Here a sac-like eversible proboscis is reliant on fluid pressure from the coelom for eversion. There is no particular development of musculature or gland. The worms tend to have reduced septa in the anterior part of the body. This allows the contraction of the posterior part of the body to exert considerable force on the buccal apparatus because of the free movement of the coelomic contents. Retraction of the proboscis is performed by muscles may be associated with a thickened first septum (gular membrane) and connected to the proboscis. There are two main feeding types among taxa with this form of proboscis: non-selective and selective deposit-feeders.

Scolecida

Non-selective deposit-feeders using a simple axial proboscis lie in Scolecida of Rouse and Fauchald (1997), including the families Arenicolidae, Capitellidae, Maldanidae, Opheliidae, and Scalibregmatidae, which, in addition to these taxa, contains, Cossuridae, Orbiniidae, Paraonidae and Questidae. These latter four taxa are also deposit feeders but have a different, usually ventral, buccal organ. A commonly cited polychaete with a simple axial proboscis is Arenicola marina the well-studied British lugworm. Arenicolidae are not found in NSW or Queensland though an Abarenicola is found in Victorian mudflats. Here the proboscis is of importance in burrowing and though papillate is unarmed. As usual in polychaetes, the proboscis is housed in an anterior asepate chamber but here the most anterior septum is especially important because in the Arenicolidae, at least, it contributes the retractor muscle. It is protruded by coelomic pressure. Other non-selective deposit feeders with a simple axial proboscis include Armandia that belongs in Opheliidae. These Opheliidae are found in clean well-sorted sand through which they can virtually 'swim'.

Spionida

Members of Spionida with a simple axial proboscis include Spionidae, Apistobranchidae, and Trochochaetidae. Feeding in Spionidae appears to involve a reduction of the proboscis and elaboration of feeding tentacles.