Snails Sea Slugs and Limpets: Gastropoda

The most conspicuous feature of many gastropods is the shell. Although sometimes flattened and caplike, most shells are cone-shaped shelters into which they can completely withdraw their bodies. The single, lopsided shells are usually made up of spiraled tubes called whorls (worlz). The shells are lopsided because whorls form below one another, instead of around each other. In spite of the fact that they are lopsided, the shell is carried over the back so that its weight is evenly balanced over the body. But not all gastropods have lopsided shells. In sea snails, known as cowries, the last whorl completely covers all the others and appears to be symmetrical. The left and right sides of symmetrical objects are the same size and shape, giving them a balanced, rather than lopsided, look. Young limpets have a distinctly lopsided and coiled shell, but as they develop into adults, the shell becomes smooth and symmetrical, resembling a Chinese hat.

The whorls form around a central line, or axis. Inside the shell, the whorls turn around a central column of shell called the columella (kol-yuh-MEL-uh). The smallest whorls are the oldest and were made while the gastropod was still in the larval stage. The last and largest whorl is the newest and ends at the opening of the shell, where the head and foot stick out. The spiraled stack of whorls above the opening is called the spire. If the whorls develop counterclockwise, the shell is said to be left-handed, while clockwise whorls are right-handed. To determine if a shell is left- or right-handed, stand the shell up so that the spire is pointed up and the opening faces toward you. If the shell opens to the right of the spire, it is said to be right-handed; if the opening is on the left, it is left-handed. Most gastropods have right-handed shells, while some species are left-handed. A few species have individuals that are either right- or left-handed. Some marine and land snails have a flat, horny disc above the back of their foot called the operculum (o-PUHR-kye-lem). The operculum is usually made of the same tough material that covers the outside of the shell. When the head and foot are withdrawn into the shell, the operculum follows to form a tight cover over the shell's opening.

In species with shieldlike shells, such as keyhole limpets and abalones, there is a notch of one or more holes in the shell that allow a current of oxygen-carrying water to reach the body and eggs, sperm, and wastes to be carried away. These animals live on rocks in the pounding surf, and their low shells and muscular feet help to keep them from being knocked off and washed away by the tides.

Gastropod shells, if present at all, come in a wide array of colors, patterns, and surface sculptures. The surface of the shell is sometimes pearly or has a highly polished, porcelainlike quality. A thin or thick covering usually protects the outer part of the shell. The shells of cowries lack this protective coating and are instead covered by the mantle. In sea slugs, the shell is small, thin, and found either inside or outside the body, if there is a shell at all. The mantle is the fleshy organ located between the body and the shell. It makes the shell by producing a hard mineral called calcium carbonate.

Like other mollusks, the bodies of gastropods are soft and fleshy. The bodies of sea slugs are often brilliantly colored and patterned and sometimes covered with fleshy, stinging outgrowths. In snails, the head and foot are withdrawn into the shell by a powerful muscle that is attached inside to the columella. The heads of all gastropods are more distinctive than in most other mollusks and may or may not have eyes and one or two pairs of tentacles. The mouthparts include a radula (RAE-jeh-leh). The radula is a tonguelike structure with rows of extremely hard teeth that are used to scrape food off rocks or pull and tear at flesh. The muscular foot has been changed in some groups to help with swimming or burrowing. The mantle forms a cavity that lies in front or to the right of the muscular foot. Inside the mantle cavity is a comblike gill used for breathing. The digestive tract is u-shaped. Both it and the nervous system are twisted. Gasropods have one or two kidneylike organs that filter out wastes from the blood. These organs, along with the anus, open into the mantle cavity above or near the head. The anus is the opening at the end of the digestive tract where solid wastes leave the body.