Gastropoda

Gastropods are a vast and diverse group of invertebrates within the phylum Mollusca, comprising the animals commonly known as snails and slugs. With an estimated 65,000 to 80,000 living species, they form the second-largest animal class after the insects. The fossil record of gastropods extends back to the Late Cambrian., 721 families are recognized—476 extant and 245 extinct known only from fossils.
Gastropods inhabit an extraordinary range of environments, including marine, freshwater, and terrestrial ecosystems. They occur in gardens, woodlands, deserts, mountains, rivers, lakes, estuaries, mudflats, intertidal zones, the deep sea, hydrothermal vents, and even in parasitic niches.
The term snail generally refers to gastropods with a large external shell into which the body can fully retract, while slugs have no shell or a small internal one, and semislugs can only partially withdraw into their reduced shell. Marine gastropods include familiar forms such as abalones, conches, periwinkles, whelks, and cowries, whose shells are usually coiled in adulthood. In contrast, limpets and related groups coil their shells only in the larval stage, retaining a simple conical form as adults.
Because of their vast diversity, gastropods exhibit remarkable variation in anatomy, behavior, feeding, and reproduction, making broad generalizations difficult. Gastropods remain one of evolution’s most successful and adaptable animal lineages.

Etymology

In the scientific literature, gastropods were described as "gasteropodes" by Georges Cuvier in 1795. The word gastropod comes from Greek and , a reference to the fact that the animal's "foot" is positioned below its guts.
The earlier name "univalve" means one valve, in contrast to bivalves, such as clams, which have two valves or shells.

Diversity

At all taxonomic levels, gastropods are second only to insects in terms of their diversity.
Gastropods have the greatest numbers of named mollusk species. However, estimates of the total number of gastropod species vary widely, depending on cited sources. The number of gastropod species can be ascertained from estimates of the number of described species of Mollusca with accepted names: about 85,000. But an estimate of the total number of Mollusca, including undescribed species, is about 240,000 species. The estimate of 85,000 mollusks includes 24,000 described species of terrestrial gastropods.
Different estimates for aquatic gastropods give about 30,000 species of marine gastropods, and about 5,000 species of freshwater and brackish gastropods. Many deep-sea species remain to be discovered, as only 0.0001% of the deep-sea floor has been studied biologically. The total number of living species of freshwater snails is about 4,000.
Recently extinct species of gastropods number 444, 18 species are now extinct in the wild, and 69 species are "possibly extinct".
The number of prehistoric species of gastropods is at least 15,000 species.
In marine habitats, the continental slope and the continental rise are home to the highest diversity, while the continental shelf and abyssal depths have a low diversity of marine gastropods.

Habitat

Gastropods are found in a wide range of aquatic and terrestrial habitats, from deep ocean trenches to deserts.
Some of the more familiar and better-known gastropods are terrestrial gastropods. Some live in fresh water, but most named species of gastropods live in a marine environment.
Gastropods have a worldwide distribution, from the near Arctic and Antarctic zones to the tropics. They have become adapted to almost every kind of existence on earth, having colonized nearly every available medium.
In habitats where not enough calcium carbonate is available to build a really solid shell, such as on some acidic soils on land, various species of slugs occur, and also some snails with thin, translucent shells, mostly or entirely composed of the protein conchiolin.
Snails such as Sphincterochila boissieri and Xerocrassa seetzeni have adapted to desert conditions. Other snails have adapted to an existence in ditches, near deepwater hydrothermal vents, in oceanic trenches 10,000 meters below the surface, the pounding surf of rocky shores, caves, and many other diverse areas.
Gastropods can be accidentally transferred from one habitat to another by other animals, e.g. by birds.

Anatomy

Snails are distinguished by an anatomical process known as torsion, where the visceral mass of the animal rotates 180° to one side during development, such that the anus is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon. Torsion is present in all gastropods, but the opisthobranch gastropods are secondarily untorted to various degrees.
Torsion occurs in two stages. The first, mechanistic stage is muscular, and the second is mutagenetic. The effects of torsion are primarily physiological. The organism develops by asymmetrical growth, with the majority of growth occurring on the left side. This leads to the loss of right-side anatomy that in most bilaterians is a duplicate of the left side anatomy. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane. The gill-combs, the olfactory organs, the foot slime-gland, nephridia, and the auricle of the heart are single or at least are more developed on one side of the body than the other. Furthermore, there is only one genital orifice, which lies on the same side of the body as the anus. Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can be retracted more easily, perhaps suggesting a defensive purpose.
Gastropods typically have a well-defined head with two or four sensory tentacles with eyes, and a ventral foot. The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls. The larval shell of a gastropod is called a protoconch.

Shell

Most shelled gastropods have a one piece shell, typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side. Numerous species have an operculum, which in many species acts as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.
Some gastropods have adult shells which are bottom heavy due to the presence of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture which may be important for gravitational stability.

Chirality

Most snail species possess a dextral shell, which mirrors their internal anatomy. However, sinistral shells are the obligate and standard form in several large families and genera, where the internal anatomy is symmetrically reversed.

Body wall

Some sea slugs are very brightly colored. This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly colored hydroids, sponges, and seaweeds on which many of the species are found.
Lateral outgrowths on the body of nudibranchs are called cerata. These contain an outpocketing of digestive glands called the diverticula.

Sensory organs and nervous system

The sensory organs of gastropods include olfactory organs, eyes, statocysts and mechanoreceptors. Gastropods have no hearing.
In terrestrial gastropods, the olfactory organs, located on the tips of the four tentacles, are the most important sensory organ. The chemosensory organs of opisthobranch marine gastropods are called rhinophores.
The majority of gastropods have simple visual organs, eye spots either at the tip or base of the tentacles. However, "eyes" in gastropods range from simple ocelli that only distinguish light and dark, to more complex pit eyes, and even to lens eyes. In land snails and slugs, vision is not the most important sense, because they are mainly nocturnal animals.
The nervous system of gastropods includes the peripheral nervous system and the central nervous system. The central nervous system consists of ganglia connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, osphradial ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.

Digestive system

The radula of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the limpets and abalone, herbivores that use their hard radula to rasp at seaweeds on rocks.
Many marine gastropods are burrowers, and have a siphon that extends out from the mantle edge. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw water into their mantle cavity and over the gill. They use the siphon primarily to "taste" the water to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.

Respiratory system

Almost all marine gastropods breathe with a gill, but many freshwater species, and the majority of terrestrial species, have a pallial lung. The respiratory protein in almost all gastropods is hemocyanin, but one freshwater pulmonate family, the Planorbidae, have hemoglobin as the respiratory protein.
In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs. Some nudibranchs have smooth or warty backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.