The Molluscs are a large phylum of about 100,000 species. They are characterised by the shell and the mantle that secretes it, the radula which is a peculiar file-like feeding structure, and the moluscan gills. The phylum includes, as well as certain primitive groups, the Bivalves which include the cockle and mussel, the Cephalopods which include the cuttlefish, squid and octopus, and the Gastropods which include the limpet, whelk, snails and slugs.
Aplacophora is a class of small, deep-water, bottom dwelling, shell-less marine mollusks found in all oceans of the world. The class comprises 28 families and about 320 species. Aplacophorans are mostly less than 5 centimetres long, cylindrical and worm-like.
Bivalves are extremely numerous and varied and there are about 8000 species, mostly in the sea but some in fresh water. Typically the shell is in two pieces with a hinge and closed by strong muscles. Siphons draw water into the large mantle both for respiration and feeding. The class includes clams, mussels, cockles, scallops and oysters.
The class Cephalopoda consists of the squids and octopuses. The name means head-foot and they have large heads with a mouth and horny beak, surrounded by muscular tentacles, sometimes with suckers or hooks. The shell may be reduced or absent. They have large eyes and part of the mantle forms a muscular siphon through which water is forced to provide jet propulsion. Some can change color by way of camouflage and several extrude a cloud of ink for protection. They are all marine and many are big, swift and intelligent.
The main feature of Gastropods is torsion. At some stage in their development part of the body containing the internal organs rotates through 90 to 180 degrees relative to the foot. Typically they have a large muscular creeping foot which they can retract into a coiled shell and often close the opening with a lid, the operculum. The head has one or two pairs of tentacles, eyes and a rasping tongue, the radula. The Pulmonates include snails and slugs, the latter having lost their shells, which live on land and can breathe air. The Nudibranchs include the sea slugs and sea hares in which the shells have largely been lost or become internal. The Prosobranchs include other aquatic species with a single shell such as the top shell, periwinkle, cowrie and whelk.
Monoplacophora is a class of mollusks thought to be extinct until 1952, when a living animal was dredged up from deep marine sediments. They have a single, flat, bilateral shell that is often thin and fragile. About two dozen living species of monoplacophorans have now been discovered, all living in deep sea trenches.
Polyplacophora includes 800 species of chitons, or coat-of-mail molluscs. Chitons have elongate, bilaterally symmetrical, dorsoventrally flattened bodies with an oval outline. The shell covers most of the dorsal surface and consists of eight overlapping calcareous plates. Chitons are adapted for life attached to hard surfaces on turbulent shores although many live in quieter deeper water.
Scaphopoda or tusk shells are a class of marine mollusks which live on the sea bottom. The usually white shell somewhat resembles a miniature elephant's tusk but is hollow and has an opening at both ends.