In these curious animals the posterior part of the body is not protected by a crustaceous covering, and therefore the animal seeks protection by inserting its soft and defenseless abdomen into some hollow object, usually the shell of a gasteropod mollusk, as the whelk or the periwinkle (/Buccinum/, /Littorina/). The hermit-crabs do not always use a shell for this purpose, as they are sometimes found in the tubes of plant-stems or in sponges. Like other organisms in the animal world, they seem [pg265] well fitted to make the best of their surroundings, the body becoming modified to suit the peculiar conditions under which they live. In those which inhabit shells the abdomen becomes spiral, in conformity to the convolutions of the shell. All the abdominal appendages are more or less atrophied; the sixth pair become like hooks, and these fasten to the columella of the shell, keeping the crab securely attached to it. In the female some of the abdominal appendages are hair-like and are adapted to carrying the eggs. The thorax, being protected, is protruded from the shell at will. The first pair of feet are much larger than the others, and are provided with claws (chelæ). The first right foot is usually much larger than the left, and, besides the usual functions of capturing and crushing prey, the claw, or hand, serves as an operculum to close the mouth of the shell when the crab retires completely within it. The small left hand is shaped to fill out the parts of the opening not covered by the right one, thus making a close fit. The next two pairs of feet end in simple hooks, and are used for walking and dragging the crab along when he travels. These crabs move about very fast, and the houses upon their backs seem no encumbrance. [pg266] [Illustration: /Pagurus bernhardus/: /ch/, chela of first right leg; /l./4, /l./5, fourth and fifth legs; /t/, abdominal terga; /up/, last pair of appendages, modified to fasten to the columella of the shell in which the hermit lives.]