes obliquely upward toward 

 the lighter end at an angle of 45°; it is evident that this is partly due to an attempt 

 at phototactic alignment with the vertical light. The significance of the horizontal 

 component of this motion is not stated. 



Daphnia does not seek an optimum but moves unhesitatingly into the most 

 intense illumination it can reach. The harmful effects of strong light are shown 



