588 



RESPONSE TO SHADING OF CHROMODORIS 



matter, since — the gill-crown being as a whole less extended, and its 

 individual plumes less turgid — the direct responses to shading (upon 

 the magnitude of which the reflex contraction of the whole crown is 

 dependent) are specifically less vigorous. But it seemed clear that 

 the photic irritability of the intrinsic reacting mechanism of the 

 plumes was also less than in water of pH = 8.2; this was tested by 

 comparing the reactions of individual plumes shaded singly. 



A 



Fig. 2. A shows the great protrusion of the gills as seen at high temperature, 

 in direct sunlight, and especially in sea water of diminished alkalinity; at x, an 

 injured plume. 



B shows how the retraction of the branchial crown normally resulting from the 

 contraction of the plumes is inhibited at low alkalinities of the sea water; the 

 plumes are here sketched as contracted maximally by touching them sharply. 



Two other influences controlling the exhibition of a response to 

 shading in Chromodoris are temperature and light intensity. Supra- 

 normal temperatures cause the branchiag to become maximally pro- 

 truded; this is seen when the nudibranchs are immersed in water at 

 temperatures of 32-35°. With the sea water cooled to 10°, the 

 branchiae become much retracted; at temperatures between 12 and 

 17°, the photic reactivity of the branchiae is much reduced. Direct 



