234 THE BIOLOGY OF THE SEA-SHORE 



behaviour in question ; (2) the impulse towards or away from 

 the light (tropism) ; and (3) the turning or oscillatory impulse 

 (differential sensitivity). 



Keeble and Gamble (1904) have made a profound research 

 into the effects of light on littoral Crustacea ; in particular 

 Macromysis, Hippolyte, and Palcemon. 



Instead of becoming inured, these animals remain highly 

 susceptible to the light conditions of their environment ; 

 thus, light affects the amount of pigment in the chromato- 

 phores, and its distribution in these structures, giving rise to 

 changes of colour ; light affects the irritability of the organ- 

 ism, forms like Hippolyte and Palcemon after remaining 

 quiet all day being seized at night by a strange restlessness ; 

 light influences the orientation and movements of various 

 Crustacea, forcing them to assume particular attitudes and 

 to move in a particular direction, and so on. 



The question of colour change is of particular interest. 

 The changes of tint so often observed are brought about 

 by the expansion and contraction of masses of pigment 

 contained in irregularly shaped cells with numerous branched 

 processes — the chromatophores (in Mysids the chro- 

 matophores are multicellular structures, in Palcemon they 

 are multinuclear). On appropriate stimulation the pigment 

 flows out into the branches in such a way that what was a 

 mere pin-point of pigment becomes spread out over a wide 

 surface, producing a change in coloration. More than 

 one kind of pigment may be present in a chromatophore ; 

 red, yellow, and blue pigments may occur together. The 

 movements of the pigment may be either of the nature of a 

 direct response to the light, or they may be modified by the 

 action of the eye and nervous system. The direct response 

 is a reaction to light intensity and is of an ephemeral nature, 

 of no value for protection. The indirect response is deter- 

 mined by background and is independent of variations of 

 intensity. In contrast to the direct responses it is ^low and 

 lasting. 



Reactions to Moisture. — The moisture conditions of 

 the shore are continually varying between the two extremes 



