Byssus

A byssus /ˈbɪsəs/ is a bundle of filaments secreted by many species of bivalve mollusc, that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including the pen shells, the true mussels and the false mussels: the Pinnidae, the Mytilidae and the Dreissenidae.