Phascolosoma nigrescens is primarily distinguished from other Phascolosomatidae, and Sipunculids by distinctive introvert hooks. These are usually 25-90 um tall, and organised in over 50 incomplete rings (Edmonds, 1955; Cutler, 1994). Its introvert hooks are distinctive in that they lack, or maintain a reduced secondary tooth (Figure 2).  P. nigrescens trunks are often 1.5-2.6 cm in length, and the introvert usually extends longer than the trunk length, which is often 0.1-0.2 mm in diamater. P. nigrescens is light brown, and has uniform dome-shaped papillae aggregating more towards the trunk and anal shield. The introvert may have distinctive black or dark brown pigment bands on the dorsal surface. The tentacular crown is not as pronounced as in filter feeding relatives, which is representative of all Phascolosomatidae (Schulze et al., 2005). 28 flattened white tentacles are arranged concentrically around the nuchal organ, which sits at the apical end of the introvert; the mouth is opposite the nuchal organ, and the anus is at the base of the introvert (Figure 3) (Cutler, 1994; Stephen & Edmonds, 1982). Figure 4 shows a full drawing of P. nigrescens.