This image shows several anatomical features of a preserved freshwater mussel.  Note the conspicuous fold of tissue called the mantle.  In molluscs,  this structure consists of a sheath of skin that hangs down in two folds around the soft body and encloses a mantle cavity, which performs many of the same functions as a coelom (body cavity) in other animals. The outer side of the mantle secretes the shell (if present), while the inner side is ciliated,  and along with gills or lungs that develop from it,  participates in gas exchange.