586 



RESPONSE TO SHADING OF CKROMODORIS 



It was desirable to consider some of the influences controlling the 

 variability in the responses of the gill-plumes when shaded; for, as 

 noted in the paper referred to above, the exhibition of the reaction 

 is by no means constant. In nature Chromodoris shows but slight 

 variability with regard to the amplitude of the response to shading. 

 With animals of medium size (6 to 10 cm. long), at about 27°, an 

 interval of 10 to 15 minutes must elapse between successive stimula- 



B 



Fig. 1. A. Semidiagrammatic view of Chromodoris zebra, showing the gill- 

 crown normally extended. The number of plumes varies from eight to sixteen, 

 with a mode of twelve. 



B. The gill-crown contracted within its pocket, the collar closed. 



tions in order to obtain the maximal reaction at each trial. The great 

 variation apparent in laboratory tests is determined by the confine- 

 ment of the nudibranchs to aquaria. In dishes of non-circulating 

 water the alkalinity is slowly reduced, owing to the excretion of CO2. 

 The nature of the response to shading was determined in a series of 

 twenty shallow vessels of similar size and shape, exposed to the 

 same conditions of illumination, in which the alkalinity had been 



