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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 ]
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 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.
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 class Gastropoda is a diverse and highly successful class of mollusks within the phylum Mollusca. It contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian .
Scaphopoda / s k æ ˈ f ɒ p ə d ə / (plural scaphopods / ˈ s k æ f ə p ɒ d z /, from Ancient Greek σκᾰ́φης skáphē "boat" and πούς poús "foot"), whose members are also known as tusk shells or tooth shells, are a class of shelled marine invertebrates belonging to the phylum Mollusca with worldwide distribution and are the only class of exclusively infaunal marine molluscs.
Other mollusks include gastropods, scaphopods and bivalves. Traditionally, the most common classification of the cephalopods has been a four-fold division (by Bather, 1888), into the orthoceratoids , nautiloids, ammonoids , and coleoids .
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