Picture a vast undersea factory with billions of shells in constant
production. Each is made slowly and entirely of lime which the
little animal inside extracts from its food, almost from the first
day of its life. Each shell builder flawlessly follows the shape
and design of the species to which it belongs.
All these sea animals come from eggs, all different according to
species, but all laid in measureless abundance--sometimes released
into the open sea, sometimes protected in homemade nests, sometimes
encased in capsules strung like beads. Hatched, most baby mollusks
swim freely for a while, their tiny, transparent bodies almost
invisible to the naked eye. Then they start building a heavier
shell and sink to the bottom.
Each shell's mantle contains a network of microscopic tubes. Each
tube secretes a tiny amount of lime which instantly adheres to the
shell. The animal builds his shell to the proper size and thickness
and determines its ridges and whorls. Some kinds of shells take
two to five years to reach maturity. Others keep growing all their
lives. Color tubes are spaced like holes on a player piano roll
allowing pigments to tint the shell at the right spots in the growing
design. Many shells are covered with a self-made brown sheath, the
PERIOSTRACUM.
[figure captions]
Most shells don't change basic structure as they grow. Young
COWRIES (l.), however, alter greatly in maturity (r.).
Tough, lozenge-shaped egg cases on this string hatch baby WHELKS
like ones shown.
Newborn mollusks are usually free swimming, moved by hairs. Shell
is there, but transparent for a few days.