It is known as the mud-burrower (_Callianassa subterranea_), and is said to burrow very deeply into mud-banks, scooping out its retreat principally by means of the second and third pairs of legs. Although found at times between the tide-marks, its principal habitat is probably in the mud that is covered by deep water, for it is not uncommonly to be found in the stomachs of fishes that habitually feed in such localities. [Illustration: FIG. 209.--THE MUD-BORER (_Gebia stellata_) (1) AND THE MUD-BURROWER (_Callianassa subterranea_) (2)] Lobsters of all kinds, and, indeed, the marine crustaceans generally, are essentially the scavengers of the sea, for they are carrion-feeders, greedily devouring flesh in all stages of decomposition. Hence the value of their work on the sea shore is very considerable. An examination of shrimps and prawns will at once show their close relationship with lobsters. The general build of their bodies is practically the same, and their appendages, though often different in form from the corresponding limbs of the lobster, will be seen to resemble them closely in arrangement and structure. The exo-skeletons of these creatures are, however, generally hardened by a horny substance (_chitin_) instead of a stony deposit of carbonate of lime. The shrimps and prawns sold for food in our markets are very similar in appearance when alive, the leading distinguishing feature being, perhaps, the presence of a sharp, serrated beak projecting forward from the front portion of the carapace of the latter. The reader is probably acquainted with the fact that the shrimps and prawns used as food have quite a different appearance when alive and in their native element to that displayed by the corresponding wares in the fishmonger’s shop--a fact that applies equally well to the edible crabs and lobsters. Most crustaceans change to a bright red colour when boiled, and, as stated in a previous chapter, the same result is produced by the action of strong spirit. [Illustration: FIG. 210.--THE COMMON SHRIMP (_Crangon vulgaris_)] The Common Shrimp (_Crangon vulgaris_) is an exception, however, for it may be distinguished when boiled by its dull greyish brown colour. When alive this species is of a very pale greenish or greyish tint, lightly spotted with brown; and its habits are so interesting that it will well repay one to watch it either in the aquarium or in a rock pool. It frequents sandy coasts, and can hide itself very quickly by burying its body in the sand, using for this purpose both its legs and its antennæ. The Prawn frequents rocky coasts, where it may often be obtained in large numbers by sweeping with a suitable net under the cover of weeds and stones. Its body is of an exceedingly pale greenish colour, and so transparent that it is quite inconspicuous when in the water. Prawns are turned to a rose-red colour by boiling, and they are captured in large numbers when young and sold as ‘red shrimps.’