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Mollusca - Class Monoplacophora - English
URL: http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/moll5.htm

Prior to 1952, the monoplacophorans were known only from fossil shells from the Cambrian and Devonian.

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Class Monoplacophora - English
URL: http://animaldiversity.ummz.umich.edu/mollusca/monoplacophera.html

Living representatives of this Class were not discovered until 1952, although Paleozoic fossil monoplacophorans had been known for some time. At present, 11 species are known. Most live at great depths and all are marine. Monoplacophorans are small and have a single, caplike shell, giving them a limpet-like appearance. A number of their organs (nephridia, heart, etc.) are repeated serially, making them resemble metamerically-arranged species such as annelids and arthropods. Whether this resemblance indicates a close relationship between monoplacophorans and phyla exhibiting true metamerism is an open question.

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