On our own shores we are familiar with the Hermit crabs making use of various kinds of univalve shells as houses to protect their softer hindparts; but in many of these cases there is a third party in the transaction, which is made to work for the crab's especial advantage. The shell of the mollusk is sometimes covered with a sort of fleshy polype-mass (_Hydractinia echinata_), which is parasitic on the shell. The shell, however, being tenanted also by the active crab, the polype, as it grows, moulds itself on the crab's body, and thus extends the dimensions of its house, so that it has no necessity either to enlarge its dwelling by the absorption of part of the interior shell-wall, or to leave this shell and search for one of ampler size, as other Hermit-crabs are obliged to do who have not the advantage of so accommodating a fellow-lodger. "One can understand," says Dr Gray, "that the Crab may have the instinct to search for shells, on which the coral [polype] has begun to grow; but this will scarcely explain why we never find the [polype] except on shells in which Hermit-crabs have taken up their residence."[233] Small Annelids and Crustaceans not unfrequently burrow into the stony walls of corals; but Dr Gray records a much more uncommon case, from the Guilding collection. "It is an expanded coral, which forms a thin surface on the top of another coral, and is furnished with a number of small, depressed, horizontal cases, opening with an oblong mouth. Some of these contain within them a small, free, crustaceous animal, a _Cymothoa_, which nearly fits the case; and it is evident that, by their moving backwards and forwards on the surface, they have caused the animal of the coral to form one of these cases for the protection of each specimen."[234] The manner in which this result is obtained is thus explained--"The animals which form their habitation in corals, appear to begin their domicile in the same way as the barnacles before referred to; they take advantage of the soft and yielding nature of the animals which form the corals, &c., and taking up a lodgement in their body, all they have to do is to keep a clear passage in it, either by the moving backwards and forwards, the exertion of their limbs, or the ingress or egress of water to and from their bodies, and in time, as the coral is secreted by the animal, it will form a wall round them; but if, by any accident, the parasite animal should not keep a passage from the coral to the surface of the body of the animal clear, which it must be constantly induced to do, since by this means it procures food, the coral animal will in a very short time close over it and bury it alive in the mass of the coral; and this, from the number of these animals, of all sizes and in different stages of growth, which are to be found in the substance of the large and massive corals, must often be occurring. Thus the Italian romance is often literally fulfilled in nature."