"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 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_.