SENSOEY REACTIONS OF CHROMODORIS ZEBRA 293 



time occupied by the contraction process is less in the case of a 

 shading response (0.6 to 1.0 seconds) than in a tactile reaction 

 involving about the same degree of muscular activity. 



The duration of the shadow must be appreciable to have a 

 detectable effect. The duration increases with the size of the 

 animal. For nudibranchs 10 to 12 cm. long, an opaque body, 

 such as one's hand, moving at about 50 to 60 cm. a second between 

 the gills and the sun provides very nearly the threshold of acti- 

 vation. With the removal of the shading the gill plumes expand; 

 they tend to remain contracted in the dark, provided the whole 

 animal or its posterior end is shaded. If, while the plumes them- 

 selves are contracting as the result of shading, the light be sud- 

 denly or slowly increased again, the retraction of the gill crown 

 is inhibited and protrusion begins. 



It was previously shown that the total response of the gill 

 crown involves two reactions : contraction of each gill plume and 

 the retraction of the whole branchial apparatus. These are quite 

 distinct things. The retraction and extension of the gill crown 

 is brought about by muscles at its base. This is determined by 

 the acting light intensity, owing apparently to the fact that bright 

 light decreases the tonus of the muscles in this region, allowing 

 fluid to accumulate there, under pressure from other portions of 

 the body. The reflex contraction of basal muscles causes the 

 crown to be retracted. The contraction of the individual plumes, 

 however, is determined by shading as such; because, after the 

 initial twitch or longitudinal contraction of the plumes they 

 become relaxed even if the state of decreased light intensity 

 continues. The plumes do not react to suddenly increased illu- 

 mination. The degree of contraction of the gill plumes when 

 shaded determines whether the whole crown shall be retracted or 

 not. The relations here are very similar to those previously 

 discussed under the head of the tactile activation of the plumes. 

 If the self-contraction of the plumes be sufficiently violent, more 

 or less complete contraction of the whole crown ensues; in this 

 case the reextension of the gill crown commences within a few 

 seconds, as under tactile activation, even though the shading re- 

 mains constant and the crown may not be fully reextended. The 



