There are over 50,000 known species of Mollusks, which makes them second only to the Arthropods in invertebrate phylum size. Among the Mollusks are some of the most well known of invertebrate sea creatures, like snails, clams, mussels, squid, and octopods. Although one might not see an obvious physical relationship between a snail and a squid, they are remarkably similar in construction.
In general, mollusks have 3 body regions: a head, a visceral mass, and a "foot." The head contains the sense organs and "brain," while the visceral mass contains the internal organs. The "foot" is the muscular lower part of the body which is in contact with the substrate. Mollusks usually have a shell (although some do not). Mollusks also have an extension of the body wall called the mantle. This portion of the animal's anatomy is responsible for secreting the shell. The mantle encloses the mantle cavity which contains the Ctenidia (gills), anus and excretory pores.
Many mollusks have a radula, a tongue of sorts, which is rough like sandpaper and is used to rasp away at food. The radula is made of a hard material called chitin, the same material of which Arthropod exoskeletons are made. In addition, some Mollusk radulae are impregnated with magnetite to give them superior wear characteristics.
Mollusks have well developed body organs (nervous system, circulatory system, respiratory system, etc.) but lack body segmentation.
There are seven classes of Mollusks: Monoplacophora, Polyplacophora, Aplacophora, Gastropoda, Scaphapoda, Bivalvia and Cephalopoda. Of these, Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures, and will not be discussed.
The class Gastropoda (meaning "stomach-foot") contains about 70% of the Molluscan species (around 35,000). These are the familiar snails, limpets, nudibranchs and abalones. The snails, limpets and abalones have a shell while the slugs and nudibranchs do not. There are a small number of land Gastropods such as some snails, slugs, etc.
The Class Polyplacophora
Members of the class Polyplacophora (meaning "many-plated") are more commonly called the chitons (pronounced Kie-tuns). These creatures have eight plates and outwardly resemble the "pill-bugs" which are common under logs and rocks in the forest (they are not related, however). Chitons crawl along rocks and forage for food (mostly algae) using their radulae to scrape it off of the substrate. Magnetite is used for hardening the teeth of the radula in chitons, so they can scrape coraline algae off of rocks. These teeth are so hard that they can etch glass!
These creatures are extremely slow moving. In a year, a chiton may not move more than ten feet. They can detect the presence of light with primitive eyes embedded in the shell plates.
The Class Bivalvia
The bivalves (meaning "two-shells") are perhaps the most well known mollusks simply because of their history as a source of food. Clams, mussels, oysters and scallops are all bivalves. There are about 15,000 known species of bivalve, with about 80% of them being marine (the rest are found in fresh water).
The bivalve may either burrow through the bottom, or attach itself to the substrate with glue-like strings called "byssal threads." Some scallops do not attach themselves because they prefer to be able to swim away with a squirt of water forced from the mantle information found at http://www.oceanicresearch.org/education/wonders/mollusk.html Mollusks comprise a group of soft-bodied animals that includes snails, clams, and sea slugs. The most common characteristic of most mollusks is their shell. One of the largest groups, the snails, is renowned for their shells. Snails are univalves, which means they have one shell. And it is this shell that for many people is the epitome of the ocean. There is perhaps no other ocean treasure that displays more diversity and beauty than the shell. Conchology, the study and collection of shells is a popular hobby the world over. The bivalves, or two-shell mollusks include the clams, scallops, and oysters. It is the oyster that is responsible for producing the most coveted of the ocean's treasures - the pearl. Still other mollusks have lost their shells altogether. The octopus, the squid, and the sea slugs have evolved their own survival strategies to replace their protective armor. Indeed, it is due to the absence of a protective shell the octopus has evolved the largest and most complex brain of all the mollusks. Below is a listing of some of the most common mollusks found on the world's coral reefs. information found at http://www.seasky.org/reeflife/sea2f.html