As of June 2011, there are about 100,000 types of mollusks that have been identified, from small snails and tiny sea slugs to palm-size oysters and giant squids. Most of them can be found in marine and fresh water, but a few species also live on land. Mollusks are extremely diverse in terms of anatomical structure, size and behaviour. However, taxonomists have categorised them into eight classes, four of which are the most commonly found. The four main classes of mollusks include Gastropoda, Bivalvia, Cephalopoda and Polyplacophora.
Gastropoda is the largest class of mollusks, which contains over 35,000 living species. More than two-thirds of them are marine creatures, whereas the rest can be found in fresh water and on land. Gastropods are so adaptable that they can live in any type of climate. Most of them have one spiral shell and a fleshy body that can be clearly defined as head, foot, mantle and visceral mass. One very distinct feature of Gastropods is their peculiar digestive tract. The anatomical process known as torsion allows the visceral mass of Gastropods to rotate 180 degrees, which causes their mouth and anus to be located at the same end. Some of the well-known Gastropods are snails, slugs, periwinkles, conchs and limpets.
Members of the Bivalvia class live in marine and fresh water. A Bivalve's soft body is hidden in a shell with two symmetrical valves. Although the valves are usually held open, they can be clammed shut in a snap once the Bevalve senses a threat. The soft body of a Bivalve consists of a hatchet-shaped foot, a pair of gills, a mantle and a visceral mass. In its rear part there is a pair of elongated tubes. One brings in water, food and oxygen while the other eliminates wastes. Some famous Bivalves include clams, oysters, cockles, geoducks and scallops.
Members of this class include squids, octopuses and cuttlefish. They only live in marine water and are said to be the most intelligent class among all mollusks. Fishermen sometimes call them "inkfish" due to their famous tactic of squirting ink at their predators and using it as a smoke screen to escape. Unlike other classes of mollusks, Cephalopods don't live in a shell and can travel through water quite rapidly using the numerous arms that are attached to their heads. Their vision is very acute, and their body can quickly change colour whenever they need to camouflage.
Polyplacophorans are better known by their other names, such as chitons, sea cradles and coat-of-mail shells. They are primitive marine mollusks with dorsal shells consisting of eight overlapping plates or valves. These plates not only provide protection but also allow a chiton to bend upwards and curl into a ball when dislodged from the surface it has clung onto. Its head is of a primitive type, which contains neither eyes nor tentacles. Its mouth and anus are located at the opposite ends of the body, with a visceral mass in the middle. Chitons move very slowly, and often times, they can be found under rocks or in rock crevices.