This small, bean-shaped animal is called an Ostracod, from the greek word for shell, because of the clam-like valves that cover the body and legs.  While an Ostracod may look superficially like a clam, the presence of articulated legs and other features makes it a crustacean, a relative of barnacles, crabs and shrimp, but placed in it's own separate major group.  

The photograph was taken through a microscope, enlarging the animal about 30 times it's natural size of about 1/8 of an inch (most species are even smaller).  The large clawed protrusion on the left side of the body is a swimming and crawling appendage.  The hairs sticking out the right side of the shell are sensory antenna.  The black smudge in the upper middle of the shell is the lateral eye.  The calcified shell valves are hinged at the outer margin like that of a clam.  

The specific kind of Ostracod shown in the photograph is not known.  Identification of various species is quite difficult requiring considerable expertise and good dissection skills due to the small size of the animal.  

Ostracods are found in both fresh and salt water with around 75% of the species occurring in marine and estuarine waters.  They are mainly benthic or bottom dwellers living in the intertidal and shallow subtidal zones.  They move about the surface of sediments, rocks and algae looking for food.  They eat a variety of things from plants and organic debris to animals, one is even parasitic on a fish.  The bivalve shell while protecting from small predators is no match for larger ones.  Ostracods are eaten by a wide variety of species from fish to benthic invertebrates.  Interestingly some Ostracods can produce bioluminescence (chemically produced flashes of light) to deter predators.