• The POLYPLACOPHORA contain about 900 living species and are commonly known as chitons.  They have a shell consisting of eight, usually overlapping plates, held together by a leathery "girdle".   The animal is bilaterally symmetrical. With a well-developed foot surrounded by a groove in which there are 6 to 88 pairs of gills.

The head lacks eyes and tentacles, but usually has light-sensitive areas and chemical receptors,for finding food and heading in the proper direction! All chitons are marine inhabitants and most make a living by grazing algae from rocks and other hard substrates. The great majority of them dwell in shallow and intertidal waters, but a few occur in depths down to 5,000+ meters. They range in length from 3 to 400+ millimeters. (1/8" to 1ft 4"+).

• The APLACOPHORA consist of about 250 living species (perhaps more, as they are relatively poorly studied!) of marine, wormlike, bilaterally symmetrical animals living at moderate, to very great depths, usually on or in soft bottoms.  They have no shell, but have calcareous spicules in the body surface.  The foot is restricted to an anterior pedal shield or to a narrow groove running the length of the body. Aplacophorans have a radula and a posterior mantle cavity.  Some are detritus feeders, others are predators.  They range in length from 1 to 300 millimeters. (If you read and go along with the views held by Brusca & Brusca (see bibliography) you will also include the Class of Caudofoveata & Solenogastres.  These are two very small classes that many scientists combine to form the Class Aplacophora. 

The Caudofoveata are aberrant molluscs that lack shells. They are quite common in the deep sea buried in the soft sediments.  The Solenogastres also lack shells, are also found in very deep water and generally live on the surface of the substratum.)

• The SCAPHOPODA comprise about 350 living species. Commonly known as "tusk shells", they are bilaterally symmetrical and their elongate, tubular, tapering shells are open at both ends. The conical foot can be protruded for use in burrowing only.  The head is rudimentary and lacks eyes and tentacles. It feeds by contractile filaments called captacula, which are withdrawn into the body cavity when they meet up with food.  All scaphopoda are marine, and live buried in muddy or sandy bottoms, where they feed on detritus.  They range from 2 to 150 millimeters (i.e., up to 6").

• The GASTROPODA  is by far the most successful class of all the molluscs, with at least 60,000 living species.  Gastropods generally have a single-valved shell, which is usually spiraled; however, this is absent in the slugs and semi-slugs.  They have a head with cephalic tentacles and a well-developed foot used in crawling. Some gastropods have lungs for respiration, others gills.

Early in their larval stage of development, the visceral mass and mantle cavity rotate up to 180 degrees counterclockwise; in a process know as torsion. (This does not occur in some of the slugs though.)  This brings their organs from a posterior position to an anterior position behind their head. In most cases, the soft animal is able to retract into their shells for protection.  Some gastropods also have an operculum (trap door) connected to their foot that they can pull in after their soft body parts sealing off their shell from the environmental hazards or as protection from enemies.  Gastropods are very successful in marine, freshwater and terrestrial habitats.  Their size is from 0.5 to 750 millimeters. (i.e., up to 2 1/2 feet long)

• The BIVALVA, or PELECYPODA (the Bivalves) comprise about 10,000 living species.  They have two valves made of calcium carbonate (in a hard form called "aragonite"), connected by a flexible ligament and an "adductor muscle" for closing the valves tightly.

The mantle cavity is enlarged, enclosing the visceral mass and other internal organs.  There is no differentiated head or cephalic region, and the radula common to all other molluscs is absent.  Most are filter feeders, with the gill acting as a food collecting and sorting organ, in addition to filling its respiratory function.  The mouth usually has a pair of labial palps on either side that handle and direct the food collected by their gills. Bivalves inhabit all of the world's marine and freshwater habitats which have a pH greater than about 5 - any more acidic than that, and they can't form a shell fast enough to prevent it from being dissolved again! They range in size from 0.5 millimeters to almost 1.4 meters (that's 1400mm, or 4'8"!!)

• The MONOPLACOPHORA are mostly are known by their fossil records; however, there are about a dozen living species today.  They have several foot retractor muscles, gills, and hearts similar to those of the annelid worms; however, their bodies are not segmented.  All Monoplacophorans are marine inhabitants grazing on algae and microorganisms on the hard ocean bottom.  They live at depths of 200 to 6,000 meters and they range in size from 2 to 35 millimeters.