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Determining whether some gastropods should be called sea snails is not always easy. Some species that live in brackish water (such as certain neritids) can be listed as either freshwater snails or marine snails, and some species that live at or just above the high tide level (for example, species in the genus Truncatella) are sometimes considered to be sea snails and sometimes listed as land ...
Some groups of snails that live in freshwater respire using gills, whereas other groups need to reach the surface to breathe air. In addition, some are amphibious and have both gills and a lung (e.g. Ampullariidae). Most feed on algae, but many are detritivores and some are filter feeders.
Some freshwater pulmonates keep a bubble of air inside the shell, which helps them to float. [1] Many of the very small freshwater limpets which live in cold water have lost the ability to breathe air, and instead flood their mantle cavity with water. [1] Oxygen diffuses from the water to the snail's body directly. [1]
Snails have complex organ systems and anatomies that differ greatly from most animals. Snails and most other Mollusca share three anatomical features; the foot, the mantle, and the radula. Foot: The foot is a muscular organ used by Gastropods for locomotion. Gastropods' stomachs are located within their foot.
These snails are popular in freshwater aquariums because they do not eat fish eggs or plants, they do not overpopulate the aquarium, and they close up if there is a water problem, giving people an indication that something is wrong a few weeks before the fish die. [15]
The shell grows with them in size by the process of secreting calcium carbonate along the open edge and on the inner side for extra strength. Although some land snails create shells that are almost entirely formed from the protein conchiolin , most land snails need a good supply of calcium in their diet and environment to produce a strong shell.
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Some saltwater limpets such as Trimusculidae breathe air, and some freshwater limpets are descendants of air-breathing land snails (e.g. the genus Ancylus) whose ancestors had a pallial cavity serving as a lung. In these small freshwater limpets, that "lung" underwent secondary adaptation to allow the absorption of dissolved oxygen from water.