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The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs. [21] In 2009, Chapman estimated the number of described living mollusc species at 85,000. [3] Haszprunar in 2001 estimated about 93,000 named species, [22] which include 23% of all named marine organisms. [23]
[2] Terrestrial mollusks comprise about 35 thousand species, most of which belong to the order (in some sources suborder or infraorder) Stylommatophora. Terrestrial molluscs occur across most of the planet, with the exception of Antarctica and some islands. They inhabit a wide range of ecosystems, from deserts and tundras to rainforests.
Gastropoda (previously known as univalves and sometimes spelled "Gasteropoda") are a major part of the phylum Mollusca, and are the most highly diversified class in the phylum, with 65,000 to 80,000 [3] [4] living snail and slug species.
The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to animals having shells composed of two valves. [3] More recently, the class was known as Pelecypoda, meaning "axe-foot" (based on the shape of the foot of the animal when extended).
Hydropower plants and dams are two examples of human ecosystem modification which contributes to loss of habitat as well as changes to channel morphology, river and floodplain connectivity and nutrient limitation. [16] Rates of extinction among freshwater bivalves are higher than those of terrestrial groups which share the same ecosystem. [15]
The mystery mollusks are hermaphrodites, which include both male and female reproductive organs. When it is time to release eggs, they descend and use their foot to temporarily attach to the seafloor.
The two major classes of molluscs have representatives in freshwater: the gastropods (snails) and the bivalves (freshwater mussels and clams.) It appears that the other classes within the Phylum Mollusca -the cephalopods, scaphopods, polyplacophorans, etc. - never made the transition from a fully marine environment to a freshwater environment.
The mollusc (or mollusk [spelling 1]) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea; many live on the land and in freshwater.