Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia † Helcionelloida † ?Bellerophontida
The molluscs (British spelling) or mollusks (American spelling) are members of the very large and diverse phylum Mollusca, which includes a wide variety of animals that are well-known for their decorative shells or as seafood. There are some 112,000 species within this phylum..
The vast majority of molluscs live in marine environments, and are found intertidally, in the shallow subtidal and on the continental shelf, although some species do live in the abyssal depths of the oceans around hot vents. Not all mollusks are marine: two groups, the bivalves and the gastropods, also contain freshwater species. Only the gastropods have representatives that live on land: the land snails and slugs.

Anatomy
There are ten classes of molluscs, eight are still living, the others are known only from fossils. These classes make up the 250,000 and more species of mollusc:
Main article: Evolution of Mollusca
Brusca & Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram above.
In this phylum's level of organization, organ systems from all three primary germ layers can be found:
All major molluscan groups possess a skeleton, though it has been lost evolutionarily in some members of the phylum. It is probable that the pre-Cambrian ancestor of the molluscs had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail or gastropod shell is perhaps the best known molluscan shell, but many pulmonate and opisthobranch snails have secondarily reduced and internalized shells, or have lost the shell completely. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonoids and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.

Class Caudofoveata (deep-sea wormlike creatures; 70 known species); now generally recognized as a subclass of Aplacophora.
Class Aplacophora (solenogasters, deep-sea wormlike creatures; 250 species)
Class Polyplacophora (chitons; 600 species, rocky marine shorelines)
Class Monoplacophora (deep-sea limpet-like creatures; 11 living species)
Class Bivalvia (also Pelecypoda) (clams, oysters, scallops, mussels; 8,000 species)
Class Scaphopoda (tusk shells; 350 species, all marine)
Class Gastropoda (sea snails with shells, such as abalone, limpets, conch, etc, and marine snails without a shell or with a reduced shell, such as nudibranchs, sea hares; sea angel, sea butterfly, sea lemon etc; land snails and slugs, freshwater snails, total estimated at 40,000 - 150,000 species)
Class Cephalopoda (squid, octopodes, nautilus, cuttlefish; 786 species, all marine)
Class † Rostroconchia (fossils; probably more than 1,000 species; probable ancestors of bivalves)
Class † Helcionelloida (fossils; snail-like creatures such as Latouchella)
Nervous System (with brain).
Excretory System (nephridium or nephridia).
Circulatory System (open circulatory system - except cephalopods which are closed).
Respiratory System (gills or lungs).