240 THE BIOLOGY OF THE SEA-SHORE 



profound effect upon the functions of the organism, the 

 movements persisting even when the external stimuH have 

 been suppressed. When Convolutas taken directly from 

 their normal habitat are placed in moist sand in a tube, the 

 up-and-down movements continue to take place at the 

 periods corresponding to those of the animals' emergence and 

 submergence on the shore. Not only this, but the variations 

 of the tide are closely followed by the Convolutas m the 

 aquarium, the movements being more sluggish during 

 neaps and more rapid at springs (Bohn, 1910). A similar 

 phenomenon can be observed in periwinkles which have 

 been exposed to the tidal rhythm. 



Inert periwinkles, even in a dry environment, can be 

 reactivated by shaking ; but, according to Bohn (1905), 

 if a collection of these molluscs has been taken from the 

 shore and isolated for a certain time in the laboratory, it 

 is easily demonstrated that, at periods of low tide, one has 

 to shake much longer to produce the reactivation than when 

 the tide is out ; that is to say, the periods of inertia in the 

 laboratory correspond to the periods of desiccation on the 

 shore. 



The impress of the external rhythm on the organism is, 

 of course, not permanent but gradually weakens with the 

 passage of time. 



Certain of the colour changes of shore Crustacea to which 

 reference has already been made, have been shown to be of 

 a rhythmic character. The colour of the i^sop prawn 

 {Hippolyte -varians) is dependent on three pigments, and 

 during the day varies greatly according to the nature of the 

 background, intensity of illumination, and certain other 

 influences. At nightfall, however, the colour of Hippolyte, 

 whatever it may happen to be at the moment, changes to a 

 transparent azure blue, the place of this blue colour being 

 taken at daybreak by the prawn's diurnal tint or tints 

 (Keeble and Gamble, loc. cit.). The animal thus exhibits 

 rhythmic colour change corresponding to the transition from 

 light to darkness and vice versa. 



