SENSORY REACTIONS OF CHROMODORIS ZEBRA 291 



There is, at any rate, an instructive correlation in these re- 

 sponses : illumination induces creeping, and also induces extension 

 of the branchial apparatus; it is to be presumed that increased 

 oxidation necessary for locomotion is in this way assisted. The 

 photopositive behavior of Chromodoris is not accompanied by 

 any reactions to changes of light intensity. It is an example of 

 phototropism in the strict sense, in which trial movements do 

 not appear. 



Long-continued observation of Chromodoris in the field has 

 shown that the positive phototropism of this nudibranch is of 

 great bionomic importance (Crozier, '16 b, '18 c), a preponderat- 

 ing element in natural behavior. On days when the sky is 

 overcast, relatively few of them are to be found. The brighter 

 the day, other things equal, the more of these nudibranchs 

 one can collect in shallow areas. In suitable spots they can be 

 observed to follow with precision the direction of the sinking 

 sun, whether it leads up-hill or down, according to the nature of 

 the bottom. In this photopositive behavior C. zebra agrees with 

 some Red Sea chromodorids, as described by Crossland ('11), and 

 differs sharply with the behavior of C. Toseapicta at Bermuda 

 and with tropical chromodorids in general as indicated by Eliot's 

 ('04, '10) experience. 



In any one locality where they abound, more of the nudi- 

 branchs are obtainable in mid-afternoon, on a sunny day, than 

 in the morning. But this is true only during the cooler months 

 of the year (October to May). During the summer, none, or a 

 few only, are so obtainable in shallow water (e.g., in the mangrove 

 creeks or on shallow grass flats) , though search among the densely 

 packed eelgrass usually shows that in June and in July a certain 

 number are still there. If one collects at early morning, before 

 sunrise, in Fairyland Creek, relatively large numbers of C. zebra 

 can be had in June and in early July. As the sun rises, the nudi- 

 branchs creep downward on the blades of eelgrass and turtle- 

 grass, and by the time the sun has risen several hours, practically 

 none are to be seen. 



In view of the positive phototropism of C. zebra, consistently 

 exhibited by individuals of every size, this phenomenon was very 



THE JOUBNAL OF EXPERIMENTAL ZOOLOGT, VOL. 29, NO. 2 



