The typical snail has a calcareous shell coiled in a spiral pattern around a central axis called the columella. Generally, the coils, or whorls, added later in life are larger than those added when the snail is young. At the end of the last whorl is the aperture, or opening. The shell is secreted along the outer lip of the aperture by the fleshy part of the animal called the mantle, first by outward additions to the shell lip and then by secretion of inner thickening layers. The outer layer, or periostracum, is a mixture of proteins known as conchin. Inner layers of calcium carbonate interlace with a network of conchin and are impregnated with a variety of mineral salts. The calcium usually is in the form of calcite crystals in marine species and aragonite crystals in terrestrial species, but mixtures of crystal types do occur. New shell is secreted by specialized mantle tissue.
Modifications and ornamentations of the basic shell are widely variable among species. Frequently, the shell is altered into a nonspiral cap or a cup-shaped limpet form as an adaptation to life in swift currents (the freshwater family Ancylidae) or amid pounding waves on rocks (the marine families Acmaeidae, Patellidae, Fissurellidae, and Calyptraeidae). In many groups, such as the abalones (the family Haliotidae), only traces of spiral coiling are evident, because the rate of successive whorl widths is so large that the last, or body, whorl occupies more than 90 percent of the shell volume. Elaborate surface sculpture, including knobs and spines, has evolved to serve as protection against predation. In a few species of the genera Leucozonia and Acanthina, a spine on the lip edge is used to wedge open clam valves so that the snail can feed. As implied earlier, land gastropods in dry regions tend to have very thick shells; on the other hand, those in very humid mountain situations have thin shells or none at all. Many carnivorous snails have the calcareous part of the shell greatly reduced.
The body
The gastropod body consists of four main parts: visceral hump, mantle, head, and foot. The body is attached to the shell either by one columellar muscle or by a series of muscles. Typical snails can withdraw the head and foot into the shell, but numerous species have shells so reduced in size as to be unable to contain the body; slugs, of course, have either an internal shell vestige or no shell at all.
The visceral hump
The visceral hump, or visceral mass, of gastropods is always contained within the shell; it generally holds the bulk of the digestive, reproductive, excretory, and respiratory systems. A significant part of the visceral hump consists of the mantle, or pallial, cavity. In both prosobranchs and shelled opisthobranchs this is a cavity completely open anteriorly; in pulmonates it is closed except for a narrow pore. The mantle tissue at the forward edge of the cavity secretes the shell. The upper surface of the mantle cavity serves a respiratory function. In marine species the ciliated lining of the mantle cavity helps produce a water current that passes posteriorly across the gill, or ctenidium, and the osphradium, which is thought to be a sensory receptor that can detect chemical changes in the environment. Both organs lie on the left anterior side of the cavity. The water current sweeps across the posterior part of the mantle cavity, where the nephridiopore, or kidney opening, lies; the water current then passes anteriorly along the right margin past the anus, through which undigested particles of food are eliminated, and usually moves past the gonopore, through which sexual products are released. Cilia on the gill play an important role in water flow through the mantle cavity; they also help some species (e.g., Crepidula) capture food particles.
The mantle cavity serves as a space for the head and foot when these organs are retracted. Many land pulmonates apparently also use the mantle cavity to retain water. Prosobranchs use the operculum, the horny or calcareous disk located on the back of the foot at the posterior end, to seal the shell opening after the head and body have been retracted.
The mantle
The mantle is the fleshy lining of the outer wall of the shell; it roofs the mantle cavity. At its anterior end lie glandular tissues that deposit the various shell layers. In terrestrial forms with reduced shells, various lobes and laps extend anteriorly over the neck and head or are reflected back over the shell surface. These are highly vascularized and probably serve both in respiration and in water balance of the body. Many carnivorous marine forms have the mantle collar extended forward and rolled into a muscular siphon, which functions in both food location and chemoreception by allowing the snail to sample water in different directions.
The head
Generally, the head is bilaterally symmetrical, bearing one or two pairs of tentacles, often with accessory palps, and the mouth in the middle of the ventral margin. In stylommatophoran land snails the upper tentacles, or ommatophores, are invaginable (capable of being rolled in), and the eyes are borne at the tips. In freshwater basommatophorans and most prosobranchs the eyes are located at the base of the tentacles, although in such forms as Strombus the eyes are elevated onto an accessory stalk. Prosobranchs have contractile (not invaginable) tentacles. In carnivorous snails the lateral lips of the mouth form lobes called labial palps, which help to locate prey. The mouth itself frequently is prolonged into a proboscis that extends well in front of the tentacles. Carnivorous species often have a proboscis capable of great extension, either invaginable or contractile.
The foot
Although the basic form of the foot is a flat, broadly tapered, muscular organ, which is highly glandularized and usually ciliated, numerous modifications occur in various groups. Frequently there is an anterior-posterior division into a propodium and a metapodium, with the former capable of being reflexed over the shell. In Strombus the foot is greatly narrowed; in limpets and abalones it is broadly expanded and serves as an adhesive disk. In pelagic gastropods, especially the heteropods and pteropods, the foot is a swimming organ. Many prosobranchs and some opisthobranchs have lateral projections of the foot called parapodia; they are used in swimming or else are reflexed over the shell surface. An unusual feature found in several kinds of land slugs, some nudibranchs, and the neogastropod marine family Harpidae is the ability to self-amputate the posterior portion of the foot, which remains wriggling violently to distract a predator while the anterior foot and visceral mass creep slowly away to safety.
The nervous system and sense organs
A series of paired ganglia (knotlike masses of nerve cell bodies that collectively function as the central nervous system) are connected by nerve cords, which are bilaterally arranged in the primitive forms. The process of torsion has twisted the visceral cords into the form of a figure eight. In more-advanced gastropods there are secondary modifications to a more nearly bilateral state, and in many groups there has been detorsion. Water-dwelling mollusks depend primarily upon ciliary water currents passing across chemoreceptors for information from the environment. The primary chemoreceptors in the gastropod body are scattered over the skin surface, protruding from tentacles or palps, and housed inside the mantle cavity in the form of the osphradium, an olfactory organ connected to the respiratory system. Sense organs are more highly developed in carnivores than in herbivores. Eyespots, located at the base (most gastropods) or tip (land pulmonates) of the eye tentacles, are primarily light-sensitive rather than image-forming. A pair of statocysts, thought to be balancing organs, is present in nonsessile taxa.
Digestive system
The radular motion conveys food particles into the mouth, and ciliary currents move the food through the digestive tract, except in carnivores, where muscular action plays an important role. Various salivary and digestive glands secrete enzymes into either or both the buccal cavity and the stomach, where digestion takes place. The apical digestive gland, or “liver,” can store digested food for use during periods of inactivity.
The excretory system
There are two kidneys, or nephridia, in primitive gastropods, such as the archaeogastropods, while, in the advanced forms, one kidney is small or lost. The kidney plays different roles, depending upon the environment in which the snail lives. Most marine gastropods have the same total concentrations of solutes as in the surrounding seawater, and thus a small osmotic differential (i.e., an equilibrium) exists between the water leaving and that entering the cell. Little energy is needed therefore to prevent the cells from losing or gaining too much water. Freshwater gastropods, however, have a higher total solute concentration than that of the surrounding water. The kidney must expend energy to control water balance (osmoregulation). The flow of water through the mantle cavity is restricted in freshwater species by the closure of the mantle cavity by the mantle collar. Land prosobranchs have an open mantle cavity and, in order to conserve water, secrete nearly crystalline urine. Land pulmonates have a ureteric groove or closed ureter that resorbs water from the urine. In both marine and freshwater species, ciliary water currents sweep the excreted matter out of the mantle cavity.
Respiration
In marine and freshwater gastropods, respiration takes place as water currents pass across the gill surfaces within the mantle cavity in most species with spiral shells, across gill elements along the sides of the bodies in most limpets, or through projections from the body surface in sea slugs or other taxa with reduced shells. The upper surface of the mantle cavity is heavily vascularized in land snails, which use muscular contractions to pump air in and out of the small respiratory pore at the anterior edge of the mantle cavity. In some land slugs or tropical snails with reduced shells, respiratory functions have shifted either to external projections from the mantle collar or to the skin as the area of mantle roof available for respiration has decreased in size.
The reproductive system
The primitive archaeogastropods retain two nephridia; the right nephridium provides the passage for eggs or sperm from the ovary or testis to the mantle cavity. The sexes are separate in nearly all prosobranchs, although in a few taxa, such as Crepidula, an animal begins life as a male and then changes to a female later. Opisthobranch and pulmonate species are hermaphroditic and often protandrous (male gonads maturing first); however, in many taxa, adults become simultaneous hermaphrodites (male and female gonads are functional at the same time). Internal fertilization is common in the more advanced marine species but mandatory in the freshwater and terrestrial groups. A very few gastropod species are parthenogenetic (gametes developing without fertilization); the progeny of these species are clones of the parent.
Features of defense
Warning coloration is found in some of the brilliantly coloured shells and bodies of carnivorous marine snails that produce highly toxic poisons. Similar bright colours characterize some land snails and slugs that secrete noxious chemicals and thus will be sampled only once by a predator. Camouflage coloration provides partial protection against predation by some European land snails.
Evolution and paleontology
The basic trends in snail evolution (aside from changes in radular and shell morphology) involve a loss of organs, a change from an herbivorous to a carnivorous diet, a shift from the ocean to freshwater and terrestrial life, and the adoption of a sluglike form through reduction or loss of the shell and visceral hump. Each change has occurred independently several times in the course of gastropod evolution.
Prosobranch gastropods are the most primitive. One group, the Diotocardia, which retains two sets of mantle organs, is nearest the generalized gastropod in structure. Gradual loss of the set of mantle organs on the right side of the body occurs in the primitive archaeogastropod superfamilies Trochacea and Neritacea, thus providing a transition to the more highly developed order Monotocardia, with only one set of mantle organs. Among the numerous changes in the Monotocardia are fewer radular teeth and a shift from grazing on algae and fungi to predation and the consumption of larger sessile organisms. The two main divisions of the Monotocardia show different evolutionary patterns. Although most mesogastropods have remained coastal marine, a number of species have invaded freshwater environments. Others crossed to land directly from the tidal zone, rather than passing through a freshwater transitional period. At the peak of prosobranch evolution is the order Neogastropoda, all marine predators with highly modified radular teeth and often well-developed poison glands to aid in capturing prey. Reduction and loss of the right mantle organs are correlated with more efficient respiration and sensory apparatuses, in which a water current crosses over the sensory organs and gills on the left side, then out on the right side, together with excretory and fecal deposits. Gill cilia are largely responsible for creating these water currents.
Opisthobranchs probably arose from an unknown group of primitive prosobranchs and have evolved extensively into different lines showing a reduction of the visceral hump and shell. In certain forms the foot is shortened, and external cerata develop to provide a respiratory surface to replace the lost mantle-cavity surface and ctenidia. Members of family Pyramidellidae (order Heterostropha) contain a mixture of prosobranch and opisthobranch characteristics.
Pulmonates show varying degrees of adjustment to freshwater and land life, with increasing union of the male and female gonoducts characterizing the more advanced groups. Similarly, the highly advanced suborder Holopoda and superfamily Limacacea show complex accessory organs on the genitalia and a more sophisticated means of water conservation through development of a closed secondary ureter and resorption of water from the excretory products. More than a dozen different groups of pulmonates have become predators, usually upon other snails or earthworms.
Fossil gastropods are known from Cambrian deposits. Since the shell is often very similar in unrelated families, fossil gastropods more than 350 million years old are not usually placed in the classification outlined below but instead are treated separately. Most neogastropod prosobranchs appeared near the end of the Mesozoic (65.5 million years ago), and many groups of land snails are known from Eocene formations (roughly 56 million to 34 million years old). Snails had their adaptive radiation early in geologic history. Living genera of marine, freshwater, and land snail families are known from Oligocene to Miocene deposits (33.9 million to 5.3 million years old). Unlike mammals, who have undergone great evolutionary change in the last 50 million years, gastropods have shown little progressive evolution during that time.