Monoplacophora

Monoplacophora (phylum Mollusca) A class of primitive, almost bilaterally symmetrical, univalved molluscs, whose limpet-shaped shell of calcium carbonate consists of an external periostracum, a prismatic layer, and an internal, nacreous deposit. The circular foot and radula are ventrally placed, and the internal organs appear to be metamerically segmented, although some authorities question the evidence for metamerism, suggesting that monoplacophoran segmentation is not true metamerism but a secondary replication of structures such as has occurred in chitons and Nautilus. Typically, Monoplacophora have paired muscle scars. All are marine, benthic filter-feeders. Fossil forms inhabited shallow water, but modern forms occur in deep water. They first appeared in the Lower Cambrian.