The shells of the members of the Gadilida are usually glassy-smooth in addition to being quite narrow and with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators. The Dentalids, on the other hand, tend to have strongly ribbed and often rather rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze. This makes them harder to detect by animals such as ratfish which can sense the electrical signals given off by the most minute muscle movement.