What is a Mollusk?

Mollusks are members of the phylum mollusca. Mollusks have been evolving in the sea for over 600 million years and there are more than 100 000 mollusks species. The mollusks phylum includes tiny snails, squid, octopus, clams, oysters, abalone, and cuttlefish. They are considered to be the most intelligent of the invertabrate species. The study of mollusks is called malacology. There are four main classes in the phylum mollusca, gastropods, bivalve, cephalopod and Polyplacophora. Gastropods include nudibranches, slugs and snails. They compose over one third of the mollusk group and most are single spiral shells but some have no shells. Bivalvia includes clams oysters and scallops. Bivalvia have two identical shells that are hinged together. Cephalopods include octopus, squid and cuttlefish and unlike most mollusks they have a closed circulatory system. They are further divided into grouops depending on the amount of tentacles and they all have the abillity to change colour making it easy for them to hide from danger. Polyplacophora include chitons and commonly clamp themselves to rocks and other surfaces. They have no eyes or tentacles. Mollusks have soft, thick and fleshy bodies and range from very small to as big as six feet accross. Mollusks have bilateral symmetry meaning that if cut down the middle from anterior to posterior end they would be the same on either side and have cephalization. Mollusks have a muscular skeleton consisting of an outer shell(s)and a soft body with a muscular foot, a mantle and a visceral mass.