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( Mollusk)
Aplacophora
Bivalvia
Caudofoveata
Cephalopoda
Gastropoda
† Helcionelloida
Monoplacophora
Polyplacophora
† Rostroconchia
Scaphopoda
† Tentaculita The phylum Mollusca is typically divided into nine or ten taxonomic classes, of which two are extinct. Cephalopod molluscs such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates – and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods, (snails and slugs), are by far the most numerous molluscs in terms of named species, accounting for 80% of the total. Gastropods are the only molluscan class which has been able to adapt to the land and freshwater as well as the oceans. Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups; the most universal feature is a mantle with a significant cavity used for breathing and excretion, and the structure of the nervous system. As a result of these wide variations, many textbooks base their descriptions on a hypothetical "generalized mollusc". This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle that covers the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom is very small, and the main body cavity is a hemocoel through which blood circulates – molluscs' circulatory systems are mainly open. The "generalized" mollusc feeding system consists of a rasping "tongue" called a radula and a complex digestive system in which slimy mucus and fine, muscle-powered "hairs" called cilia play various important roles. A striking feature of molluscs is the use of the same organ for multiple functions the gills both "breathe" and (in bivalves) produce a water current in the mantle cavity which is important for excretion and reproduction; the heart and nephridia ("kidneys") are important parts of the reproductive system as well as of the circulatory and excretory systems respectively.
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