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Life of the Clams 43
HOW THEY SENSE AND SEE
Bivalves are the least "brainy" of the moUusks and, although the central
nervous system forms a rather complicated latticework throughout the body,
its three pairs of "brains" are merely swellings or ganglia in the larger nerves.
The pair of so-called cerebral gangUa control the actions of the lip palps
near the mouth, parts of the mantle, and they also receive "nerve notices"
from the tiny organs of balance, the otocysts. The second major pair of
ganglia are the pedals which supply the foot. This pair is large in the clams
that use the foot for digging or burrowing, but it is extremely small or
aborted in the oysters in which the foot is not used. The third pair, or vis-
ceral ganglia, is usually the largest and supplies the adductor muscles and
the visceral mass. The remarkable eyes of the scallops are connected with
this pair of visceral ganglia.
Many of the bivalve larvae possess true paired eyes, but in all cases these
are lost when the animal transforms into the adult stage. The adults of a
number of clams and mussels have developed pigment spots sensitive to
changing light, but in the scallops true eyes are well-developed. When the
shell of a scallop is open there can be seen just within the margin of each
valve a line of small, brilliant, emerald-like dots on the mantle, each of which
is a small eye fully equipped with cornea, focusing lens, receptive retina and
conducting nerves.
HOW THEY BURROW AND SWIM
There are bivalves that swim, leap, crawl and burrow deeply in mud,
sand or clay, and some that bore into wood, rock and even lead casings of
submarine cables. Even the rock-bound oyster and the stuck-in-the-mud
clam have their days of wandering about as free-swimming lan^ae before
they settle down to a life of permanent attachment or clumsy crawling.
The habit of swimming among adult bivalves is rare. The scallops and
the Lima File Clams not infrequently swim. Only under the abnormal condi-
tion of finding themselves "unearthed" do the Eiisis Razor Clams and the
Solemya Veiled Clams practice jet propulsion through the water. The Razor
Clam swims backward in quick, short jerks by first extending its long cylin-
drical foot out from the shell and then suddenly withdrawing it with great
force. This action, together with the closing of the shell valves, quickly
forces the water within the mantle cavity out through the openings at the
anterior or foot end. Thus the razor clam darts through the water with its
pumping foot to the rear. In Solemya, the foot is in front of the animal
as it swims. In this case the water is admitted around the foot but is ex-
pelled from the opposite end through the siphons.