Class Bivalvia (a.k.a. class Pelecypoda) includes all of the bivalves. These are marine animals with a hinged shell divided into two halves. The hinge is head together by a ligament and one or two adductor muscles. Most live in sand or mud, and use their foot for digging and anchoring to surfaces. These bivalves use the hinges to take in food, and are also able to jet some distance away by closing the hinged shell and squirting the water taken in out of the mantle cavity. Bivalves lack a head and have a spacious mantle cavity as well. Some examples include clams, oysters, scallops, and mussels.

Bivalves are in the phylum Mollusca. All mollusks have a muscular foot used for moving and a mantle, an outgrowth that covers the animal. Many mollusks have an external calcium carbonate shell that is produced by the mantle. The gill of a mollusk extracts oxygen from the water and disposes of waste. All species of the phylum Mollusca have a complete digestive tract, spanning from the mouth to the anus. Many also have a radula, a unique organ, mostly composed of chitin, in the mouth that allows the animal to scrape food from surfaces, especially the ocean floor, by sliding back and forth. Mollusks have a coelom, but the coelom is made from cell masses, making all species in this phylum protosomes. All organs are suspended in this coelom, between the outer covering and the digestive tube of the animal. However, animals in this phylum are unique to coelomates in that they lack body segmentation. There are seven classes in this phylum.