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42 American Seashells
of Nucula Nut Clams discharge their eggs freely into the water, one New
England species, N. delphinodoiita, 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 develop-
ing 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, Fardstarte, Psephidia, Transennella,
Kellia, Lepton and Lasaea. The odd Dwarf Milner Clam of California {Mil-
Jieria 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 suffi-
ciently to fend for themselves, the sheath is "unzipped," and all tumble out
into the free world.
Figure 16. The shipworm, Bavkia goiildi, in the act of fertilizing its neighbor.
The spotted siphons are shown projecting from the wood in which these bivalves
live. Arrows indicate the direction of water currents. X5. (Redrawn from W. F.
Clapp 195 1.)
In practically all cases, the sperm from bivalves is liberated into the
water where it comes in contact with unfertilized eggs that have been pre-
viously 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 Labora-
tories observed Gould's Shipworm {Teredo) placing their exhalant siphons
down into their neighbors' inhalant siphons and discharging what is presumed
to have been sperm.