Mollusks are tasty. They are also fairly diverse. There are about 100,000 different extant species and 35,000 extinct ones. The reason why we can identify so many extinct species is because most mollusks have some sort of shell that mineralizes very easily. It leaves a great fossil.
Mollusks have been dated to 600 MYA.
the typical example is a CHITON. A small ovular species with eight plates on its dorsal side (back). It has:
These are the tasty ones. This group includes mussels, clams and oysters. They are called "bivalves" because their shell is made up of two hinged segments. They don't move very far from their homes on the bottom of the various bodies of water they inhabit. Like other mollusks, these things have huge amounts of GONADS - why? Because they aren't built like other animals. They can't copulate like other animals can. They have external fertilization. They spew large amounts of sperm and eggs throughout the water and hope that they meet. It doesn't seem very efficient but that also explains why clams and other mollusks live in beds. A group of these things got together and thats where mating occured and thats where the next generation was born. They don't move great distances but they DO move around. Zebra mussels for instance, are just about everywhere in this part of the world. As larvae, they can swim, so they can inhabit other lakes and rivers. They clog up intakes and are just a plain nuisance. They also have cleared Lake Erie tremendously. In doing so, they've altered the food web and are outcompeting native specimens. Not bad for a mussel who wasn't even in North America until less than 15 years ago.
Snails are the archetypical example of gastropods. Snails are noteworthy in that they are NOT gonochoristic. They share all the typical "molluscian" traits with chitons - the shell, the mantle, the radula, the open circulatory system etc. In addition, they are HERMPAPHRODITIC. There is no such thing as a male snail or a female snail. When they mate, they pick who will be the male and who will be the female.
Cephalopods are the most "advanced" of the mollusks. Squids and octopi are the prime example of cephalopods. They have a closed circulatory system so they can move much faster - the closed circulatory system allows more efficient "pinpointing" of blood. Squid also have the largest nerve fibers in the animal kingdom. This enables them to move pretty quick. Squid can hit speeds of about 15 meters PER SECOND in short bursts (~50% faster than a world-class olympic sprinter). They move through jet propulsion - water is taken in and expelled under pressure.
Squid also are notable for their eyes. Their eyes are very similar to that of the vertebrates. This is an example of convergent evolution - two species distantly related moving closer together.