The polydontids are a rare group of large tubiculous worms living in sand or mud. The tube is composed of long silky threads secreted by the spinning glands which is tough and extend a metre or more below the mud. The are either scavengers or omnivores. They extend their anterior flattened part of the body well out of the tube in their search for prey. They are large solid-bodied scale-worms covered with thick loosely constructed tubes consisting of tubes of thin threads filled with clay or sand particles. One polydontid (Polydontes lupinus) is known to harbour gastropod (Cochliolepis parasitica) which lives under the elytra of the worm or within its tube. The gastropod is reported to perform a cleaning function. They are renamed the Acoetidae