It has been mentioned above that the Common Shrimp is protected, not only by its habit of lying half buried in sand, but also by its close resemblance in colour to the sand among which it lives. There are many others among the shore Crustacea which show what seems to be a "protective resemblance" in colour and form to their surroundings. It is necessary to be cautious in interpreting these resemblances as necessarily protective, since the fish and other enemies which prey on these Crustacea see them with eyes very different from ours, and probably, in many cases, are guided to their prey by the sense of smell rather than by sight. The "masking" habit of the Spider Crabs, already described, strongly suggests, however, that concealment from sight is an important protection to some shore Crustacea, and helps to make it probable that the same end is reached in other cases by modifications of form and colour. There can be no doubt, at all events, that many Crustacea are very inconspicuous to human eyes when they remain motionless in their natural surroundings. Thus, for example, the Caprellidæ, or "Skeleton Shrimps" (see Fig. 22, p. 54), are hard to detect without very close search, as they cling to the feathery branches of the hydroid zoophytes among which they are usually found. They are strangely modified Amphipods, in which the body is slender and thread-like, and generally of a semi-transparent, whitish or yellowish colour, like the zoophytes on which they live. They clamber about among the branches with a movement like that of a "looper" caterpillar, and often remain clinging by means of the hooked claws of the hinder pairs of legs, with the fore part of the body gently waving about. The little Crabs of the family Leucosiidæ (Oxystomata), of which the British representatives are several species of the genus _Ebalia_, are often extremely like pebbles of the gravel among which they live. In many tropical species the carapace is pitted and eroded, so as to resemble a worn fragment of coral shingle. One of the most striking cases among the Crabs, however, is that of _Huenia proteus_ (Fig. 40), one of the Spider Crabs (Oxyrhyncha), which is found in the Indian and Pacific Oceans. In this little Crab the carapace is flat, and is extraordinarily variable in form. In most of the males it is triangular in outline, but in most of the females and in some males it is broadened by leaf-like expansions of the side edges. Borradaile has pointed out that these broad individuals are usually found among the sea-weed _Halimeda_, and that they closely resemble the fronds of this weed in form and in their greenish colour. [Illustration: FIG. 40--A, A PIECE OF A TROPICAL SEA-WEED (_Halimeda_); B, A CRAB (_Huenia proteus_) WHICH LIVES AMONG THE FRONDS OF _Halimeda_, AND CLOSELY RESEMBLES THEM IN FORM AND COLOUR. REDUCED. (After Borradaile.)]