132 
president’s address. 
suits it best, without adapting itself in succession to a series of 
differently coloured weeds. 
A very different result was obtained by altering the intensity in 
the illumination of the specimens ; and definite colour-changes 
were rapidly induced by changes in the intensity of the light. 
Some evidence was obtained that the periodic rise and fall of the 
tides thus affects the colour. Specimens taken at low spring-tides 
were of a lighter colour than those which were trawled in the same 
places in higher states of the tide, when the prawns were covered 
with several feet of muddy water. 
The most striking individual change is, however, that which is 
associated with nightfall. Every evening, at a time which alters 
with the seasons, a reddish tint makes its appearance in each 
Hippolyte. This is followed by a change to green, which “ gradually 
melts into blue,” the body becoming at the same time more trans- 
parent. “Thus, as darkness falls, Hippolyte 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, the more deeply 
pigmented specimens becoming more blue than those which have 
less pigment. This nocturnal blue colour may be replaced by the 
diurnal colour in less than a minute when the prawn is exposed to 
incandescent gas-light. 
The colour-changes are due to the existence of “ chromatophores,” 
which occur in the connective tissue, in definite relation with the 
skin, the digestive organs, or the vascular and muscular systems. 
Each chromatophore is a body which passes into excessively fine, 
branched, tubular processes ; and it contains red, yellow, and blue 
pigments. In nocturnal specimens the red and yellow pigments 
are for the most part retracted into dense masses whose superficial 
area is so small as to produce no great effect on the general color- 
ation ] the blue pigment, on the contrary, being spread out through 
the whole of the chromatophore and the fine reticulum of tubules 
which surrounds it. During the day, the chromatophore usually 
consists of red and yellow pigments ; and it may hardly be possible 
to distinguish any trace of the blue colour. 
