3o8 • A.M. Banta 



dark as it could be made under the conditions, there being a sharp 

 plane of demarkation between the halves: 



1 Asellus is more active in the illuminated region than in the 

 dark one. 



2 The photokinetic effect upon Asellus is such that for a time 

 it very generally recoils from the end of the darkened half of the 

 tank and reenters the illuminated region. 



3 Asellus showsimmediate responsiveness to the sudden change 

 in the intensity of illumination, these responses manifesting them- 

 selves in inhibition of the animal's movements. Sometimes it 

 stops on entering the light region — whereupon it occasionally 

 turns back, but more often this happens when it first enters the 

 dark region. Sometimes Asellus shows immediately accel- 

 erated movements when it enters the illuminated region, but when 

 this is not the case increased activity usually appears very soon. 



4 The animals (Asellus ) ultimately collect and remain within 

 the dark region. 



5 The reactions of Caecidotea resemble those of Asellus, but 

 Caecidotea reacts at the bounding plane much less often and less 

 decidedly than Asellus. Caecidotea collect within the dark region 

 more slowly than Asellus and do not remain there as exclusively 

 as do the latter animals. 



III. With Illumination by Direct Sunlight 



To the sun's rays at right angles to the long axis of the tank 

 both species are more responsive than to an illumination by arti- 

 ficial (Nernst lamp) light of 6983 CM. intensity, although they 

 respond in much the same manner. As under the oth^r methods- 

 of illuminat'on, Asellus is decidedly more respons've than 

 Caec'dotea. 



