Haplotaxidae Michaelsen, 1900 s.s. (clade 1 in Fig. 1)

Type genus: Haplotaxis Hoffmeister, 1843

(with type species Lumbricus gordioides Hartmann in Oken, 1819);

Phreoryctes Hoffmeister, 1845 is a junior synonym to Haplotaxis.

Diagnosis [modified from the Haplotaxis description in Brinkhurst (1988)]

Body form narrow and elongate, body wall with well-developed cuticular layer, brain anterior to mouth within elongate peristomium, the latter usually biannulate with a distinct transverse groove, although not in all species (Brinkhurst and Marchese 1987, Brinkhurst and McKey-Fender 1991) (Figs 2A–C, 3C). Ventral chaetae large, single, usually straight in basal part and curved to strongly hooked distally (Fig. 3I–J); simple-pointed, but may have distal ornamentation or keel; modified genital chaetae in at least two species. Dorsal chaetae smaller than ventrals, single, sigmoid to nearly straight; may be absent in many or all segments. Foregut without a dorsal, pharyngeal pad; mouth opens into a capacious, thin-walled oral cavity (possibly eversible) (Fig. 3B–C); mouth and oral cavity with strongly developed radial muscle bands extending to the body wall, interspersed with diagonal muscle bands (Figs 2A–C, 3A); conspicuous pharyngeal glands absent. Oral cavity terminates in a thick-walled, muscular pharynx (or ‘gizzard’, a disputed term here—see ‘Gizzard vs. muscular pharynx in Haplotaxis ’ in the discussion), consisting of transverse-circular muscle bands, interspersed with radial muscle, usually in IV– V (Fig. 3B–D). Nephridia present in all middle and posterior segments; ducts with large, irregular cells, forming a compact mass filling much of the segment (Fig. 3G). Conspicuous coelomocytes absent. Dorsal and ventral blood vessels connected by simple commissural vessels, often in each segment; those in anterior segments long and convoluted. Small clusters of glandular cells (‘Timm’s glands’) may be present midventrally in some segments, approaching or surrounding the ventral nerve cord (Fig. 3H). Clitellum one cell thick. Spermathecae in 2–4 segments, in front of gonadal segments, typically petiolate, with pores anterior and lateral or dorsolateral. Usually with two pairs of testes (typically in X– XI), followed by one or two pairs of ovaries with the second pair rather than the first being lost where reduction has occurred; eggs mesolecithal. Gonoducts simple, without attached prostate glands, but there may be glands around the pores; male ducts tubular, plesioporous.