o, the fish is seen moving freely in
the cavity of its body. If it be taken out, it immediately seeks the
shade. If the two halves of the _Culcita_ (still alive) be placed in the
water, the fish will soon be seen to draw towards them, in order to get
into the cavity of the star-fish. When exposed to the light, it is
uneasy, and its iris contracts excessively. The author never found two
fishes in the same star-fish.

"In most of the fishes examined by him, the author found the stomach
empty; it was full only in one. The contents of the stomach had the
appearance of a lump of fat, and consisted of half-digested muscle.
Under the microscope, striated muscular fibres could be detected, and
the author thinks that they belonged to the muscles of a fish. This
circumstance proves that _Oxybeles_ does not feed upon the chyle of the
star-fish, but that its nourishment is analogous to that of other
fishes. Whether it seizes upon the fishes taken by the star-fish for its
own nourishment must be determined by further investigations.

"The author's observations establish--

"1. That _Oxybeles gracilis_ is not a true parasite.

"2. That it passes the greater part of its life in the stomach of
_Culcita discoidea_, as is also indicated by the unusually pale colour
of the fish.

"3. That, however, it can come out, either to seek nourishment, or for
the purpose of reproduction.

"4. That it returns to the mouth along the furrow on the ventral surface
of the arms.

"5. That it is very sensitive to light.

"6. That it feeds upon other animals.

"In fresh water the animals live for about half-an-hour. The pigment
upon the peritoneum exhibits under the microscope the most beautiful
stellate forms. The fish possesses a swimming-bladder."[232]

Some very curious instances of parasitism occur, in which one kind of
creature compels or induces another creature to labour for its special
benefit. Indeed, in all cases, the parasite is benefited by the
functions of the supporter; but, in the cases I refer to, the slavery is
more special and more apparent.

There is a large species of Crab (_Dromia_) found in the West Indies,
which is invariably found covered with a dense mass of sponge. The
sponge is found to have grown in such a manner as to fit every
prominence and cavity of the crab, exactly as if a plastic material had
been moulded on it, yet it is not adherent to it, but is merely held in
position by the hindmost pair of feet, which