Mollusk

A mollusk is a soft-bodied invertebrate (an animal without a backbone) that is often protected by a hard shell. Most mollusks live in water and make up the second largest group of invertebrates (next to arthropods). Mollusks are represented by such diverse invertebrates as clams, slugs, snails, and octopuses.

There are about 100,000 species of mollusks, most of which live in the water. All are soft-bodied, nonsegmented, and usually enclosed in some sort of covering made of calcium carbonate. Whether a land snail or an underwater clam, all mollusks share certain traits. They all have a "mantle," or a covering of tissue that protects their internal organs. Many species secrete a substance that forms a hard shell, protecting them from predators. Their digestive systems are made up of a mouth, throat, gullet, stomach, intestine, and anus. Mollusks that live underwater use gills to get their oxygen, while land-dwelling mollusks have lungs. All have a two-chambered heart that pumps blood through vessels that branch throughout their bodies. The mollusk nervous system consists of two pairs of nerve cords. Some mollusks have eyes and other sense organs. Most mollusks also have a muscular "foot" that is used for slow crawling or digging. In an octopus, this foot is divided into arms or tentacles lined with suction cups.

There are at least eight species of mollusks known, but most fit into three main groups—gastropods, bivalves, and cephalopods—that are grouped according to the shape of their muscular foot. The name gastropod means "stomach-foot" and describes those mollusks, like snails, that

have a sucker-like foot. This is the largest group of mollusks and is best represented by the common garden snail. Most gastropods, excluding slugs, have a single shell often shaped like a spiral, or coil. They also have a large, flat foot that they use to slide along on top of the mucus they secrete. This is why they leave a visible, slimy trail behind them. A gastropod has a distinct head with tentacles that can move up and down like a periscope and act as eyes or sense organs. A snail also has rows of tiny teeth. Not just confined to land, there are many ocean-dwelling gastropods like periwinkles, abalones, and sea slugs.

A bivalve is a mollusk that has a two-part shell joined by a hinge. Bivalves are water animals and are best represented by clams, oysters, and scallops. All bivalves are filter feeders. They filter water through large gills that catch bits of food and absorb oxygen. To do this, however, they must keep their shells open, and as a result, expose their vulnerable soft body. A mollusk, like a clam, moves on the ocean's bottom by thrusting its large, hatchet-shaped, muscular foot between the open shells, thus pushing it along slowly. Unlike gastropods, bivalves have no head, and their sense organs are not well-developed. Bivalves do have a powerful adductor muscle that they use to clamp their shells tightly together.

Members of the cephalopod group of mollusks, like the octopus and squid, are free-swimming and have no shell. Unlike typical mollusks, they are fast-moving hunters and prey on other animals. An octopus has a sharp beak, like a parrot's, that it uses to rip its food and break open shells. In squid and octopuses, the mollusk foot has evolved into long arms, or tentacles, around the head. They use these tentacles to capture fish. In some ways, cephalopods resemble vertebrates because they have a solid internal support structure, like a skeleton, and a large head with eyes, as well as a good-sized brain. Cephalopods move by a form of jet propulsion in which they forcefully squirt water. They also can eject a cloud of dark-colored chemicals that covers their escape as they zoom away. A giant squid may extend its tentacles as far as 60 feet (18.29 meters) and has the largest eyes in the animal kingdom—up to 1 foot (0.3 meters) wide. Some squid and octopuses can change color like a chameleon. The only cephalopod with a shell on the outside of its body is the nautilus whose shell is divided into chambers. It lives in the largest chamber and adds another when it grows. It fills the empty chambers with a gas that makes it easier to swim. All three groups of mollusks reproduce sexually through the union of male sperm and female eggs. Unlike gastropods and cephalopods, however, bivalves are not necessarily either male or female. Instead they may contain both male and female organs.

[See alsoInvertebrate ]