A number of Crustacea are known to possess a chameleon-like power of changing their colour. The mechanism by which this change is effected is similar to that found in other animals, such as fish and frogs, which have the same power. The pigment which gives its colour to the animal is lodged in microscopic star-shaped bodies known as _chromatophores_, lying for the most part just below the skin. Each chromatophore consists of a central body from which a number of branching filaments radiate. The pigment may contract into the centre of the chromatophore, forming a minute and hardly visible speck, or it may spread out into the branching filaments, forming a distinct spot of colour. Each chromatophore may in some cases contain several colours of pigment, and these may expand or contract independently of each other, so that a whole series of changes may be produced by a single chromatophore. In the larger Crabs and Lobsters the visible colour of the animals depends on pigment in the shelly exoskeleton, which is thick enough to hide the chromatophores in the living tissues underneath, and no very rapid or considerable changes are apparent; but in the smaller forms, in which the exoskeleton is thin and translucent enough to allow the underlying colours to appear through it, the changes in the chromatophores may produce striking effects. Thus, Fritz Müller describes a species of Fiddler Crab of the genus _Gelasimus_, in which the hinder part of the carapace was brilliantly white, but five minutes after the Crab was captured it had changed to a dull grey. Many other cases of colour change have been described, but most remarkable and the most fully studied is that of the Prawn, _Hippolyte varians_, which is very common on our own coasts, and has recently been the subject of a very elaborate series of researches by Professors Keeble and Gamble. The specimens of this Prawn show "a bewildering variety of colour and of colour-pattern"; they may be uniformly coloured in various shades of brown, green, or red, or they may be "blotched," "barred," or "lined," with colour. These different varieties are generally found among sea-weeds, which they resemble in colour and pattern, the "lined" forms, for instance, frequenting finely branched and feathery weed. Like many other protectively coloured animals, they are of sedentary habits, clinging to the weed, and seldom moving by day. If a specimen be removed from its habitat and placed in an aquarium with different kinds of sea-weed, it will take refuge among that which it most closely resembles. It appears that this resemblance in colour-pattern is acquired during the growth of the Prawn, and that a young specimen kept among finely branched sea-weed will acquire the "lined" pattern, while others, living among coarser weed, become "barred," "blotched," or "monochrome." Even in the adult Prawns the colour (though not the pattern) becomes changed in a day or