Sexual Differentiation in Mollusks. I. Pelecypods

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Abstract
The various aspects of sexuality occurring in the bivalve mollusks are summarized, with a list of the recent papers pertaining thereto. The author recognizes the following 5 categories of sexuality: 1, functional ambisexuality (functional hermaphroditism) throughout life; 2, consecutive sexuality, characterized by a single change of sexual phase (usually [male] followed by [female]); 3, rhythmical consecutive sexuality, with alternating [male] and [female] phases throughout life; 4, alternative sexuality, in which it seems impossible to predict during any sexual phase whether the following phase will be [male] or [female] and 5, uni-sexuality (dioecism, gonochorism). Not all individuals of the species, however, experience exactly the same aspects of sexuality, because differences in genetic constitution or environmental influences may lead to the functional development of true [male][male] or true [female][female] or functional hermaphrodites, or some modification thereof, in spp. with dominant consecutive sexuality. As many as 5 or even 7 variations in the sequence of sexual phases or in the extent of ambi-sexuality may be found within a single species. There are also local races with differing sexual tendencies. There is at present no satisfactory exptl. proof of truly phenotypic sex determination in this group.