The Columbia Encyclopedia, Sixth Edition. 2001.

snail


name commonly used for a gastropod mollusk with a shell. Included in the thousands of species are terrestrial, freshwater, and marine forms. Some eat both plant and animal matter; others eat only one type of food. Respiration is carried on by gills in the aquatic species; terrestrial forms have a pulmonary sac, or lung, in the mantle cavity. A few terrestrial species have returned to the sea, and consequently must rise to the surface to breathe. Eyes are borne on stalks or tentacles. Many snails, including all land snails, are hermaphroditic, but the majority of the marine species have separate sexes. A snail secretes a slimy path over which it progresses slowly by rhythmic contractions of the muscular base, or foot. Marine and terrestrial snails are eaten in various parts of the world. Snails are considered a delicacy in Europe and were eaten by primitive man and raised for food by the Romans. Certain harmful freshwater species harbor flukes and other parasites that cause disease in humans. Although some land snails cause economic losses by destroying vegetation, even more harm is done to gardens by slugs. Snails are classified in the phylum Mollusca, class Gastropoda.



The Columbia Encyclopedia, Sixth Edition. 2001.

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 The foot and other soft parts of the body can usually be completely withdrawn into the shell and the opening covered by a permanent plate called the operculum.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.