To remove the creature from its home is no easy matter as a rule. To pull it out by means of its legs or its antennæ would probably be to sever some portion of its body; but if you thrust the creature, shell and all, among the spreading tentacles of a large anemone, it will at once grasp the peril of the situation; and, if the shell has already been secured by the clinging petals of this dangerous marine flower, the hermit will speedily quit its home and endeavour to rush from the many snares in order to secure its freedom. Or, it not infrequently happens that the occupied shell is one that has withstood many a storm, but not without the loss of the apex of its cone. In this case the insertion of a very flexible fibre into the opening thus made will cause the hermit to leave its home in the possession of the enemy.
Having, by some means or other, managed to drive the crab from its shell, we place it in a shallow rock pool, or in a vessel of sea water, and observe the chief features of its structure.
The first thing that strikes one is the absence of a calcified skin on the extended abdomen, which is so soft that, remembering with what eagerness fishes will attack and devour crabs of all kinds, we can at once understand the necessity of such a home as the creature selects. Again, we observe the presence of appendages at the tip of the abdomen by means of which the crab is enabled to hold itself securely in the shell. Also, when we note the general form of the armoured portion of the body, and the position of the soft-skinned abdomen, we can see how well adapted the whole is to fit snugly into the spiral shell of a whelk or winkle.
We also observe that one of the pincers is much larger than the other, and the value of such an arrangement may be estimated when we see the animal at home. The smaller claw, together with the other appendages used for walking or prehension, can be retracted within the shell, but the large claw, which constitutes a formidable weapon of attack and defence, is not only in such a position as to be ready for immediate use; but, lying as it does in front of the body, with other portions hidden more or less behind it, it serves the purpose of a shield when the animal retires.
If we place a homeless hermit crab in a rock pool, the behaviour of the creature immediately suggests a feeling of uneasiness—a sense of danger—for it moves about in a very erratic fashion that is quite different from the straightforward and deliberate action of the same animal when properly protected; and very amusing results may be obtained by making it the subject of a few harmless experiments. For instance, drop down before it an empty whelk-shell that is much too large to properly accommodate its body. It will immediately approach the untenanted house, search and probe it well with its antennæ and other appendages, and then, finding it uninhabited, and having no apartment of more suitable size at hand, will abruptly gives its body a turn and hastily thrust itself backwards into it.
If at the time of this experiment the advancing tide disturbs the water of the pool, the result is somewhat ludicrous, for the shell, too cumbersome to be controlled by the creature within, is, regardless of its attempts to maintain a normal position, turned over and over as each wave advances and retreats.
Again, supposing the shell supplied to be too small for the intended occupant, it will, after the usual examination of the interior, thrust its soft abdomen as far in as possible, and make the best of the unsatisfactory circumstances until a more suitable home can be found. And if, at this distressing period, we drop before it a shell of just the right size—the one from which the creature was originally expelled for instance, it is astonishing how quickly the change of houses will be accomplished. After a brief examination of the shell with the object of determining whether all is right within, during which the crab continues to avail itself of the imperfect accommodation afforded by the previous shell, it rapidly extracts its body from the one and thrusts itself backwards into the other. Its normal habits are at once resumed, all its movements being now suggestive of confidence and contentment.
We have already referred ([p. 153]) to the fact that a large anemone (Sagartia parasitica) is commonly found attached to a whelk shell, which at the same time forms the home of the hermit crab, and ([p. 44]) that a marine worm (Nereis) is also a common associate of the hermit, taking up its abode in the interior of the same shell; and we also briefly discussed the mutual advantage of such an arrangement to the parties concerned. These triple combinations are not so frequently met with on the shore between the tide-marks, but are dredged in considerable numbers by the trawler; and the reader will find it repay him to secure one in order that he may be able to watch the interesting habits of the associates. The movements of the hermit crab are always pleasing, particularly the manner in which it seizes and manipulates its food; and still more so is the occasional appearance of the head of the worm, always in exactly the same place, for the purpose of deliberately stealing the food from the very jaws of the crab.
Hermit crabs are easily kept in captivity, and may be fed on any kind of animal food, but care should be taken not to allow an excess of food to remain in the water and render it putrid by decomposition. As long as the crabs are active and remain within their shells you may assume that the conditions are favourable; but when they become sluggish in their movements, and leave their homes, the sanitary condition of the aquarium should be regarded with suspicion; for hermit crabs, like many of the marine tube worms, generally quit their homes when the conditions are unfavourable, as if they preferred to die outside.
The Common Hermit Crab (Pagurus Bernhardus), also known as the Soldier Crab, on account of its very pugnacious habits, is common almost everywhere on our coasts, and may be distinguished by the numerous little tubercles on the claws and on the upper edge of the front legs; and there are several other species, belonging to the same genus, distributed more or less locally on the various shores. All are similar in general structure and habits, the various species being identified principally by means of their colour, the variations in the form of the appendages, and the general character—smooth, tubercular, spiny, &c.—of the exo-skeleton. One species, found in the sandy bays of Cornwall, burrows rapidly in the sand.