CILIARY ACTIVITY 45 



Hippolyte. Hippolyte, as we have seen, displays a remarkably 

 close resemblance of bodily colour to the hue of the seaweeds 

 on which it resides. While this similarity is striking, there are 

 in reaHty two sharply defined categories of colour forms, greens 

 on the one hand, and reds or browns on the other. The green 

 state corresponds to an " expansion " of the yellow pigment, 

 the red being withdrawn into the cell bodies whose branches 

 are suffused with the blue " nocturnal " substance ; and the 

 brown or red colour forms display " extension " of the red 

 pigment into the cell branches ; we should be tempted to 

 presume that sympathetic coloration depends on the wave- 

 length of the light reflected respectively by green or brown 

 weeds ; this interpretation is not, however, compatible with the 

 results of experiments in which the effect of light of different 

 wave-lengths has been put to the test. 



In carrying out experiments on the effect of monochromatic 

 light of different qualities, it is evident from what has already 

 been stated that the direction of the incident rays must be taken 

 into account by controlling each experiment by comparison of 

 the response evoked both by absorbing and dispersing back- 

 grounds. Monochromatic lights, whether red, yellow, green, 

 or blue, acting on a black background, produce pigment 

 expansion, the yellow pigment reacting more readily than the 

 red. Monochromatic lights, whether red, yellow, green, or 

 blue, acting on a white background, induce, like white light, 

 the transparent condition. Thus the quality of the light does 

 not seem to be the effective factor which evokes '' sympathetic " 

 coloration in Hippolyte. It would, therefore, seem that the 

 slowly effected transition from one colour to another, when 

 individuals are transferred from green to red weeds or vice 

 versa, is in all probability a reaction to the different intensities 

 and not to the predominant wave-lengths of the light reflected 

 from the differently coloured surfaces of the green, red, or 

 brown weeds. It should be added, however, that the diurnal- 

 nocturnal rhythm of colour change is in part an automatic 

 process whose periodicity is independent of external stimuli 

 once there is an appreciable tendency to contract by night or 

 expand by day, when the animals are kept in continuous light 



