Most are active benthic creatures, moving over the surface or burrowing in sediments, or living in cracks and crevices in bedrock.

[2] Phyllodocida are segmented worms and range in size from a few millimetres long to over a metre.

The next few segments tend to differ from those further back in having enlarged dorsal and ventral cirri (fine appendages) and reduced parapodial lobes and chaetae (bristles).

Some species have appendages with specialised functions but most have many segments that are similar to each other but which vary in size and shape along the length of the body without abrupt changes in the chaetae and parapodia from one to the next.

There is agreement on the monophyly of a group including the Hesionidae, Nereididae and Chrysopetalidae families but opinions differ about Pilargidae, and molecular and morphological studies continue.