Class of Mollusca - Monoplacophora
Bilaterally symmetrical, with broad flat foot and single shell, mantle cavity has five to six pairs of gills; six pairs of nephridia of which two functions as gonoducts; radula present; sexes separate. Extant species are included in three genera. These are found at great depths in the sea. Also many fossil forms belonging to Cambrian and Devonian times are known. The monoplacophorans can he regarded as the ancestors of all molluscs. Monoplacophorans have following salient characters: The shell varies in shape from a flat shield to short cone: length of the animals ranges from 3 mm to 3 cm.