The praesoma of the acanthocephalan parasite Paratenuisentis ambiguus was studied at the light and the electron microscope level, with special reference to the lateral sense organs and the musculature, in order to substantiate the basal pattern of the Acanthocephala and to analyse the phylogeny of the taxon. The study includes the first ultrastructural description of a lateral sense organ in the Acanthocephala. Two sensory support cell ducts extend from the binucleate pericaryon of the sensory support cell to the lateral sense organs. On their way to the lateral sense organs the ducts penetrate the receptacle and join the anterior ventral nerves. Each lateral sense organ consists of a conical termination of one of the sensory support cell ducts, in which the neuronal fibres and dendritic terminations of the equilateral anterior ventral nerve are embedded. An analysis of the available data of praesomal sense organs in Acanthocephala suggests that lateral and apical sense organs are absent in the basal pattern of the Acanthocephala. It is likely that two lateral sense organs, a binucleate sensory support cell with two ducts and two anterior ventral nerves evolved within the stem-line of some Palaeacanthocephala, all Eoacanthocephala and all Archiacanthocephala, whereas two apical sense organs, a quadrinucleate sensory support cell with four ducts and two apical sensory nerves presumably represent an autapomorphic character of the Archiacanthocephala. Furthermore, it can be derived from data in the literature and the present study that the praesomal hooks are totally covered by epidermis in the basal pattern of the Acanthocephala, whereas the ontogenetic loss of the epidermal covering can be regarded as an autapomorphy of the Archiacanthocephala.