The ‘shipworm’, is a mollusc that looks like a worm. At microscopic size it penetrates underwater timber and there spends the rest of its life, eating your boat away. Remaining within the thickness of the planking it can grow to a foot or more in length and as fat as your finger. The galleries made by teredo will destroy the structure of a wooden ship. But they do not abound in temperate waters. They can be held at bay by a sound coat of Anti-fouling paint, or by copper sheathing, and they die if the boat is removed from salt water for a fortnight or more.