300 A. M. Banta 



the effects of unequal illumination upon different parts of the 

 animal's body. 



12 Caecidotea sometimes reacts in a characteristic manner to 

 the sudden influence of the light, and this occasionally assists in 

 directing the individual into the dark region. 



13 The general reactions, consisting in increased activity 

 within the light region and an ultimate assembling in the dark 

 region, were due to photokinesis. The assembling and remaining 

 within the dark region occurred when photokinesis was not strong 

 enough to cause the animals to recoil from the dark end of the 

 tank so as to reenter the illuminated region. 



Nagel ('94) has called the reactions of animals to sudden illum- 

 ination "photoskioptic" and to sudden shading "skioptic." 

 Applying Nagel's terms here, it may be said that both Asellus and 

 Caecidotea are both photoskioptic and skioptic, since both species 

 (or certainly Asellus) sometimes immediately respond to the sud- 

 den change in illumination encountered in passing from a dark 

 region to an illuminated one, and also on passing from an illumi- 

 nated to a dark region. Yerkes ('03, p. 306) states that with the 

 jelly fish Gonionemus murbachii, an "increase in light intensity 

 uniformly causes a motor reaction in quiescent individuals, and 

 the inhibition of movement in active individuals. Decrease in light 

 intensity usually causes the inhibition of movement in active in- 

 dividuals, but rarely does it act as a stimulus to activity in case 

 of resting animals." 



With Asellus and Caecidotea the photoskioptic and skioptic 

 responses are more variable than in Gonionemus. Both species 

 of Crustacea when active sometimes respond to a sudden increase 

 in the intensity of illumination by an inhibition of movement, at 

 other times by an abrupt acceleration of movement; when quies- 

 cent, Asellus often responds to increased intensity of illumination 

 by moving at once. If in motion, Asellus very often responds 

 to a sudden decrease in the illumination by coming to an abrupt 

 stop. 



These experiments with vertical illumination have shown that 

 both species respond in a photokinetic way and ultimately collect 

 in a dark region. Asellus responds more quickly and more gener- 



