Gastropods are the largest, most successful class of mollusks. Of over 50,000 species, 35,000 are still living. once confined to seas, snails later became adapted to life in ponds, streams, and on land and now been able exploit a wide variety of marine, freshwater, and land habitats. Gastropods have a head with eyes and a mouth, a flattened foot for crawling, and organs that are generally coiled and carried in spiral shell A few groups cease the coiling at some stage, and some have abandoned the shell all together. Shells are composed of calcite. The shell opening may be closed by a lid when the animal draws in, as you may have seen snails. An indescent layer in the inner shell is characteristic of some more primitive forms, but the organic outer coat of many living gastropods is usually lost in fossils