gastropod, member of the class Gastropoda, the largest and most successful class of mollusks (phylum Mollusca), containing over 35,000 living species and 15,000 fossil forms. The shell of gastropods is of one piece (called univalve) and usually coiled or spiraled as in snails, periwinkles, conches, whelks, limpets, and abalones; however, in some forms, as in slugs and sea slugs, it is reduced or completely absent. There is usually a definite head, bearing one or two sensory tentacles and a mouth that is often equipped with a rasplike tongue called a radula. The lower surface of the animal is modified into a large, flattened foot, used by bottom-dwelling forms for creeping about. The foot and other soft parts of the body can usually be completely withdrawn into the shell and the opening covered by a permanent plate called the operculum. Ancient gastropods were probably bilaterally symmetrical, but living species undergo a process known as torsion in which most of the body behind the head rotates 180° so that the anal and urinary openings are relocated behind the head, and the digestive tract and nervous system become U-shaped. Most gastropod species are marine but many groups, notably the pulmonate (lung-bearing) snails, have successfully invaded freshwater and moist terrestrial habitats.