Defining Characteristics

An important characteristic of most molluscs is the head-foot region. Most molluscs are strongly cephalized; that is, they have a well-developed head, in which is located a mouth and a concentration of nervous and sensory functions. Adjacent to the head is a large, muscular foot formed from the ventral body wall. Used primarily in locomotion, the surface of the foot is sometimes ciliated and and laden with numerous mucous glands.

Another characteristic of most molluscs is the mantle. This sheath of tissue is formed from the dorsal body wall. It surrounds the mantle cavity, which houses the gills or lungs if they are present, and its surface may assist in gas exchange. The mantle also secretes the shell in those forms that possess one.

Systematics

Internally, molluscs have a complicated digestive system, with a mouth in the head and the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is present in all groups except bivalves and Aplacophora. The radula is usually toothed and its structure may be very complex. It is adapted to a wide variety of feeding styles in different species, including scraping, tearing, stabbing, and cutting.

Molluscs have an open circulatory system (except cephalopods, in which it is closed), complete with a heart, blood vessels, and respiratory pigments. Gas exchange is via gills, lungs, or the body surface. Excretion takes place by means of kidneys that, like the digestive tract, pump waste into the mantle cavity.

A fairly complicated nervous system is present, including several ganglia and a system of nerves; sensory and nervous systems are cephalized in at least some kinds of molluscs. Some molluscs have complex, extremely sensitive eyes.

The majority of molluscs have a shell of some kind. This calcareous structure is secreted by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its shape, size, and color are widely used by taxonomists, and they has also made molluscs a popular pursuit of collectors. Further, shells decay slowly and fossilize well, and the fossil record of molluscs is excellent and ancient.

Most molluscs are dioecious. Many pass through free-swimming larval stages, called trochophore and veliger larvae.

Habitat

The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats.

Feeding

Molluscs are extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs.

Other Information

Molluscs are an extremely diverse group of organisms. Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein. Oysters produce pearls. Other species are pests in gardens and to crops. Some are essential components in the life cycles of human parasites, including devastating diseases such as schistosomiasis. And a few, such as oyster drills, cause problems by preying on other molluscs that are important to us.

Mollusca Links

http://animaldiversity.ummz.umich.edu/mollusca/monoplacophera.html