Monoplacophorans: Monoplacophora

Monoplacophorans (mon-oh-plak-oh-FOR-ans) look like limpets, but their single round shell, or valve, is bilaterally symmetrical. This means that the valve has a distinct left side and a distinct right side. The point of the valve is located in the middle, just over the front edge. Depending on the species, the caplike valve varies from flat and shieldlike to a short, squat cone. The valve is mostly thin but becomes slightly thicker toward the edges. They measure 0.25 to 1.25 inches (3 to 30 millimeters) in length.

The head is small but distinct. There are no eyes, and the only tentacles are found next to the mouth. Behind the mouth is a round, muscular foot. The foot is located on the end of a short column and is not very strong. It is surrounded by a small groove called the mantle cavity. The mantle cavity has five or six pairs of gills used for breathing and separates the foot from the fleshy mantle. The mantle produces the mineral calcium carbonate that makes up the valve. There are three to seven pairs of kidneylike organs. Males and females have a pair of reproductive organs. The anus is located opposite the head at the end of the digestive system. The anus is the opening where solid waste leaves the body.