Hodgson, A.N. and Jamieson, B.G.M. 1992d. Spermatogenesis in the earthworm Microchaetus pentheri (Oligochaeta, Microchaetidae). Zoomorphology 112: 57-66.

Spermatogenesis in Microchaetus pentheri (Microchaetidae) follows the familiar pattern known for other oligochaetes. Spermatogenic stages   develop around an anucleate cytophore from which they separate as mature   spermatozoa. During spermiogenesis the nucleus elongates and becomes   surmounted by a complex, elongate acrosome: the flagellar axoneme develops   from the distal centriole. The centriole is positioned posterior to the   mid-piece, which consists of six mitochondria radially adpressed to form a   cylinder about 2 um long. Microchaetus shows many plesiomorphic features   in the structure of its acrosome, which are also seen in two other taxa of   the Diplotesticulata, Haplotaxis (Haplotaxidae) and Sparganophilus   (Sparganophilidae, Aquamegadrili), each of which has the greater number   of plesiomorphies in spermatozal characters in its group. The   Aquamegadrili constitute the sister-group of the Terrimegadrili which   contain the earthworm families including the Microchaetidae. The numerous   symplesiomorphies in spermatozoal characters do not, however, establish   monophyly of microchaetids with haplotaxids and sparganophilipids. An   apomorphy in the acrosome of Microchaetus is its greater length (3.8 um  vs less than 1 um in Haplotaxis and 1.5 um in Sparganophilus), in   their respect resembling other investigated terrimegadriles, the   lumbricids, hormogastrids and megascolecids. The axial rod of the acrosome   of Microchaetus appears apomorphic relative to that of Haplotaxis,   Sparganophilus, lumbricids and megascolecids in lacking an anterior   expansion, the capitulum. It ends positively in a cylindrical body,   somewhat resembling the note diagnostic of the axial rod of megascolecid   earthworms.