114 COLORATION OF THE SKINS OF FLAT-FISHES. 
given. Besides the wooden tank already described and seen in the 
figure, there is also a large bell-jar. Both vessels are supported on 
trestles, and the large mirrors are placed beneath, upon the floor of 
the Laboratory. The vessels are placed in front of the tanks on the 
south side of the Laboratory and opposite the south windows, the 
supply of water being conveyed into the vessels by siphons from the 
Laboratory tanks. The fish are seen by reflection in the mirrors, 
At present the entrance of light is absolutely prevented by coverings 
of black cloth, or wooden covers lined with black cloth, except 
through the bottoms of the two vessels. The smaller fish are some 
reared from the brood of 1892, and the results exhibited by these 
are not yet published anywhere; the two larger fish are sole 
survivors from two separate experiments, and each of these is almost 
completely pigmented on the lower side. 
During the period of time over which these experiments have 
extended, I have been studying, in collaboration with Dr. MacMunn, 
the anatomy and the physical and chemical properties of the elements 
to which the coloration is due. The results of these studies are fully 
described and illustrated in the memoir communicated to the Royal 
Society. A general account is all that can be given here. In the 
skins of flat-fishes the chromatophores have been described by Pouchet 
and other zoologists. They are of two kinds. Those of one kind are 
black or dark brown, have very definite outlines, and are contractile. 
They are stellate in form, having, when expanded, branching pro- 
cesses stretching out from the centre in all directions ; but these pro- 
cesses can be partially or entirely retracted, and when completely 
contracted the chromatophore has a circular outline, being really 
nearly spherical in shape. The chromatophores of the other kind 
are yellow in colour. In the flounder the yellow deepens to orange 
at the centre. Usually the outlines of the coloured chromatophores 
are much less distinct and definite than those of the black, and it 
generally looks as though the pigment had diffused to some distance 
into the surrounding tissue. Nevertheless, contraction and dilata- 
tion of these yellow chromatophores takes place. 
The chromatophores are of considerable size, easily seen with a low 
power of the microscope when a slice of the fresh skin is examined. 
But there are present much smaller elements which affect the colora- 
tion. These are angular plates of opaque substance of fixed form, 
having no colour, but reflecting leght strongly. They are called 
iridocytes. 
In the flounder, in the skin of the upper side a layer of chromato- 
phores and iridocytes occurs close beneath the epidermis outside the 
scales, which are small and rudimentary. In the deeper part of the 
skin there are scarcely any chromatophores and no iridocytes, but > 
