Cnidarians have a true mouth and true digestive cavity. A cnidarian has two definite layers of cells, one covering the outer body, and the second forms the lining of the digestive cavity. Between these cell layers is a noncellular layer called the mesogloea; however cells or muscle strands from the inner or outer layer may sink into the mesogloea. (Kozloff, 2000)

References:

Brusca, R.C., and Brusca, G.J. (2003). Invertebrates. Library of Congress Cataloging-in-Publication-Data: USA.

Kozloff, Eugene N. (2000). Seashore Life of the Northern Pacific Coast. University of Washington Press: Seattle and London.