HIPPOLYTE VARIANS. 623 



reddish tint — a sunset glow — the foreshadowing of the 

 change, makes its appearance. This is followed by a green 

 tinge which spreads fore and aft from the middle of the body. 

 The green colour gradually melts into blue, and a general 

 increase of transparency sets in. Thus, as darkness falls. 

 Hip poly te is seen to become of a wonderful azure blue 

 colour and absolutely transparent, except in the region of the 

 liver and stomach, which are now very clearly visible. The 

 depth of the blue colour varies in different specimens ; in 

 some it is almost indigo, in others the faint azure of a sky at 

 sunset (PI. 83, figs. 10, 11). 



During August or September specimens of Hippolytein 

 the laboratory or tankroom undergo this change in one and a 

 half to two hours, during December in about one hour. 



Prawns in this nightly condition, exhibiting a deep yet 

 transparent blue colour, we propose to call nocturnes; those 

 which become more transparent, greyish or almost colourless, 

 "grey" or "colourless" nocturnes. The term semi- 

 nocturne may be used to designate the antecedent phase 

 during which the animal is green and semi-transparent. 



We give a few cases to illustrate the change. A "red 

 liner " (fig. 14) becomes green-lined in twilight : and in dark- 

 ness, faintly blue-lined on a highly transparent ground. In a 

 "black-barred" specimen (PI. 33, figs. 12, 13) the transverse 

 markings become deep blue, standing out boldly against the 

 transparent intervals. Dark brown forms become bluish 

 green and ultimately deep transparent blue. Red specimens 

 become first mauve, then colourless or bluish. Green forms 

 pass quickly into the blue nocturnal condition. 



To show that this nocturnal change occurs in Hippolyte 

 living under natural conditions we give the following extracts 

 from our diary : 



I. August 20th, 1898. Dredged with strong tow-nets over 

 the Halidrys bed at Piel, 9 to 9.30 p.m. Tide low, night 

 dark but clear. Eesult : — All Hippolyte were nocturnes 

 when examined at the moment of capture by the light of 

 lamps carried in the boat. During the return trip they were 



