In simultaneous hermaphrodites, the best known example of the transfer of an accessory gland substance is probably the shooting of so-called love-darts in land snails. This substance is not transferred along with the sperm, but rather by injection through the partner��s skin using a calcareous dart loaded with the substance. This introduced allohormone increases the shooter��s paternity, probably via the inhibition of sperm digestion. There are similarly idiosyncratic behaviors achieving analogous goals in other hermaphrodites as well as in CGI1746 separate sex species. Strikingly, the transfer of substances via the seminal fluid has not been convincingly demonstrated in simultaneous hermaphrodites, even though this seems the method of choice in species with separate sexes. The existing theoretical framework predicts a vital role for sexual selection in simultaneous hermaphrodites despite the fact that the sexes are joined within each individual, and there is reason to believe that substances transferred by the ejaculate have an important role to play. Recent evidence demonstrates the existence of sexual selection in hermaphroditic H-89 organisms, even though Darwin initially did not envision this. In order to test the presence and the function of seminal fluid substances, we used the hermaphroditic pond snail Lymnaea stagnalis. We already know that in this species there is a clear scope for sperm competition. These animals donate and receive sperm frequently, and their egg masses usually contain eggs fathered by multiple partners. In addition, although donors can donate sperm repeatedly, they prefer to inseminate novel partners each time and they transfer more sperm to virgins than to previously-inseminated partners, which is in accordance with sperm competition theory. Most importantly, there are clear indications that as yet unidentified seminal peptides and/or proteins, produced by the prostate gland, are transferred to the partner that affect egg laying behavior. Sexual selection favors the transfer of substances, called allohormones, that enhance the reproductive success of the sperm donor.