Introduction to Snail
Snail, a mollusk that bears a univalve (one-piece) shell. A typical snail shell is cone-shaped and spirally coiled. There are thousands of species of snails, no two species having shells of exactly the same shape or color. Some shells are drab, varying from gray to brown; others are brightly colored with orange, red, and yellow designs. They range in size from those barely visible to the unaided eye to shells that are two feet (60 cm) long. Snails are found in freshwater, in salt water, and on land.Snails creep along slowly on a large flattened foot.
A few snails have no means of locomotion and spend their lives attached to rocks or other objects; most, however, are able to move about. They creep along slowly on a large flattened foot, in most cases gliding along a layer of mucus that they secrete. Usually a snail can withdraw its body into the shell. Some snails have a hard, flat plate (called the operculum) on the rear part of the foot. When the snail withdraws into the shell, the operculum covers the opening.
A snail gets around on its foot. But it’s not the kind of foot you might be thinking of. A snail’s foot is actually a long, muscular organ that spreads out under the snail’s body.
A land snail moves forward by making a rippling motion with its foot. As it moves, the snail releases a slippery slime. The slime coats the ground under the snail. This helps the snail move along a little more easily. If you watch a snail cross a yard, you can see the trail of slime it leaves behind.
Slime or no slime, snails are very slow animals. It would take a snail moving at top speed more than a day to move from one end of an American football field to the other.
Snails have one or two pairs of tentacles, depending on the species. The eyes may be at the base or on the tips of one pair of tentacles; a few species have no eyes. Respiration is through gills or lungs, or through the mantle (a thin layer of tissue that envelops the soft parts of the body).
Snails do have eyes, but not all snails have eyes in the same spot on their bodies. A land snail has two pairs of tentacles that stick out from its head. On the tip of the two longer tentacles are its eyes. A land snail can move its tentacles up and down and from side to side to help it get the best view. But a sea snail’s eyes are located at the base of its tentacles. And they always stay in one place.
Not only do land and sea snails see things differently, but they breathe differently, too. A land snail breathes using its lung. Its lung is located in a fold in the mantle called the mantle cavity. Sea snails breathe through gills. Like fish, they use their gills to get oxygen from the water. A sea snail’s gills are located in its mantle cavity. Some freshwater snails have gills, too. Others have lungs, just like land snails. When they need to breathe air, they must come to the surface.
Some snails eat plant material, some eat other animals, and others eat both. Some snails are scavengers; others are parasites. Most snails are dioecious (an individual produces either eggs or sperm), but some species are hermaphroditic (one individual produces both eggs and sperm).
Snails eat many types of foods. Land snails mostly eat dying plants. They also eat fruits, living plants, and sometimes bark, too. Freshwater snails eat algae (AL jee) and the remains of dead animals and plants. Sea snails mostly feed on algae. Some also eat other small animals.
How do snails eat? Snails have teeth, but they are not like any teeth you’ve ever seen. Nearly all snails have radulas (RAJ u luhz). Radulas are hard, ribbonlike organs that look like tongues. Radulas contain rows of tiny teeth. Some snails have just a few teeth while others have thousands. As these teeth wear away over time, they are replaced by new ones. Snails don’t chew their food. Instead, they use their radulas to grind, grate, and tear it.
Some species of snails, such as abalones and some land snails, are edible. In some parts of Europe, especially France, land snails are raised for food. At one time a species of snail was the source for a very expensive dye known as Tyrian, or royal, purple. (This dye is now obtained synthetically.) Some species of freshwater snails play an important role in the transmission of a serious human disease known as schistosomiasis. Snails are often used in aquariums as scavengers to scrape debris from the sides of the tank. Some species of land snails can become garden pests.