Fig. [117].—Remipes scutellatus, dorsal and ventral views, × 1. (From original drawings prepared for Professor Weldon.)
Fam. 2. Hippidae.—The first legs are simple, the carapace is subcylindrical with expansions covering the legs. Remipes (Fig. [117]) and Hippa in tropical or sub-tropical seas.
Tribe 3. Paguridea.[[142]]
The ordinary Hermit-crabs, common on the English as on every coast, are characterised by the fleshy asymmetrical abdomen from which all the hard matter has disappeared, and which is carried tucked away in an empty Gasteropod shell. The abdomen is spirally wound in accordance with the shape of the shell, and a firm attachment is effected by means of the sixth pair of pleopods, especially that of the left side, which is fashioned into the form of a hook and is curled round the columella of the shell; this attachment is so secure that in trying to pull a Hermit-crab out of its shell the body is torn apart before the hold gives way. The other pleopods are in a much reduced condition, being generally altogether absent from the right side of the abdomen, and often greatly reduced on the left side, especially in the male, though in the female they are still used for the attachment of the eggs.
The last two pereiopods are much reduced and are concealed inside the shell, which they help to carry. The great chelae are usually asymmetrically developed, that on the right side being much larger than that on the left, and often serving the purpose of shutting the entrance to the shell when the crab is withdrawn inside.
The constant association of a large group of animals like the Hermit-crabs with the appropriated empty houses of another group is sufficiently curious, but it does not stop there. In almost every case there are present one or more Sea-anemones growing on the outside of the shell, and each kind of Hermit-crab generally carries a special kind of Anemone. Thus at Plymouth, Eupagurus bernhardus is generally symbiotic with Sagartia parasitica, or else with a colony of Hydractinia echinata, while E. prideauxii is usually associated with Adamsia palliata. In the latter case the shell is frequently absorbed, so that the Anemone comes to envelop the crab like a blanket. Instead of Anemones carried turret-like and imposing aloft, or enveloping the inmate of the shell like a blanket, some of the Hermits have Sponges, an unexpected association; and it is a common sight at Naples to find the little red round Sponge, Suberites, running around animated by its Hermit within. It is held that Anemone and crab mutually assist one another, that the Anemone stings the crab’s enemies, and that the Hermit-crab carries the Anemone to new feeding-grounds. It is also said that when a crab grows too big for its shell, and is forced to seek another, it persuades the Anemone to loosen its attachment to the deserted shell and to be transplanted to the new one, and that there is something mesmeric in its power, because nobody else can pull an Anemone off a shell without either cutting it off at the base or tearing it to pieces. Other animals as well sometimes enter into this partnership. At Plymouth a Polychaet worm, Nereis fucata, frequently inhabits the Whelk’s shell, together with Eupagurus bernhardus, and puts out its head for a share of each meal; and at Naples the Amphipod Lysianax punctatus is almost always present in the shells of Eupagurus prideauxii.
Fig. [118].—Pylocheles miersii, × 1. A, End view of a piece of mangrove or bamboo, the opening of which is closed by the great chelae (c) of the Pagurid; B, the animal removed from its house. (After Alcock.)
Besides the ordinary twisted Pagurids which inhabit Gasteropod shells, there are a few which preserve the symmetry of the body. The interesting Pylocheles miersii[[143]] (Fig. [118]), taken by the Investigator in the Andaman Sea at 185 fathoms, inhabits pieces of bamboo; it is perfectly symmetrical, with well-developed pleopods and symmetrical chelae, which, when the animal is withdrawn, completely shut up the entrance to its house (Fig. [118], A).