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.).
On the hypothesis that such a form as Dinophilus (see Haplodrili) has preserved the characters of the primitive Chaetopod more nearly than a any existing Polychaet or Oligochaet, it is clear that the nephridia in the Oligochaeta have preserved the original features of those organs more nearly than most Polychaeta.
These structures appear to be absolutely distinctive of the Oligochaeta, unless the sacs which contain sperm and open in common with the nephridia of Saccocirrus (see Haplodrili) are similar.
They are divided as follows: (i) Haplodrili or Archiannelida; (2) Chaetopoda; (3) Myzostomida, probably degenerate Polychaeta; (4) Hirudinea (see CHAETOPODA and LEECH); (5) Echiuroidea.
HAPLODRILI (so called by Lankester), often called Archiannelida (Hatschek), the name provisionally given to a number of FIG.
There appears to be little either in the development or in the structure of the Haplodrili to warrant the view held by Hatschek and Fraipont that Polygordius and Protodrilus are exceedingly primitive forms, ancestral to the whole group of seta-bearing Annelids (Oligochaeta, Polychaeta, Hirudinea and Echiuroidea).