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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.
List of mollusc orders illustrates the 97 orders in the phylum Mollusca, the largest marine animal phylum. 85,000 extant species are described, [1] making up 23% of described marine organisms. [ 2 ] Class Aplacophora
Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks [a] (/ ˈ m ɒ l ə s k s /). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda . [ 5 ]
The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods , bivalves , scaphopods , cephalopods , and several other groups.
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.
The taxonomy of the Gastropoda as it was revised in 2005 by Philippe Bouchet and Jean-Pierre Rocroi is a system for the scientific classification of gastropod mollusks (Gastropods are a taxonomic class of animals which consists of snails and slugs of every kind, from the land, from freshwater, and from saltwater).
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).
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 (minimum 50,000, maximum 120,000). [9]