The Molluscan Skeletal System:
The skeletal systems of all organisms within the Mollusca Phylum use an external shell as a means of protecting themselves. Due to the multiple differences between the shells of any two different mollusks, there are three main sub-categories that group these organisms based on general shell design and location:
The Glory scallop, found in Australia, is a bivalvia mollusk, meaning that it has a two-part shell connected by a hinge that allows it to move about the ocean floor and open its mouth.
Cephalopods, such as the blue-ringed octopus shown above, have an internal shell that surrounds an protects the vital organs inside its head.
The distinct shape of the tusk shells, like the one above, is what defines the scaphopoda class of mollusks.