Bivalves, meaning "hatchet-foot", is also called Pelecypoda that includes animals such as clams, mussels, scallops, and oysters. Approximately 8,000 living species of bivalves live mostly in marine; however, many of them live in fresh water as well. They are found in almost every marine environment, from the intertidal zone to the deepest marine habitats.
Feeding:
Bivalves are filter-feeder that use their gills to extract organic particles from the water; thus, there is no need for a muscular head-foot or radula for locomotion and grinding respectively. Cilia, covering the gills and labial palps, filter food from the water and intake the food particles toward the mouth. Particles that are too large are flushed along with the water forced out by the closing of the valves.
Respiration:
Most bivalves have a single pair of large gills used for respiration. Enlarged gills form lamellae or folds with cilia to increase the surface area for gas exchange between the water and the blood vessels. The cilia create currents over the gills moving water through the incurrent opening into the mantle cavity. Water is also move through the siphon to the gills before the exchange, and finally exiting through suprabranchial chamber and excurrent opening.
Digestion:
Food particles are carried to the esophagus and down into the stomach after entering the mouth. Cilia and a mucoid food string move the food against the gastric shield, which is an abrasive structure. As the food is being ground up, enzymes are released. The stomach further breaks down the food. The indigestible particles are taken to the intestine and excreted through anus. Partially digested food goes to digestive gland for more intense digestion.
Circulation/Internal Transport:
They have an open circulatory system which means that blood is pumped by a heart, but not all blood are contained in blood vessels. Oxygen and nutrients are carried by the blood to all parts of the body.
Excretion:
The solid wastes are removed as feces through the anus.The metabollic waste materials are removed by nephridia. Nephridia are invertebrate organs which have function similar to kidneys.
Reproduction:
Most of pelecypods are dioecioius, which means that individuals are either male or female. Most species simply just shed their eggs and sperms directly into the sea all at the same time. A free swimming larvae is developed after the sperm and egg fertilized.
Adaptation/Response to Environment:
Bivalves have two shells that are hinged together at the back and held together by one or two powerful muscles. They also secretes a layer called "Mother of Pearl". Most bivalves are sessile; however, some, such as scallops, can move around rapidly by flapping their shells when threatened. Some of them will close up their bivalves and dig into mud. Mussels will secrete sticky threads to attach themselves to rocks. They have simply nervous system since they have inactive lives burrowing in mud or sand.
Movement:
Bivalves have a highly muscular organ called the foot, which muscle fibres run in all direction. They use it for digging and anchoring. This is an advantage because it helps to anchor the organism to the ground. Its disadvantage is that it's hard to move freely and quickly.
Internal Structure of a Clam
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