The rotatory
motion in them all is produced by an apparatus that exists not only
in all animals, but in plants also, and is a most important agent in
sustaining the freshness and vitality of their circulating fluids and of
the surrounding medium in which they live. It consists of soft fringes,
called Vibratile Cilia. Such fringes cover the whole surface of these
little living beings, and by their unceasing play they maintain the
rotating motion that carries them along in the water.
The Mollusks, the next great division of the Animal Kingdom, also
include three classes. With them is introduced that character
of bilateral symmetry, or division of parts on either side of a
longitudinal axis, that prevails throughout the Animal Kingdom, with the
exception of the Radiates. The lowest class of Mollusks has been named
Acephala, to signify the absence of any distinct head; for though their
whole organization is based upon the principle of bilateral symmetry, it
is nevertheless very difficult to determine which is the right side and
which the left in these animals, because there is so little prominence
in the two ends of the body that the anterior and posterior extremities
are hardly to be distinguished. Take the Oyster as an example. It has,
like most Acephala, a shell with two valves united by a hinge on the
back, one of these valves being thick and swollen, while the other is
nearly flat.
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