The sea-water with which a shipworm inflates its mantle cavity is expelled posteriorly through its dorsal siphon or released anteriorly into its tunnel through the modified pedal opening. Radiographs of shipworms tunnelling through air-filled softwood confirm the presence of water in the blind ends of tunnels. Retracted shipworms can extend themselves and resume tunnelling where they left off, but they must get rid of tunnel water first. They do this by forcing it into the grain at the blind end. It is suggested that tunnelling is continually interrupted by small shrinking movements which ensure that the valves are lubricated. The tendency for shipworms to tunnel with the grain in softwood blocks is accounted for in terms of their need of lubrication. Tunnelling at an angle to the grain and turning round before the solid boundary is reached are accounted for in terms of a reaction to decreased hardness.