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All snails belong to the phylum Gastropoda (literally "stomach-footed"). Nearly all are covered with a single spiral shell. Given that there are around 75,000 species around the world, plus several thousand more extinct forms known from their fossils, there is obviously going to be quite a bit of variation in this group of univalves. Including some that are completely shell-less, such as land and sea slugs, and others whose shells are so rudimentary that they are barely noticeable. So I’ll be using some pictorial examples that I hope will illustrate some of the basic common features.
