A second peculiarity of the male Crustacea consists not unfrequently in a more abundant development on the flagellum of the anterior antennæ of delicate filaments which Spence Bate calls “auditory cilia,” and which I have considered to be olfactory organs, as did Leydig before me, although I was not aware of it. Thus they form long dense tufts in the males of many Diastylidæ, as Van Beneden also states with regard to _Bodotria,_ whilst the females only possess them more sparingly. In the Copepoda, Claus called attention to the difference of the sexes in this respect. It seems to me, as I may remark in passing, that this stronger development in the males is greatly in favour of the opinion maintained by Leydig and myself, as in other cases male animals are not unfrequently guided by the scent in their pursuit of the ardent females. Now, in our _Tanais,_ the young males up to the last change of skin preceding sexual maturity resemble the females, but then they undergo an important metamorphosis. Amongst other things they lose the moveable appendages of the mouth even to those which serve for the maintenance of the respiratory current; their intestine is always found empty, and they appear only to live for love. But what is most remarkable is, that they now appear under two different forms. Some (Fig. 3) acquire powerful, long-fingered, and very mobile chelæ, and, instead of the single olfactory filament of the female, have from 12 to 17 of these organs, which stand two or three together on each joint of the flagellum. The others (Fig. 5) retain the short thick form of the chelæ of the females; but, on the other hand, their antennæ (Fig. 6) are equipped with a far greater number of olfactory filaments, which stand in groups of from five to seven together. Fig. 3. Head of the ordinary form of the male of Tanais dubius (?) Kr. magnified. The terminal setæ of the second pair of antennæ project between the cheliferous feet. Fig. 4. Buccal region of the same from below; lambda, labrum. Fig. 5. Head of the rarer form of the male, magnified. Fig. 6. Flagellum of the same, with olfactory filaments, magnified.