Note: Within the shell is a pair of ``arms,'' often long and
spirally coiled, bearing rows of ciliated tentacles by
which a current of water is made to flow into the
mantle cavity, bringing the microscopic food to the
mouth between the bases of the arms. The shell is both
opened and closed by special muscles. They form two
orders; Lyopoma, in which the shell is thin, and
without a distinct hinge, as in Lingula; and
Arthropoma, in which the firm calcareous shell has a
regular hinge, as in Rhynchonella. See {Arthropomata}.