by Size range and structure variety
Some adults sea snails (Homology) and forest litter snails (Stenopilis, Punctum) is less than one millimeter (0.04 inch) in diameter. At the other extreme, the largest land snail, the African Achatina Achatinait forms a shell that is almost 20 centimeters (eight inches) long. The largest freshwater snails, Pomacea from South America, reaching almost 10 centimeters in diameter, and the largest sea snail, the Australian Syrinx aruanusoccasionally grows to more than 0.6 meters (two feet). It is probably the longest snail Parenteroxenos doglieliwhich lives as a parasite in the body cavity of a sea cucumber: it grows almost 130 centimeters (50 inches) long, although it is only 0.5 centimeters (0.2 inches) in diameter. Most snails are much smaller; probably 90 percent of all adult snails are less than an inch in maximum size.
Snails show a huge variety of shapes, based mainly on the logarithmic spiral. They can be evenly rounded in one plane, as in Planorbis; becoming globose with the whorls rapidly increasing in size, as in Pomacea; that the sparrows would become elongated and rapidly larger, as i How and Scaphella; have a couple of flat coil strips that greatly increase in width, ie Haliotis; become elongated and spike-shaped, as in Turritella; or bent to form a barrel, as in Fissurella. Several such shell shapes can often be found among species within a single family, but marine families such as the Terebridae, Conidae, and Cypraeidae have a conservative shape. The thickness of the shells of different species varies considerably, and the heads of many species have prominent spines and ridges, probably as an evolutionary adaptation to predation.
Quiz Britannica
Animal Group Names
Traditionally, the three main gastropod groups are the prosobranchs (subclass Prosobranchia), the opisthobranchs (subclass Opisthobranchia), and the pulmonates (subclass Pulmonata); however, many authorities classify the lungfish as a subgroup within the subclass Opisthobranchia. The prosobranch branches usually provide a substantial shell in which the animal can retreat. The operculum, a disc that is often flattened on the back of the leg, fills the shell opening when the snail is inside the shell, protecting the animal from predation and desiccation. Opisthobranchs are marine species that often have a reduced or absent shell and very colorful bodies. Lungworms are snails and slugs that do not have an operculum but show complex and highly diverse body structures. They have a “lung” or lung cavity that also acts as a water reservoir. Gastropods have a fossil record that stretches back over 500 million years.
Distribution and abundance
Of the more than 65,000 species, about 30,000 are marine, 5,000 live in fresh water, and 30,000 live on land. In general, oceanic gastropods vary in number of species and shell structures in tropical waters; hundreds of species (each represented by a small number of individuals) can be found in a single coral reef habitat. This is in contrast to the arctic or sub-arctic coasts, where the few species present are represented by many individuals. Several deep-sea species are known, and hydrothermal vents are associated with a significant snail fauna. Most marine species have large ranges.
Freshwater snails are common in ponds, streams, marshes and lakes. Usually only a few species are found in one place, but each species will have a fairly wide range. Most species are common and feed on algae or dead plant material. In a few relatively old river and lake systems – notably, Lake Baikal in Siberia, Lake Titicaca in South America, Lake Ohrid on the North Macedonia-Albania border, the Mekong basin in Southeast Asia, and the African Rift lakes – are extensive and complex. . a radiation of snails has occurred in recent geological time, producing a large number of species.
Land snails are slightly, but very successfully, terrestrial. When they are actively moving, they will continuously lose water. During periods when water is not available, they retreat into their shells and remain inactive until conditions improve. They hibernate during winter periods, when water is locked up in snow or ice, and grow during dry summer periods. Land snails were found above the snow; a species of Showcase crawl on snowbanks in alpine meadows. Other species live in restless deserts and must remain inactive for years between rains.
Fewer than 10 species live in the same area together across most of North America. On the other hand, in such favorable areas as New Zealand, Jamaica, northeast India, and the wet forests of Queensland (Australia) 30 to 40 different species can be found together. In some parts of western Europe 20 species can be found together. Only one or two species are found in many desert regions, and they have dramatic feeding specializations.
The local abundance of snails and slugs can be amazing. Millions of several saltwater and freshwater species can live on small mudflats. An acre of British farmland could contain 250,000 slugs, while a Panamanian mountain forest was estimated to have 7,500,000 land snails per acre. Despite this abundance, snails and slugs often go unnoticed. Terrestrial and freshwater species often stay hidden during the day and are active at night. Most marine species are also nocturnal, and the shells of many of these species are so covered with algae and other fouling organisms that they could be mistaken for pieces of rock.
