Carrying their homes on their backs, snails are important in aquatic ecosystems, in which their grazing and scavenging of diatoms, algae and bacteria enhance nutrient recycling. The class Gastropoda in the phylum Mollusca includes the snails and slugs, encompassing many freshwater and marine species.

Freshwater species are largely in the order Archaeogastropoda, within the subclass Prosobranchia, and feature an operculum and usually a spiral, coiled shell. Torsion is the developmental process that results in the rotation of the mantle and organs of the snail on its foot, creating the unusual and potentially undesirable positioning of the anus adjacent to the head and mouth. Although the larvae of aquatic snails are initially bilaterally symmetrical, little evidence remains in the asymmetric adult body form. Some species are distinguished from others on the basis of the direction of the shell coiling and body twisting as sinistral (left-handed) or dextral (right-handed). The well-developed eyes of aquatic snails are mounted on a pair of tentacles and connect to paired nerve bundles known as ganglia. Gastropods have a muscular foot that enables locomotion, which includes creeping, climbing, swimming, and burrowing. As the muscles of the foot ripple, a lubricating slime is secreted, helping the snail glide over any terrain.

A very diverse group, freshwater species are not as colorful as their marine relatives and usually exhibit direct development from egg to adult form, skipping the free-swimming veliger larval stage characteristic of marine snails. The unidentified snail photographed here was collected from a Northern Florida pond and was observed laying eggs in a gelatinous mass that adhered to glass and rock surfaces. As the snail offspring develop in the clear egg case, an observer can watch the progress, including the growth of miniature transparent shells and organ systems such as the two-chambered hearts of the circulatory systems. Some pond snails actually carry their developing young with them, at least until the babies emerge and fend for themselves as tiny models of their parents. Many species are hermaphroditic, but avoid self-fertilization by transferring sperm in packets. In aquaria, snails consume excess food and detritus as well as scrape algae from the tank sides, thus preserving a healthy and attractive artificial microcosm, while sometimes providing the fishes a nutritious snack. Whether in ponds, wetlands, or streams, aquatic snails rarely become a nuisance because of natural population controls such as disease, competition, and predation. However, several significant human health problems, such as schistosomiasis and "swimmer's itch", are dependent on aquatic snails as disease vectors or parasite hosts. In many underdeveloped nations, molluscides and habitat alteration are prescribed to prevent parasite and disease outbreaks.

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