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Rh the eggs are fertilized within the mantle cavity, and the young complete development to the adult form in brood pouches on the gills of the parent. Usually self-fertilization does not occur, for in the majority of these species the sperm is discharged before the eggs are mature in the same individual (protandric hermaphroditism).

Other kinds of bivalves are accustomed to practicing sex reversal in which the early part of their lives is spent as males and their “adulthood” as females. In the Quahog (Mercenaria mercenaria), nearly all individuals experience a male phase in which functional sperm is produced while the clam is only a few months old. Following this initial male phase, about half of the population turns female to produce eggs, while the other half remains male. No further sex change takes place.

Sex reversal is apparently very popular among some of the oysters, such as our native Pacific Coast Ostrea lurida. In this species there is a series of male and female phases. There may be three changes within a single year. Usually the male phase comes on first. Alternating sexuality also occurs in our Atlantic Oyster (Crassostrea virginica), but the early sex organs are capable of turning toward either male- or femaleness. It is not known, at present, to what extent environmental conditions determine the direction of sex change. It has been shown, however, that under unfavorable circumstances, when circulation of water is poor and the food supply low, there are more female oysters in a colony. When conditions improve, the percentage of males increases considerably.

Thorough studies have now been made to show that normally no sex reversal occurs and that the sexes are separate and of equal numbers in a given colony in the following species: Modiolus demissus, Mytilus californianus, Septifer bifurcatus, Anomia simplex, Mytilus edulis, Petricola pholadiformis, Donax gouldi, Mya arenaria (Soft-shell Clam) and the Angel Wing, Barnea costata.

The number of eggs produced by the female bivalve may vary considerably depending upon the species and environmental conditions. Species which retain the fertilized eggs within their bodies for further development invariably produce fewer eggs than those species which discharge them into the water. The oysters are probably among the greatest molluscan producers of eggs. C. R. Elsey estimates that one female Crassostrea gigas of Japan and our northwest Pacific Coast may discharge into the water each year eggs numbering 1000 to the eighth power. If all survived in five generations, the aggregate would be large enough to make eight worlds like ours. Needless to say, enemies and unfavorable conditions kill off most of the young.

In contrast to this prodigious effort on the part of the oyster, the Dwarf Turton Clam (Turtonia minuta) deposits only 12 to 20 eggs which are neatly encased in oval egg masses of gelatinous material. While most species