Gastropods (gas'-troh-pods) commonly are called snails. The snail carries its shell on its back and retreats into it whenever danger threatens. As a snail grows larger, the shell expands and lengthens. Most commonly the shell is coiled in a spiral, but some are shaped like a Chinese coolie hat.
There are many kinds of gastropods. Some live in the sea, some live in rivers, and still others live on land. The ones that live in water have gills like fish, but those that breathe air have simple lungs. Gastropods have a distinct head, feelers, eyes, and a mouth. Some of the snails have a rasp-like tongue and may use it for boring into other shellfish, which they eat.
Snails are common as fossils in the rocks of Ordovician and Pennsylvanian ages in Illinois. Those that lived during the Ice Age are abundant in the loess along the bluffs of the major rivers, and their shells may be recovered by washing the loess through a coarse screen. The oldest snails lived during the Cambrian Period, more than 490 million years ago.