Description:
Abstract Hemiplectus muscorum, the type and single representative of its genus, is redescribed on the basis of abundant new material collected in the UK, Canada and the USA using both light and scanning electron microscopy. The phylogenetic relationships of the species are inferred from morphological as well as molecular data. Maximum parsimony, neighbour joining and maximum likelihood analyses of small subunit (SSU) rRNA sequences support a position nested among the Plectidae. This conflicts with our morphological assumptions of character polarity, as it implies that the absence of a valvate bulb in Hemiplectus is a reversal rather than a plesiomorphy. The excretory system of Hemiplectus is described more precisely. Its structure is highly reminiscent of the system in Plectus but differs in the presence of an anterior and posterior pair of pseudocoelomocytes flanking the renette cell. A pair of lateral somatic setae is identified as possible homologues of the 'deirids' in Plectus and Rhabditida. Measurements and descriptions are given of all four juvenile stages.