300 Proceedings of the Royal Society of Edinburgh. [Sess. 
acts as a double structure, the upper half functioning separately from the 
lower. His discovery in brief was this. When one half of the eye — upper 
or lower, it did not matter — was illuminated, the other half being kept dark, 
the colour of the animal as a whole became dark. When the whole eye 
was simultaneously either darkened or illuminated, the change to a dark 
colour did not occur. His results, of course, incidentally explain how 
Idotea changes colour in response to a change of background ; on a dark 
background the eye is unequally illuminated, on a white background it 
is wholly illuminated. 
Colour change of isopods is due to retraction or expansion of chromato- 
phores in the hypodermis. Ligia and Sphceroma also undergo colour 
change in response to change of background, though some other isopods 
do not — see Tait (1910, 1911). A recent worker, Pieron (1914), whose 
paper I have failed to obtain, has apparently dealt further with colour 
change in isopods. The presumption is that the phenomenon is widely 
distributed throughout the order. 
Examination of the spirit specimens of Glyptonotus shows not only 
that black chromatophores are present under the cuticle, but that these 
are retracted in the lighter coloured animals and expanded in the darker. 
In other words, the animal is capable of changing colour — and this it 
probably does according to the mode of illumination of the eyes. From 
the literature I have been unable to discover whether Chiridotea possesses 
chromatophores. It need hardly be said, however, that the whole question 
of the structure of the eye in relation to colour change is worthy of study. 
During part of the year at least Glyptonotus lives under ice, and there 
are no records to show in what proportion light can penetrate through the 
frozen surface of the sea. Hellen-Hansen (1912) has found that the 
extreme limit at which light can affect a photographic plate exposed 
under open sea- water lies between 1000 and 1700 metres. Lest one 
should imagine that colour change of isopods can occur only under the 
incidence of intense light, I may mention that Ligia exposed on appro- 
priate light or dark backgrounds under feeble red illumination in a 
photographic dark-room completely changes colour within the space of 
an hour or two. 
Summary. 
1. Advantage has been taken of the large size of Glyptonotus to study 
certain structural features, especially skeletal, which cannot be so readily 
investigated in smaller isopods. Where possible, an attempt has been made 
to correlate structural peculiarities with functional use. 
