Their bodies are generally cylindrical with two wider regions separated by a narrower region. There is a large extendible, scoop-shaped proboscis in front of the mouth which gives the animals their common name. This proboscis resembles that of peanut worms but it cannot be retracted into the body. It houses a brain and may be homologous to the prostomium of other annelids. The proboscis has rolled-in margins and a groove on the ventral surface. The distal end is sometimes forked. The proboscis can be very long; in the case of the Japanese species "Ikeda taenioides", the proboscis can be long while the body is only . Even smaller species like "Bonellia" can have a proboscis a metre (yard) long. The proboscis is used primarily for feeding. Respiration takes place through the proboscis and the body wall, with some larger species also using cloacal irrigation. In this process, water is pumped into and out of the rear end of the gut through the anus.