General As voracious animals, furnished with powerful rasping organs, many gasteropods play an important part in the struggle for existence among marine organisms. Oyster beds, for example, are infested by thou sands of industrious "borers)) of several kinds, that perforate the shells of oysters, clams, etc., fatally sucking out the life-blood of their occu pants. Certain terrestrial forms, as slugs, are most destructive devastators of vegetable and flowering plants. From very early times, vari ous gasteropods, such as whelks, have been utilized for human consumption and also as bait, while yet more frequently the shells, often so beautiful in form and color, have been used for the decoration of the person and the dwelling, for the basis of cameos, as domestic utensils, or even as weapons, and in many other ways, From the mucous glands of the roof of the gill-cavity in the genera Purpura and Murex, there exudes the famous secretion, at first colorless, but afterward becoming purple or violet, which furnished the ancient Tyrian dye. See SHELLS.
Geological few gasteropods oc cur in strata as far back as the Cambrian, from which remote period they have continued with a steady increase. Almost all the Palaeozoic genera are now extinct, and during these ages the siphon-possessing forms seem to have been almost, if not altogether, unrepresented. A host of new gasteropods appeared in the Juras sic period, and many of the modern families have their origin in Cretaceous times. Numer ous as the fossil forms are, the number of types wholly extinct is comparatively small; both as regards persistence of types and in crease of numbers, the gasteropods are a peculiarly successful class.
The grouping of forms within the class is as follows, according to the latest conclusions of naturalists, as summarized by Cooke in the third volume of the Cambridge 'Natural History' (1894) : Class GASTEROPODA ; order Amphineura; sub orders, Polyplacophora, Aplacophora; order, Prosobranchiata; sub-orders, Diotocardia, Mo notocardia; order, Opisthobranchiata; sub orders, Tectibranchiata, Ascoglossa, Nudi branchiata, Pteropoda; order, Pulmonata; sub orders, Bas.ontmatophora, Stylommatophora.
(For the characters of the orders see Classi fication in the article ANATOMY). The sub divisions are based upon different anatomical categories in each order. Thus the first sub order of Amphineura embraces all the ordinary chitons having a foot and plated shell, both of which are absent in the degraded Aplacophora. Among the Prozobronchiata (which embrace the ordinary marine shells) two auricles in the heart characterize the Diotocardia, a single auricle the Monotocardia, of which the strange pelagic Heteropoda are now regarded as only a subordinate group. The Opisthobranchs are classified according to gill-features; and the Pulmonata, according to relative position of the eyes.
See MOLLUSCA ; SNAIL; WHELK, etc., and consult the works cited thereunder, especially Cooke, 'Shells' (New York 1896), in which will be found many instructive references to other authorities.