Meet the MollusksSome people are saying "Yummy" right about now. Not us. Clams, snails, slugs, mussels, oysters, and clams are all considered to be mollusks. Squid and octopi are also mollusks but we put them on a different page because they are very cool and actually somewhat intelligent. Clams are not known for their intelligence. Mollusks are known to be the first animals to have developed gills.
Snails and SlugsYou can find them on land and in the ocean. Bivalves are usually only found in water. What are some characteristics of gastropods? They have tentacles on their heads. Think about a snail for a moment. It has two stalks for the eyes and two shorter ones towards the front of the head. It can actually move those to look around. They aren't tentacles like an octopus.
Gastropods might or might not have shells. Slugs are basically snails that have lost their shells over years of evolution. Snails have those big spiral shells that can come in a variety of shapes and sizes. They are usually made out of a compound called calcium carbonate. The shells have little compartments inside where the creature can place its digestive system.
Clams and Mussels with Two ShellsLet's talk bivalves. This is the place where you'll find clams and mussels. These invertebrates have two shells. The shells are connected at the base by little ligaments that the creatures control. They are able to open and close the shells for protection, to help them move, or to open up and eat. These creatures even have tiny eyes. They will see you coming if you are trying to sneak up on them.
Snails Time-lapse (BBC Video)
Encyclopædia Britannica (Gastropods):