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42 of Nucula Nut Clams discharge their eggs freely into the water, one New England species, N. delphinodonta, deposits from 20 to 70 tiny, opaque brown eggs in a gelatinous sac which is attached to the posterior end of the valves of the shell. Small bits of debris and mud stick to the outside of this sac, which probably serve as a camouflage. Many bivalves keep the developing young within the mantle cavity or in the meshes of the gills until the tiny shells are quite well advanced in development. With the aid of a high-powered lens one may readily see tiny juvenile clams inside the translucent adult shells of such genera as Gemma, Parastarte, Psephidia, Transennella, Kellia, Lepton and Lasaea. The odd Dwarf Milner Clam of California (Milneria minima) incubates about 50 young in a peculiar external pouch. The valves are indented on the ventral margins to form a neat exterior pocket. To prevent the young from dropping out, a sheath of periostracum is stretched over the entrance. When the small clam shells have grown sufficiently to fend for themselves, the sheath is “unzipped,” and all tumble out into the free world.

In practically all cases, the sperm from bivalves is liberated into the water where it comes in contact with unfertilized eggs that have been previously released. In cases where eggs are retained by females, the sperm is sucked in through the inhalant siphon of the mother. Only one instance of pseudo-copulation is known. In 1951 workers at the W. F. Clapp Laboratories observed Gould’s Shipworm (Teredo) placing their exhalant siphons down into their neighbors’ inhalant siphons and discharging what is presumed to have been sperm.