There are over 100,000 different species of mulusks alive today and they come in a variety of shapes and sizes from microscopic snails to 500 pound clams and 65 foot giant Squids. Mollusks are divided into 8 subgroups. The three largest are bivalves, gastropods and cephalopods. A bivalve is an animal that makes a shell with valves hinged together. Clams, oysters, mussles and scallops are all bivalves.
Bivalves open their shell to breath,eat, or to move around.
A snail is a gastropod. Octopus squid, cuttlefish, and the chambered nautilus are all cephalopods. They all have a gland that makes or produces the shell. Some mollusks, over thousands of years, have lost their shell. Octopuses and squid no longer have a shell, while cuttlefish have a remnant of their former shel inside their body.