The snail’s soft body is divided into three distinct sections. A well-defined head is connected to a large muscular foot. The foot is the most visible external feature of the snail’s body and is the animal’s mode of locomotion. A snail propels itself with this single foot, which is controlled by hydrostatic action inside the snail’s body. Additionally, the foot surface is covered with tiny projections (cilia) that assist the gliding motion. Snails feed along the bottom as they move, partially clearing a path for the foot. As the foot passes over the substrate, it leaves a “trail” behind the snail. These trails are commonly visible in soft sediments or across hard surfaces such as rocks or submerged trees.
The third distinctive section, the internal organs, is concealed inside the shell. The internal organs of a snail include a heart for circulating “blood;” a complete digestive system with a distinct mouth, stomach, and intestine; a reproductive system; and a gill or other respiratory surface used for oxygen exchange. When a snail is threatened by a predator, the head is retracted into the shell first, followed by the foot; thus the entire body can retreat inside the shell. Some snails even have a hard “lid” on their tails called an operculum that is made of protein. The hard operculum covers most soft tissue of the foot exposed to a predator when the snail has retracted into its shell.
North American freshwater snails represent a rich natural heritage of global importance. The number of North American freshwater gastropods is one of the highest in the world, rivaled only by the river systems of Southeast Asia. North America once had approximately 700 species of native freshwater snails from 16 families. Unfortunately, 67 species (10%) are considered likely extinct, 278 (40%) endangered, 102 (15%), threatened, 73 (10%) vulnerable, and 26 (4%) have uncertain taxonomic status. The remaining 157 (26%) species are considered stable. Along with freshwater mussels, freshwater snails are the most severely imperiled group of animals in the world, and they face the same issues as their bivalve cousins. The conservation status of freshwater gastropods of Canada and the United States is summarized in Johnson et al. (2013)