Terms or phrases in bold in the following text are further explained in the Glossary.

Achaeta totally lack setae and may have one, two or three pairs of epidermal glands in each segment, called epidermal gland sacs; these sacs are lacking in the single marine species, Achaeta littoralis. Spermathecae are in V or extend posteriorly as far as X and the ampulla is without an ental duct or other connection to the esophagus; the brain is convex posteriorly; the sperm sac is small and compact and the penial apparatus includes several small, glands around the male pore. The head pore of species of Achaeta is near the tip of the prostomium.

Species of Enchytraeus possess paired, unbranched post-pharyngeal peptonephridia; setae are straight with 2 or more in a bundle; glands are present at the spermathecal ectal pore; the brain is convex posteriorly and the penial apparatus has either a single compact gland or a longitudinal row of glands along the ventral body wall.

Species of Lumbricillus have a lobed testis sac; setae are usually sigmoid, more than two per bundle; glands are present at the spermathecal ectal pore; and the brain is incised posteriorly.

Species of Marionina may lack setae or have straight or sigmoid setae, which are single, paired or multiple per segment; spermathecae may have glands along the spermathecal ectal duct and/or at the spermathecal ectal pore; many marine species also have sperm rings, sometimes in distinct diverticula of the ampulla; the brain is incised or convex posteriorly; the nephridial canal extends anterior to the septum; and in most species the anterior dorsal blood vessel bifurcation is posterior to the brain with circumpharyngeal vessels looping anteriorly, lateral to the brain (Coates & Ellis 1981).

Stephensoniella species have bundles of only two or three slightly sigmoid setae; single compact penial glands; large compact pharyngeal glands and thickened muscular septa from 5/6 to about 8/9; and each spermatheca has a globose or saccate ampullar diverticulum.

Randidrilus species have single, ventrolateral setae present as far forward as II; one or two pairs of spermathecae; and each penial apparatus with separate, compact anterior and posterior glandular masses at the male pore.

Grania species have stout, single setae; sperm rings in the spermathecal ampulla; elongate posteriorly extending sperm and egg sacs; and a diversity of complex penial apparatuses, sometimes with penial stylets.