The largest and in many ways the most succesful of the main divisions into which the phylum Mollusca is split. More than 60.000 species in marine, freshwater and in terrestrial habitats. Body asymmetrical. Head well-developed, with eyes and sensory tentacles. Visceral mass rotated through 180° in early embryogeny, a process known as torsion, giving the looped gut and nervous system, and anteriorly situated mantle cavity characteristic of the class. Secondary detorsion occurs in some groups. Shell of one piece, prematively conical, typically spiralled. Ventral foot well-developed, used for crawling, burrowing, and swimming. Feeding modes diverse.