The Lobster's habit of seeking shelter in rock-crevices or under stones is one which is shared by a very large number of shore Crustacea. From some primitive kind of Lobster which discovered the advantages of a portable shelter have been derived the Hermit Crabs. In rock-pools one may often see whelk or periwinkle shells tumbling about with an activity quite foreign to the nature of their original molluscan inhabitants, and closer examination will show that each contains a Hermit Crab, which retreats into the shell when disturbed. If extracted from the shell, the Crab (Fig. 36) can be seen to be most beautifully adapted to its peculiar mode of life. The abdomen is soft and spirally twisted to fit into the interior of the spiral shell, and the uropods, instead of forming a tail-fan, are modified into holding organs, with roughened, file-like surfaces which can be pressed outwards against the walls of the shell, and wedge the body so firmly that an attempt to drag the animal forcibly from its retreat often results in tearing it in half. The front part of the body, which is exposed when the animal is walking, retains its shelly armour. One of the pincer-claws, most commonly the right, is much larger than the other, and serves to block the opening of the shell when the body is withdrawn into it. The next two pairs of legs are long and slender, and are used for walking; but the last two pairs are short, with a roughened surface at the end, and serve to steady the body in the mouth of the shell. The swimmerets on the right side of the body, which is pressed against the central pillar of the shell, have disappeared, but those of the left side remain.