It can crawl, but that is about all; it is
neither swift nor energetic. Even the earthworm can outcrawl it. It
has feelers and eyes, and is thus better provided with sense-organs
than almost any worm. It has a supra-oesophageal ganglion of fair
size.
The clams and oysters show even more clearly what we might call the
logical results of molluscan structure. They increased the shell
until it formed two heavy "valves" hanging down on each side of the
body and completely enclosing it. They became almost sessile, living
generally buried in the mud and gaining their food, consisting
mostly of minute particles of organic matter, by means of currents
created by cilia covering the large curtain-like gills. Their
muscular system disappeared except in the ploughshare-shaped "foot"
used mostly for burrowing, and in the muscles for closing the shell.
That portion of the body which corresponds to the head of the snail
practically aborted with nearly all the sense-organs. The nervous
system degenerated and became reduced to a rudiment. They had given
up locomotion, had withdrawn, so to speak, from the world; all the
sense they needed was just enough to distinguish the particles of
food as they swept past the mouth in the current of water. They have
an abundance of food, and "wax fat." The clam is so completely
protected by his shell and the mud that he has little to fear from
enemies.
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