The Colour-Physiology of Hippolyte varians. 465 



the females, which are more resourceful in adjusting their colour to 

 their surroundings. From the point of view of " protection " this is 

 what might be expected owing to the greater sluggishness of the 

 female, which in turn is partly due to the large mass of eggs or 

 developing zoea?, which she almost invariably carries. 



V. The Nocturnal Colour. " Nocturne*." 



"Whatever the diurnal colour of Hippolyte may be, it changes at or 

 soon after night-fall to a wonderfully beautiful transparent blue or 

 greenish-blue colour. Prawns in this condition we designate as 

 Nocturnes. The depth of the nocturnal tint is directly proportioned to 

 that of the diurnal colour ; dark brown prawns becoming deep blue 

 and light ones pale blue. Under natural conditions the nocturnal 

 colour persists until daybreak. At the first touch of dawn the colour 

 vanishes and that of the preceding day re-appears. Specimens trawled 

 at night and in the early morning before daylight show that the 

 nocturnal colour is perfectly normal and is assumed by Hippohjte 

 mrians while still on its food plant. Other Crustacea too, show a 

 peculiar nocturnal colour. Mysis, for example, of different species and 

 possibly even of distinct genera, show a transparent and blue colour- 

 phase at night, giving place during the day to a deeply pigmented 

 condition. 



Nocturnes are remarkable chiefly but not solely for their peculiar 

 pigmentary condition. The red and yellow pigments are maximally 

 contracted, while the blue is present in a very diffuse homogeneous 

 condition forming a network which traverses the connective tissue of 

 all the chief organs, particularly the muscles. The peculiar trans- 

 parency which accompanies this nocturnal condition is, however, not 

 entirely explicable by the retraction of the red and yellow-coloured 

 pigment. It is only one of a number of profound changes affecting 

 the body as a whole. Indeed we are prepared to say that the noc- 

 turnal state opens up a new chapter in biological investigation, and 

 that by a study of this condition increased knowledge of the succes- 

 sion of metabolic processes may be gained. 



VI. Period kit ij of Colour-change. 



Under normal conditions Hippolyte variant passes through a daily 

 colour-cycle. Its diurnal colour gives place to a slight increase of 

 reddishness — a sunset-glow — just before night-fall, and this ushers in 

 the nocturnal phase. These changes are periodic in the strict sense 

 of the word. Though often modified by external agents the}^ exhibit 

 a certain independence of them. In constant darkness a Nocturne 

 recovers its diurnal colour. In constant light (of certain kinds at 



2 M 2 



