Bivalve Respiration
Bivalves are mollusks that are composed of a shell divided from front to back into left and right valves. This shell comes from the mantle, which surrounds the foot. Between the body of the bivalve and the mantle lie the ctenidia, the type of gills present in mollusks. The ctenidia are split into two branches, inner and outer, and these branches have their own sets of lamellae, which are thin, plate-like structures that allow for the bivalves to filter out as much oxygen as possible from the surrounding water (1).
|
Pictured is the inside of a clam. Some of the parts labeled are the mantle, where the gill is located, and the gill and the siphons, which work together to filter out oxygen from the water.
|
Evolution
Bivalves are closely related to their fellow mollusks, the gastropods. These two groups share many similarities. They both have an outer shell protecting their visceral mass and they both diffuse oxygen out of their environment into their bodies, in which they both have open circulatory systems. One gastropod, the snail, has another similarity with a bivalve, the clam; they both use their muscular foot to move around. The adaptation from a marine environment to a terrestrial environment for gastropods must have been recent relative to most evolutionary adaptations because this is one of the only differences between these two classes.
(1) Morton, Brian. "Bivalve." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 15 June 2016.