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.

• The buccal cavity comprise a radula and a subradular organ. The stomach has a crystalline style. There is a long and coiled intestine.
• The foot is very broad and flat.
• Sexes are separate. Two pairs of gonads are situated in the middle of the body. Fertilisation is external. Example Neopilina
• The mantle cavity comprises 5 or 6 pairs of monopectinate gills.
• The mouth lies in front of the foot and the anus opens posteriorly in the mantle cavity.
• 6 pairs of kidneys are present.
• Heart has two pairs of auricles and a pair of ventricles and is surrounded by paired pericardial coelom.
• Nervous system is typical along with a pair of cerebral ganglia, circumoral nerve ring, a pair of visceral nerves and a pair of pedal nerves.
• There is a preoral fold in front of the mouth extending laterally as a larger ciliated palp like structure. Another fold projects behind the mouth on either side as postoral tentacles.