Size range 1.0 mm to 50 ft. Mantle also secretes calcareous shell with organic matrix.
Exhalent currents remove waste products.
Gills - cilia on gills create water movement. In class Pelecypoda, gills also used as feeding structure. Digestive tract - Mouth, jaws, radula, stomach. Pelecypods lack jaws and radula. Under X nicols, small xls seen.