ctenidia

Attached to the dorsal surface of the visceral mass is and hanging freely down the sides of it is the mantle, often called the skirt or pallium. There is a space between the mantle and the viseral mass, this space is greatest towards the rear of the animal where it is called the mantle cavity or the pallial cavity. The mantle cavity generally contains the gills or ctenidia, a water current, generated by beating cillia, enters the mantle cavity at the sides, passes over the gills and departs centrally, i.e. the outward bound current runs out between the two inward bound currents. Near the head, just behind the mouth is a pair or more of ganglia and a nerve ring from which two nerve chords arise that reach out through the body. Molluscs are true coelomic animals, though the coelom they have is small, enclosing only the gonads and the heart where it is called the gonodial cavity and the pericardial cavity respectively.