Mollusks

Mollusk, pronounced MAHL uhsk, is a soft-bodied animal that has no bones. Snails, slugs, clams, mussels, oysters, squids, and octopuses are mollusks. Most mollusks have a hard shell that protects their soft bodies. Some, such as cuttlefish and squids, have no outside shell. A special shell grows inside their bodies. This shell is called a cuttlebone in cuttlefish and a pen in squids. A few kinds of mollusks, including octopuses and certain slugs, have no shell at all. For additional information on mollusk shells and how they are formed.

All mollusks have a skinlike organ called the mantle. In mollusks with outside shells, the mantle makes the shell. The edges of the mantle release liquid shell materials and add them to the shell as the mollusk grows. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.

Mollusks live in most parts of the world. Some kinds of mollusks live in the deepest parts of oceans. Others are found on the wooded slopes of high mountains. Still others live in hot, dry deserts. Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.

The importance of mollusks

Mollusks are used mainly for food. People in many parts of the world eat mollusks every day. Most Americans do not eat them nearly so often. The most popular kinds used as food in the United States are clams, oysters, and scallops. Mollusk shells are made into many useful products, including pearl buttons, jewelry, and various souvenir items. Perhaps the best-known mollusk products are the pearls made by pearl oysters.

Some mollusks are harmful to people. For example, certain small, freshwater snails of the tropics carry worms that cause an often fatal disease called schistosomiasis. Shipworm clams drill into rope, wooden boats, and wharves and cause millions of dollars worth of damage a year.

Kinds of mollusks

Mollusks make up the largest group of water animals. There are about 50,000 known kinds of living mollusks, and scientists find about 1,000 new species every year. The fossils of about 100,000 other species of mollusks have also been found.

The mollusks make up a phylum (major division) of the animal kingdom. The scientific name of the phylum is Mollusca, a Latin word meaning soft-bodied. To learn where the phylum fits into the animal kingdom, see ANIMAL.

There are seven classes (large groups) of mollusks. They are (1) univalves or Gastropoda, (2) bivalves or Bivalvia or Pelecypoda, (3) octopuses and squids or Cephalopoda, (4) tooth shells or Scaphopoda, (5) chitons or Polyplacophora, (6) Monoplacophora, and (7) Aplacophora.

Univalves or gastropods (Gastropoda) are the largest class of mollusks. They include limpets, slugs, snails, and whelks. Most kinds of univalves have a single, coiled shell. The name univalve comes from Latin words meaning one shell. But some kinds of univalves, including garden slugs and the sea slugs called nudibranchs, have no shells after the larval stage.

The name Gastropoda comes from Greek words meaning belly and foot. Gastropods seem to crawl on their bellies, but actually they use a large, muscular foot. The foot spreads beneath the body, and its muscles move in a rippling motion that makes the animal move forward. Most sea snails and some land snails have a lidlike part called an operculum on the back of the foot. When danger threatens, the snail draws back into its shell and the operculum closes the shell opening.

Certain kinds of univalves have two pairs of tentacles (feelers) on their heads. One pair helps the animals feel their way about. Some species have an eye on each of the other two tentacles. Others have no eyes at all. A univalve also has a ribbon of teeth. This ribbon, called a radula, works like a rough file and tears apart the animal's food. Most univalves that eat plants have thousands of weak teeth. A few kinds eat other mollusks, and have several dozen strong teeth.

Bivalves (Bivalvia or Pelecypoda) form the second largest class of mollusks. They include clams, oysters, mussels, scallops, and shipworms. All bivalves have two shells held together by hinges that look like small teeth. The shells of bivalves are usually open. When the animals are frightened, strong muscles pull the shells shut and hold them closed until danger has passed.

Bivalves have a strong, muscular foot. Many kinds of these animals move about by pushing the foot out and hooking it in the mud or sand. Then they pull themselves up with the foot. Some bivalves, such as the geoduck and razor clam, use the foot to dig holes. They push the foot downward into mud or sand. First the foot swells to enlarge the hole, and then it contracts and pulls the shell into the burrow. The Pholas clam can dig holes even in hard clay or soft rock.

Bivalves have no head or teeth. They get oxygen and food through a muscular siphon (tube). The siphon can be stretched to reach food and water if the animal is buried in mud or sand. Bivalves feed on plant cells material, which is filtered from the water by the gills.

Octopuses and squids (Cephalopoda) are the most active mollusks. The argonaut, cuttlefish, and nautilus also belong to this group. All the species in the group live in the ocean.

The word Cephalopoda comes from Greek words meaning head and foot. A cephalopod seems to be made up of a large head and long arms that look like feet. Octopuses and squids have dome-shaped "heads" surrounded by arms. Octopuses have eight arms, and squids have eight arms and two tentacles. The arms grow around hard, strong, beaklike jaws on the underside of the head. These jaws tear the animal's prey, and are far more dangerous than the arms. Octopuses use their arms and squids use their tentacles and arms to capture prey and pull it through their jaws. Octopuses and squids eat fish, other mollusks, and shellfish.

Tooth shells (Scaphopoda) have slender, curving shells that resemble tusks. These mollusks are often called tusk shells. The word Scaphopoda comes from Greek words that mean boat and foot. A tooth shell has a pointed foot that looks somewhat like a small boat. All tooth shells live in the ocean, where they burrow in the mud or sand. The top of the shell sticks up into the water. Tooth shells have no head or eyes. They feed on one-celled organisms that are swept into the mouth by tentacles.

Chitons (Polyplacophora) have flat, oval bodies covered by eight shell plates. The plates are held together by a tough girdle. The name Polyplacophora comes from Greek words that mean many, shell, and bearer. This name refers to the eight overlapping pieces of a chiton's shell. Chitons have a large, flat foot. They can use the foot to move about, but they usually cling firmly to rocks. When they are forced to let go of the rocks, they roll up into a ball. Chitons have a small head and mouth, but they have no eyes or tentacles. Their long radula has many teeth, which some chitons use to scrape seaweed from rocks for food.

Monoplacophora live in the deep parts of the ocean, and most are found only as fossils. The name Monoplacophora comes from Greek words meaning single, shell, and bearer. Monoplacophorans have one shell that is almost flat, like a limpet shell. They are unusual because they have several pairs of gills, six or more pairs of kidneys, and many ladderlike nerve centers. Like other mollusks, they have a mantle. They also have a radula. Little is known about their habits.

Aplacophora are rarely seen, wormlike mollusks covered with small spines. The name Aplacophora comes from Greek words that mean no shell.