a class of marine mollusks. Until 1952 they were known only from fossil shells from deposits of the Lower Paleozoic. In 1952, Danish zoologists discovered the first modern representative of Monoplacophora—Neopilinagalatheae. The body consists of a head, a foot, and a visceral hump covered with a mitriform or flat-spiral shell lined with a mantle. The head and foot can retract into the shell by means of one to eight pairs of muscles. Between the mantle and the foot is a mantle groove, in which there are five or six pairs of feathery gills; into the groove open the ducts of six pairs of kidneys and the anal opening. The digestive system includes an esophagus with a radula, a loop-shaped gut with a stomach, and a paired liver. The heart consists of two ventricles and four auricles. The nervous system consists of four longitudinal cords united by an esophageal ring and transverse commissures. There is an organ of equilibrium—the statocyst. There are two pairs of sex glands; the sexes are separate.

Of the approximately 60 original species of Monoplacophora, six are extant. They are united in the genus Neopilina. Extinct forms from the Cambrian to the Devonian are grouped in three orders: Tryblidiida (miter-shaped forms with many body retractors; Neopilina are included here), Cyrtonellida (flat-spiral or miter-shaped forms with two pairs of retractors), and Sinuitopsida (flat-spiral forms with one pair of retractors).