1. Contrary to the assertion of Giard that Leander males when parasitized by Bopyrus undergo sex reversal, Leander xiphias and L. squilla show no modifications of the secondary sex characters, while the testes, although reduced in size, never produce oocytes. This is probably true also of L. serratus.

2. The power of a parasite to induce sex reversal is probably limited by the degree of sex separation of the host species. The species which show sex reversal ("castration parasitaire") are thus considered to possess a less complete sex-separating mechanism than those, such as Leander, which are not reversed by parasites.

3. Breeding females of Leander develop an incubatory chamber under the abdomen, involving many structural changes.

4. These "breeding characters" are not formed by parasitized females, nor by females castrated by X-rays.

5. It is possible that there is a female sex hormone in Crustacea, but the evidence is not yet conclusive.

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