SENSORY REACTIONS OF CHROMODORIS ZEBRA 289 



ppsitive prototropism, although many individuals may for a time 

 be inactive and may appear insensitive to light. This is notably 

 true when two or more individuals are together in a dish, in which 

 case, if they be ready for pairing, they may stimulate one another 

 to conjugation. Even here, however, photic irritability is evi- 

 denced in one respect, for in the dark (in well aerated water) the 

 gill crown is almost invariably contracted; upon illumination, 

 even by the light of a match, the gills become extended; this 

 occurs also during the progress of copulation in the dark. 



The directive effect of light is manifest not only in horizontal 

 creeping, but also when the illumination is from above. Sunlight 

 reflected from out-of-doors was caught by a second mirror and 

 reflected vertically downward into a vessel containing Chromo- 

 doris. Active locomotion ensued, and many animals on reaching 

 the side of the jar climbed it until they met the water surface, 

 continuing then along the water line; whereas, on creeping outside 

 the path of the light beam, the nudibranchs tended to return to 

 the bottom of the dish. In successive tests the animals could be 

 forced to creep upward when illuminated, going downward again 

 during intervals of shade. 



/The effect of illumination is distinctly a kinetic one. In a 

 shaded dish the nudibranchs become quiet, but are set into activ- 

 ity at once if light be thrown on the dish. In spite of their 

 positive phototropism, these nudibranchs tend under some con- 

 ditions to collect in the shade. In a shallow dish well shaded 

 on one-half, the dish being in a chamber admitting direct sun- 

 hght from above on one-half, there seemed to be a decided ten- 

 dency for the Chromodoris to collect in the shade; whereas when 

 diffuse light was used the light compartment was the one more 

 frequented. This is explained by the fact that strong hght in- 

 duces greater activity, leading automatically to wandering move- 

 ments, which become less pronounced in the shade, while with 

 diffuse light the photopositive behavior is not interfered with 

 by photokinetic effects. If originally placed in the dark com- 

 partment, the Chromodoris wanders into the light. Even with 

 direct sunlight, falling vertically into the light compartment, no 

 reaction other than increased speed of creeping is detected. 



