Runnegar, Bruce Department of Earth and Space Sciences, University of California, Los Angeles, California.
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A class of the phylum Mollusca. Although fossil monoplacophorans had been known since the end of the nineteenth century, it was the discovery of a living species in deep water off Costa Rica in the 1950s that led to universal acceptance of the class. Monoplacophorans are bilaterally symmetrical, univalved mollusks that vanish from the fossil record at the end of the Paleozoic, about 240 million years ago. The living species, such as Neopilina galatheae, are rare and inhabit deep water, which may explain their absence from Mesozoic and Cenozoic rocks. Living monoplacophorans have a limpet-shaped shell, a circular foot attached by pairs of retractor muscles, and several gills on each side of the body (Fig. 1). An anterior mouth, but no distinct head, is evident, along with a radula for obtaining food, a central gut, and a posterior anus. Many of the organs are repeated as many as eight times down the length of the body, suggesting that the monoplacophorans are derived from some kind of segmented animal. An alternative view, which relates the monoplacophorans and other primitive mollusks to unsegmented flatworms (platyhelminths), has been challenged recently by both anatomical and molecular evidence. See also: Mollusca
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