Phylum Mollusca
There are between 50,000 and 200,000 different species in the phylum Mollusca. Called mollusks, these animals are characterized by their soft bodies having a head and a foot area which may or may not be covered by an exoskeleton or shell. They inhabit a wide range of environments, from the thermal vents at the bottom of the sea to the tops of mountains. In addition to the bivalves such as oysters, scallops and clams, the phylum includes octopuses, snails and squids. The smallest mollusk is the aplacophora, a worm-like creature only about a millimeter long, and the largest is the giant squid, reaching lengths as great as 20 meters.
Class Bivalvia
Oysters belong to the class Bivalvia which is characterized by a completely enclosing shell. The shell is typically made up of two halves called valves that are mirror images of each other, and they are hinged together. The bivalves are a very diverse group with a wide range of habitat. Besides oysters, the class includes clams, scallops and mussels.
Order Ostreoida
The order Ostreoida is made up of 11 families, including the oyster's family Ostreidae. Scallops, for instance, are of the order Ostreoida, family Pectinidae. The other families in this order are Spondylidae, Entoliidae, Propeamussiidae, Syncyclonemidae, Anomiidae, Placunidae, Gryphaeidae, Plicatulidae and Dimyidae.
Family Ostreidae
All true oysters are in the family Ostreidae. The oysters in this family typically have irregularly shaped shells due to their cementing themselves to rocks and other objects. These animals may be either egg bearing or larva bearing. They are all hermaphrodites, with the larva-bearing oysters alternating their sex and the egg-bearing oysters being hermaphrodites, both male and female at the same time.
Genera
There are eight genera of the family Ostreidae: Crassostrea, Cryptostrea, Dendostrea, Lopha, Ostrea, Ostreola, Saccostrea and Teskeyostrea. The two genera used for food are the Crassostrea and Ostrea genera. Genus Crassostrea is the American or eastern oyster, and genus Ostrea includes European or flat oysters.