Snails or Gastropods, are soft bodied animals enclosed in a hard protective coiled shell. They are very common animals, and have diversified to fill wide range of estuarine environments. Snails can be found from the weeds and algae in shallow waters, from soft muddy sediments in flowing waters, attached to plant material, rocks or pebbles of slow flowing waters and amongst leaves and debris in ponds and lakes. Snails have a unique feeding structure called a radula, which is a belt of chitinous teeth stretched over a cartilage base. This is scraped over the surface of rocks and plants to collect food. Some Freshwater snails have gills inside the shell, while others obtain oxygen from an air filled space within the body. More strangely, some species are thought to use a triangular shaped surface for breathing. Snails secrete their own shell, and it grows with age, so that the edge represents the youngest part of the shell. Freshwater snails play an important role in the breakdown of detritus, which is dead plant and animal material. Freshwater snails are generally very tolerant to water pollution.