in this genus of crabs, are
turned upwards, and apparently serve no other purpose than that of hooks
to hold on the sponge _in situ_.

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 th