Acorn worms are worm-like invertebrates that live alone. They mostly reside in burrows and are deposit feeders, however some species are pharyngeal filter feeders, and the Torquaratoridae family are free-living detritivores. Many are well known for their synthesis and accumulation of different halogenated phenols and pyrroles. Pterobranchs are filter feeders that reside in a coenecium, which is a collagenous tube structure. Acorn worms are detritus feeders that eat sand or mud to obtain organic debris. Others feed on organic material suspended in water that they may drag into their mouth with the cilia on their gill bars. Acorn worms have a worm-like body with an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a ciliated and muscular organ that aids in movement as well as the collecting and transport of food particles. The mouth is located between the proboscis and the collar, while the trunk is the animal's longest component. It has a perforated mouth with gill slits, an oesophagus, a lengthy intestine, and a terminal anus. It also houses the gonads. A young member of the acorn worm family Harrimaniidae has a post-anal tail. Acorn worms breathe by sucking oxygenated water into their mouths. The water subsequently comes out of the animal's trunk-mounted gills. As a result, the acorn worm breathes similarly to fish.