• Squid are among the many members of the Phylum Mollusca.squid image by tamon from Fotolia.com
  
  Phylum Mollusca is host to an incredibly diverse range of creatures. The Phylum's three major classes--Bivalvia, Gastropoda, and Cephalopoda--run the gamut from common sea food restaurant menu items such as scallops and oysters to the giant squid.
  
  

Class Bivalvia

• Oysters are a member Bivalvia class.oysters for sale sea food image by david hughes from Fotolia.com
  
  One of the three major classes of the Phylum Mollusca, Bivalvia has a strong relationship with humans. The bivalves derive their name from the two-valved shells in which the organisms live. Common members of the class are clams, mussels, scallops and oysters. The majority of bivalves are shallow marine water dwellers, the oldest known relatives of whom, the Fordilla and Pojetaia, date to the Tommotian period--roughly 535 to 512 million years ago.
  
  

Class Gastropoda

• Aquatic snails are members of the Gastropoda class..Escargot image by Goldfish from Fotolia.com
  
  Class Gastropoda, the second of Phylum Mollusca's major classes, is a slimy affair. The largest class of the Phylum, Gastropoda--which means "stomach foot" in Latin--is home to over 40,000 living species of mollusk. An estimated 75 to 80 percent of mollusks are members of the Gastropoda class. Members of the class are all shell-dwelling creatures, many of which live in fresh water, that slither on their stomachs to get around, hence the "stomach foot" label. Aquatic snails and slugs are members of class Gastropoda.
  
  

Class Cephalopoda

• The octopus typifies the intelligence of the Cephalopoda class.octopus image by Matthias Kr??ttgen from Fotolia.com
  
  Class Cephalopoda, as per its Latin, is a class of "head foots." While the "stomach foots" of Gastropoda slither, the "head foots" of Cephalopoda have tendrils protruding from their often missile-shaped heads. Considered the most advanced class under the Phylum Mollusca, common members of Cephalopoda are octopi, squid, and the cuttlefish. The cephalopods are aggressive hunters with keen senses and highly developed brains. Indeed, the intelligence and problem-solving skills of the octopus has been well documented. It's cranial capacity has been compared to that of a house cat.