622 F. W. GAMBLE AND F. W. KEEBLE. 



Hippolyte exhibits marked sensitiveness to changes of 

 light-intensity, but offers no rapid positive response to light 

 in virtue of its colour. At least three kinds of colour-change 

 must be distinguished. First, the slow sympathetic colour- 

 change which accompanies a change in the colour of its weed. 

 Second, the rapid changes produced by altering the light 

 intensity. Third, a periodic habit of changing from the 

 motley of the daytime to blue at night. We deal with this 

 habit in the next section. 



Section V. 

 Nocturnal Colour. — Nocturnes. 



Hippolyte varians, living in the so-called ^''Lami- 

 narian zone,'^ is subject, owing to tidal movements, twice 

 every twenty-four hours to change in illumination. At high 

 tide, the animals may be living in comparative obscmnty 

 beneath twenty feet of muddy water ; whereas later in the day, 

 in the season of spring-tides at all events, the prawns may be 

 exposed in shallow pools to full daylight. Since Hippolyte 

 seems not to wander far from its food-plants, and since, 

 moreover, as we have shown, it is very sensitive to change 

 in light-intensity, there will doubtless be daily changes of 

 coloration in the prawns in response to these periodic altera- 

 tions in light. "VVe find, in fact, that at very low spring- 

 tides in summer, which always ebb about 6 a.m. and 6 p.m., 

 prawns from the Halidrys beds are of a lighter tint than 

 those trawled from the same spot in several feet of water. A 

 careful study of colour-changes of the littoral fauna would 

 probably bring to light many cases showing response to 

 recurring alterations in illumination. 



A much more important colour-change than that which we 

 have just recorded is effected by the daily alternations of 

 light and darkness. Every evening, as darkness comes on, 

 Hippolyte gradually loses its distinctive diurnal colour. In 

 summer the change begins about 9 p.m., in winter at about 

 5 p.m. Toward this or that time, according to the season, a 



