208 



CHARLES HARLAN ABBOTT 



TABLE 2 

 Summary of reaction of Oniscus A 11, to directive light of 12.955 CM., shoiving 

 results of daily tests for 15 days. Tests ivere made ivith the animal facing the 

 light {A), facing away from the light (B), and illuminated on the right (C) and 

 left (D) sides 



typically negative as that of either of the other individuals just 

 described. Table 3 shows how this isopod gradually changed 

 from indifferent or positive to negative. 



As an indifferent animal would be about 25 per cent positive, 

 the 45 per cent recorded on the first day shows a considerable 

 degree of positiveness on that day. 



This method of classification shows that these isopods, when 

 stimulated by directive light, usually avoid the light by locomo- 

 tion to regions of lower intensity. As the light stimulus is re- 

 ceived through the eyes, it is further important to know the result 

 of unequal stimulation of the two eyes. This is shown by a 

 study of the angle of the course taken by the animal after it 

 has been exposed to the light. 



2. Orientation — second method 



As was shown in figure 2, the circle on the light field was 

 divided into sixteen sectors, numbered 1 to 8 on each side of 

 the central axis m n. If the homologous classes on the right and 

 left sides are grouped together, the responses of any individual 

 isopod can be divided into eight degrees of negativeness, accord- 

 ing to the sector of the circle which the animal crossed after 

 exposure to the light. 



The same records which were summarized in tables 1 and 3 are 

 shown graphically in figures 6 and 7, after tabulation according 

 to this second method. Figure 6 shows the degree of negative- 

 ness of Oniscus A 10, the numerals on the abscissae corresponding 



