HTPPOLYTE VARIANS. 635 



Hippolyte differs in an important manner from that which 

 obtains during the day. Indeed, we are prepared to say that 

 the nocturnal state opens up a new chapter in biological 

 investigation, and that by a study of this condition increased 

 knowledge of the succession of metabolic events may be 

 gained. 



Section VI. 



Periodicity of Colour-change. 



Hippolyte of the most diverse hues become nocturnes 

 at evening. Daylight invests them once again with their 

 motley. Change of light-intensity ushers in either colour- 

 change. As light waxes the diurnal colour appears ; as it 

 wanes, the blue. Change of light-intensity is, we have shown, 

 an efficient stimulus to colour- change. No other factor of 

 their environment, as far as we have discovered, so profoundly 

 affects the colour-state. It might therefore be concluded 

 that the morning and evening changes are due, and solely 

 due, to increase or decrease of light-intensity. Kecovery 

 from the nocturnal state seems, however, at times to anticipate 

 the dawn, though this might be supposed to be due to the 

 exquisite sensibility of Hippolyte in the nocturnal phase. 

 But the colour-change occurs, though light-conditions are 

 constant ; in other words, the change is periodic. The idea 

 of periodicity is familiar to the botanist and physiologist ; 

 the former knows it to be a function of growth itself, and of 

 the ^^ sleep" and other movements of many sensitive plants; 

 the latter recognises it as characteristic of the metabolic pro- 

 cesses in higher animals. The only case among Invertebrates 

 in which periodicity has been claimed to manifest itself — 

 though it cannot be said that it has been demonstrated — is 

 that of the ascent of small pelagic animals to the sea-surface 

 at night, and their descent to deep water during the day. 

 The argument in favour of the periodicity of this movement 

 lies in the behaviour of the Southern Plankton borne into 

 northern waters during the short arctic summer. It is stated 

 (Walther, 1893, p. 148) that, though the light-intensity re- 



