The tract of seashore which is laid bare by the retreat of the tide offers on most coasts a rich collecting-ground to the student of Crustacea. In places where shelving, weed-covered rocks run out to sea, innumerable Crustacea have their home in the rock-pools, or lurk in crannies awaiting the return of the tide. On sandy beaches, at first sight apparently barren of life, a closer search will reveal a whole fauna, amongst which burrowing Crustacea of various orders are prominent. Further, the shore collector will find from time to time stray specimens of forms that have their proper habitat beyond low-tide mark, and occasionally their remains are thrown in quantities on the beach by storms. It is convenient, therefore, to treat the Crustacea of the shore as a sample of those inhabiting the shallower waters of the ocean. In these shallower waters—down to the limit where light no longer penetrates from above, where vegetable life ceases, and where the strangely modified inhabitants of the deep sea begin to appear—the sea-bottom is perhaps the most densely populated of all parts of the earth's surface. Nowhere, at all events, do we find so wide a range of animal forms, from the simplest organisms (Protozoa) up to highly-organized Vertebrates. Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin.

Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "shifts for a living"—of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the shore and of shallow water.

Though it might seem that the heavily armoured Lobsters and the larger Crabs would be sufficiently protected against most enemies when once they have attained their full size, yet they are preyed upon by the Octopus, which seizes them with its suckers and pierces their armour with its powerful beak, injecting a poison that paralyzes its victims. Some years ago a "plague" of Octopus very seriously affected the Lobster fishery in the English Channel. To escape from enemies such as these, the Lobsters and many Crabs have the habit of lurking in crevices of the rocks, while in case of sudden alarm the Lobster may escape from danger by swimming, or rather darting, with great swiftness, tail foremost, through the water by powerful strokes of the abdomen and tail-fan. In the more lightly armed Prawns and other Crustacea of the tribe Natantia, which are characteristically swimmers, the power of rapid motion is probably the chief means of protection against enemies. There is reason to believe that the Lobsters have been derived from prawn-like swimming forms which have sacrificed some of their agility in developing their heavy armour-plating, retaining, however, the power of sudden and rapid motion in emergency. This power, again, has been lost by the typical Crabs (Brachyura), in which the abdomen is reduced in size and without a tail-fan, so as to be useless for swimming. While most of the Crabs, however, are somewhat slow of movement, trusting to their armour and their powerful pincers for defence, the Swimming Crabs (Portunidæ—[Plate XIII].) have reacquired the power of swimming by means of the paddle-shaped legs of the last pair. Some of the tropical species of Portunidæ are probably the most expert swimmers among the Crustacea, and are described as shooting through the water like fish.

Fig. 36—A Common Hermit Crab (Eupagurus bernhardus) removed from the Shell

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Fig. 37—Pylocheles miersii, a Symmetrical Hermit Crab. (After Alcock.)