292 W. J. CROZIER AND LESLIE B. AREY 



puzzling — especially since it was found that nudibranchs ob- 

 tained (in Fairyland Creek) before sunrise, and tested immediately 

 with lateral hght, were without exception photopositive. 

 No reversal of the customary phototropism occurs, under these 

 conditions, at the time of sunrise. Nor is C. zebra photonega- 

 tive immediately after long exposure to the dark ; but even if this 

 should be true, it would not explain the natural behavior 

 described. 



Several possibilities were considered, among others the possible 

 reversion of phototropism by rise in temperature. The tempera- 

 ture of the water in Fairyland Creek was 24° to 25°C. just before 

 sunrise. The water is very shallow, and is rapidly heated by the 

 vsun's rays so that it quickly reaches a temperature of 27° to 28° 

 as the sun rises. But the phototropism of C. zebra is not altered 

 at any temperature between 17° and 31°; at the higher tempera- 

 tures orientation is quicker, and still toward the light. Other 

 possibilities are dealt with in a preceding section (p. 277). 



The positive phototropism of C. zebra is not affected by pro- 

 longed starvation (four months; Crozier, '18 c), and is the same 

 in sense with animals dredged at various depths down to 8 fathoms ; 

 nor does it vary with the reproductive condition of the animal. 



2. Shadi7ig 



The gill crown of C. zebra reacts to shading, after a detectable 

 latent interval. No other portion of the animal's surface is 

 sensitive in this respect. The gill plumes must themselves be 

 shaded in order to produce a response. The reaction in question 

 is in the form of an incomplete retraction of the gill crown, ac- 

 companied by longitudinal contraction of the individual plumes. 

 The responses are exceedingly variable. The first reaction ob- 

 tained from an animal which has for some time been undisturbed, 

 in the light, is likely to be the most pronounced. This is not 

 always true. Subsequent successive shadings commonly evoke 

 a faint contraction of the plumes, the crown as a whole being 

 little if at all retracted. This reaction is precisely similar to that 

 which may be induced by tactile irritation of the gills, but the 



