Bivalves comprise a taxonomic class within the phylum Mollusca. They have a unique exoskeleton comprised of two shells from which they get their name. They also share a number of other body parts common to all mollusks. These aquatic animals that are known for the delicacies they make as food, their beautiful inside shell, and fun memento to collect on the beach. Bivalves were created on the 5th day of creation, because they make their homes in water. Water is necessary for many bivalves to eat and respire.(Levine and Miller p 701).
Many common bivalves that you have probably heard of are clams, scallops, cockles, mussels, and oysters. Oysters, in fact, are all born as males, but may change sex if environmental changes permit .
Anatomyphylum Mollusca. They all have a foot, mantle (a small layer of tissue that covers the majority of the body, shell, and visceral mass (internal organs).(Levine and Miller, p 702, 706). Bivalves are composed of two shells held together by an elastic ligament. The shells open and close through two abductor muscles. Their shells have three different layers created by the mantle. The easily recognizable inside of an oyster is the nacreous layer.
Inside of the two shells is the hatchet-shaped foot. The foot is used for moving, burrowing, and swimming. The gills in a bivalve are collapsed, thin pieces of tissue suspended in the mantle cavity. These gills aid the bivalve in respiration when water passes through them, and in feeding. 
At rear of a bivalves' body comes two siphons, the incurrent and excurrent siphon, used for the inflow and outflow of water. Every bivalve has bilateral symmetry because they are a class in the phylum mollusca. (Levine and Miller, p 702, 706)
Reproduction occurs externally when packets of sperm released from the gonads (the male reproductive organ) mix with the female eggs. A female bivalve can release millions of eggs throughout her life. . The majority of bivalves are either male or female, but in rare cases some are found to be hermaphroditic(having both male and female reproductive organs).
Once the eggs become fertilized small, little free-swimming larvae are formed and after a few days they attach themselves to a rock or hard surface where they start to form a shell. It has been found that some larvae can stay in the plankton stage for up to 60 days! 
Bivalves can inhabit a variety of places depending on the species. They make their homes in freshwater lakes, streams and rivers, coastal lines, banks of mud or sand, tropical water, and estuaries. Bivalves are indigenous(originated in a certain area) to many different parts of the world. The Northeastern Atlantic Oyster is from the Atlantic waters and the zebra mussel is from Western Asia and Eastern Europe. 
Predators of bivalves include lobster and octopi. When they are being attacked they close up their shells to protect their inside tissues . They also burrow into the mud or sand using their foot. The majority of bivalves are either filter feeders or decomposers(organisms that break down organic substance). Bivalves have gills with mucus which trap plankton and other organisms.
Human Usesclams and mussels do as well. A pearl begins to form when an unknown substance slips between the mantle and the shell. The mantle becomes irritated and as a result begins to cover the surface with nacre, the same substance the shell is made out of. Pearls can be made from either freshwater or saltwater bivalves.  We now use pearls for jewelry for bracelets, necklaces, etc.
Bivalves are also considered a delicacy by many and are a part of our food system. For example,Clams are eaten raw and steamed. Scallops, mussels, and oysters are also consumed.
Scientists are able to check filter feeding bivalves and monitor the quality of the water. This is because certain bivalves concentrate unhealthy pollutants inside of their tissues. They are used as environmental monitors, because they warn of the problems in the water before other organisms. (Levine and Miller, p 708)
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- How do Oysters make pearls? How stuff works.
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