Although the walls of most serpulid tubes are homogeneous, tubes of certain species may contain up to four ultrastructurally distinct layers. Some of these layers are made of densely packed large crystals and others are composed of sparsely packed fine crystals. In almost all (16 of 17) examined species having layered tubes, the dense layer is located in the outer wall part and the layer(s) composed of fine and relatively sparsely packed crystals are positioned in the inner wall part. Two species have transparent tube walls made entirely of densely packed crystals. Fossil serpulid tubes with dense outer layers (DOL) are known from the Late Cretaceous (Pentaditrupa subtorquata) and the Eocene (Pyrgopolon cf. mellevillei and Rotularia spirulaea). DOL gives a characteristic smooth shiny appearance to the tube surface and presumably evolved as an adaptation against drilling predation by gastropods and to delay shell dissolution in the waters of the deep-sea under-saturated with calcium carbonate.