Most mollusks of the class Bivalvia use their gills to filter the water for food. Most mollusks use gills for breathing. Aquatic mollusks use gills located in their mantle cavities. However, mollusks that live on land use an adapted mantle cavity lined with blood vessels to breathe. This surface must be kept moist so the mollusk can breathe.
Mollusks have a simple heart, which pumps the blood throughout the body.
Mollusks of lower complexity have an open circulatory system, which means that blood does not always travel through blood vessels. However, since closed circulatory systems are more efficient, mollusks of high complexity use closed circulatory systems. Undigested food becomes solid waste that leaves through the anus in the form of feces. Mollusks remover ammonia from their blood by using simple tube-shaped organs called nephridia. It is then released to the outside.