☞ 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

.