They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits.
These chitinous, rod-like, rarely squat and then hook-like structures are found in the majority of the Chaetopoda, being absent only in certain Archiannelida, most leeches, and a very few Oligochaeta.
Among the Archiannelida, in Aeolosoma and some Polychaetes, the whole central nervous system remains imbedded in the epidermis.
They are divisible into the Haplodrili or Archiannelida, the Polychaeta containing the marine worms, the Oligochaeta or terrestrial and fresh-water annelids (see Earthworm), the Hirudinea or leeches (see Leech), and a small group of parasitic worms, the 11-Tyzostomida (q.v.).
There is a definite Annelid group (see Annelida), including the Archiannelida, the bristleworms (see Chaetopoda), of which the earthworm (q.v.) is the most familiar type, the Myzostomida, Hirudinea (see Leech) and the armed Gephyreans (see Echiuroidea).