Reactions of I so pods to Light 305 



2. Ccecidotea 



Under the same conditions of illumination several experiments 

 were made with Caecidotea. The results of one of these, in which 

 21 individuals were employed, are shown in detail in Table XIV. 



This table shows that under the conditions of illumination de- 

 scribed, Caecidotea, like Asellus, tended to collect in the dark region. 

 The photokinetic effect appeared quite promptly with the ani- 

 mals in the illuminated region, though it was less prompt than 

 with Asellus. However, most of the animals began moving 

 within from three to five minutes, and within eight minutes this 

 activity had led a majority of them into the dark region. After 

 entering the dark region they returned to the illuminated one 

 much more than Asellus did; but even with Caecidotea the num- 

 ber which kept returning was comparatively small. The number 

 which started into the illuminated region and turned back within 

 a few seconds was such as to suggest that the influence of the 

 change in illumination at the plane of division between the two 

 regions was in many cases immediate. A more careful study of 

 this point seemed desirable, but an unobscured sun at mid-day 

 came so seldom at that time, that the necessary observations were 

 not made. 



The other experiments made with Caecidotea under the same 

 conditions gave results in entire accord with those of the experi- 

 ment discussed above. The average per cent of Caecidotea in the 

 illuminated area for the whole time of all the experiments of this 

 series was 22.4. 



Caecidotea, then, was quickly afi^ected in a photokinetic way by 

 direct sunlight, though not so quickly as Asellus. This activity 

 incidentally led the animal into the dark region, from which it 

 very generally did not return. Caecidotea just entering the illum- 

 inated region from the dark one sometimes turned back into the 

 dark region at the plane of division between the two regions. 



Comparing Asellus with Caecidotea when subjected to illumina- 

 tion by direct sunlight with the rays at right angles to the long 

 axis of the tank, Asellus proved decidedly the more respon- 

 sive. It was afi^ected more quickly by the sunlight, sooner 



