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.).
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.
- Excretory organs which are undisputed nephridia are practically universal among the Oligochaeta, Hirudinea and Archiannelida, and occur in many Polychaeta.
HAPLODRILI (so called by Lankester), often called Archiannelida (Hatschek), the name provisionally given to a number of FIG.