Taking Geoffrey Smith’s case of _Inachus-Sacculina_ as typical, the changes brought about are as follows: The parasites attach themselves to the young crabs before the external secondary sexual differences have appeared. In the females, the effect is to cause them to develop prematurely the distinctively female characters. In the male, on the other hand, the narrow abdomen of the male changes after a molt into the broad abdomen of the female, which also develops ovigerous appendages on its ventral surface like those of the female in every detail. The larger claw of the male changes into that of the female, which is different in form as well as in size. Some years ago I ventured to raise the question as to whether these effects on the male might not be interpreted as retention of the juvenile characters rather than development of the female characters in the male. This might appear more especially the case in the somewhat more juvenile shape of the anterior abdominal appendages and possibly also in the shape of the broader abdomen; but Smith has later shown that the results can not be interpreted as juvenile, for when the changed organs are examined in detail they are found to differ from the same organs in the juvenile condition, and to be identical with those of the adult female. I think, therefore, that we must accept this interpretation of Giard and of Smith as correct. But Smith goes further and believes that the effects may be carried so far that eggs develop in the old testes; in other words, that the testis changes to an ovary. It seems to me that the evidence to support this last point should be much stronger than that advanced by Smith before we can accept this interpretation, for we lack the essential control for this evidence. In only a single case were eggs found--in the testis of a male that had been infected, but from which the parasite had fallen off, and which was presumably recovering from the effects of its presence. Now, it is known that in the testes of some male animals a few eggs may occasionally be found where there is no suspicion that the animal has changed its sex. In some crustacea, in scorpions, and in insects, isolated instances of this kind have been found. Abnormal division of a spermatogonial cell, of such a kind that both sex chromosomes (in the case of insects at least) got into the same cell might be expected to cause such a cell to become, even in the male, an egg-cell rather than a sperm-cell. The degenerative changes of the testes in the hermit crab caused by the parasite might be imagined to favor such abnormal division with its consequences. More significant, however, is the fact that the parasite causes the absorption of the ovary when it infects a young female, so that even all its eggs disappear. In other words, the parasite is as injurious to the peculiarly female organ as it is to the testis. Why then, one can not but ask, should an influence that causes