Grazers such as chitons, limpets and many snails have a rasping radula that removes minute algae from surfaces or cuts through large seaweeds. Their relatively simple digestive system can efficiently process large amounts of hard-to-digest plant material. Digestion is partly extracellular in the gut cavity and partly intracellular in the digestive glands. Some shell-less gastropods that feed on seaweeds keep the seaweeds' chloroplasts intact. The chloroplasts are kept in the digestive gland, where they can photosynthesize and provide nourishment for the gastropod.
Carnivorous snails have a radula modified to drill, cut or even capture prey. The radula and mouth are contained in a proboscis that can be protruded to strike the prey. Jaws may even be present. In these snails digestion is extracellular and takes place in the stomach.
Bivalves ingest food particles that are filtered and then sorted out by the cilia on the gills. The radula is absent, and food enters the mouth trapped in long strings of mucus. An enzyme-secreting rod in the stomach, the crystalline style, continually rotates the food to help in its digestion. The stomach contents eventually pass into a large digestive gland for intracellular digestion. Giant clams not only filter food but also obtain nutrients from zooxanthellae living in tiny branches of the gut extend into the clam's expanded mantle. This extra nourishment may allow the clams to attain their giant size.
Crystalline Style of bivalves
All cephalopods are carnivores that have to digest large prey. The stomach is sometimes connected to a sacin which digestion is rapidly and efficiently completed. It is entirely extracellular.
Molluscs have a circulatory system that transports nutrients and oxygen. A dorsal, muscular heart pumps blood to all tissues. Most molluscs have an open circulatory system in which blood flows out of vessels into open blood spaces. Cephalopods, on the other hand, have a closed circulatory system in which the blood always remains in vessels and can be more effectively directed to oxygen-demanding organs such as the brain.