Not all molluscs have all the same parts however they do have one part in common - a hard outer protective body - the shell itself. This outer body, or shell, is created by the animal itself. Some malacologists promote the idea that the shell is part of the body and they have a point because without the shell the animal within would soon die, much as we would without our protective shell (our skin). However as a conchologist it is my opinion that the shell is separate part of the animal because the animal within must attach itself at some point within the shell either by use of tendons as a conch or tusk shell does by affixing its tendon to one part deep within the shell, or membranes such as chitons and bivalves do or by simply wrapping a portion of its body onto the inside of the shell and holding on tightly the way cephalopods (such as the nautilus) do. But in each case each mollusc has a hard out protective casing we call the shell.
Conch and tusk snails have a cartilaginous foot called the operculum which is the outer most part of the body itself. When the animal withdraws into the shell the snail will draw the operculum tight into the aperture or opening securing the soft tender body from most predators. Basically the operculum acts as a doorway the snail can close shut or open to come out.
Bivalves have no such feature, instead their body is encased between two opposing hard shells that are hinged at one point - thus the name "BI-valves." When the animal is in danger it uses its powerful membranes within to close fast the two hinged shells making it impenetrable from most predators.
Chitons are positively one of the most archaic looking creatures found in the sea. They are generally found attached to large rocks and boulders along a rocky shoreline. The chiton actually has several hard outer shells that form plates much like an armadillo. These plates are attached on the underside my strong muscular membranes. The chiton uses other muscular membranes beneath itself to create a powerful suction against the rock and pull the plates down to where they are fastened hard against the rock and extremely difficult to pry off.
Most conch and tusk shells have what is called a siphonal canal of some kind through which the snail can extend an apparatus to siphon seawater inside in order separate the oxygen in order to breath. It will then pump the oxygenless seawater back through the same siphon.
Since not all conchs and tusk are the same this size, shape, length, location and manner in which they breath are different but most have some form of siphonal canal.
Bi-valves have a siphon but no canal on the shell. Most bi-valves use their muscles to pump oxygenated seawater in and out but the siphon is contained within the safety of the shells.
These are the three main parts of most sea shells. When dealing with specific classes each will have different sub-basic parts of the shell. The diagram shown is of a basic gastropod or uni-valve sea shell and breaks down the parts of the shell itself. Even with the gastropods specific families, genus and even as far down as species will all have additional parts not found on other shells.
Perhaps in the future we'll examine and break down shell parts even further.