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. [Illustration: FIG. 36--A COMMON HERMIT CRAB (_Eupagurus bernhardus_) REMOVED FROM THE SHELL]