Organisation of the praesoma of Paratenuisentis ambiguus (Van Cleave, 1921) (Acanthocephala : Eoacanthocephala), with special reference to the lateral sense organs and musculature

The praesoma of the acanthocephalan parasite Paratenuisentis ambiguus was s tudied at the light and the electron microscope level, with special referen ce to the lateral sense organs and the musculature, in order to substantiat e the basal pattern of the Acanthocephala and to analyse the phylogeny of t he taxon. The study includes the first ultrastructural description of a lat eral sense organ in the Acanthocephala. Two sensory support cell ducts exte nd from the binucleate pericaryon of the sensory support cell to the latera l sense organs. On their way to the lateral sense organs the ducts penetrat e the receptacle and join the anterior ventral nerves. Each lateral sense o rgan consists of a conical termination of one of the sensory support cell d ucts, in which the neuronal fibres and dendritic terminations of the equila teral anterior ventral nerve are embedded. An analysis of the available dat a of praesomal sense organs in Acanthocephala suggests that lateral and api cal 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 Archiacanthoceph ala, whereas two apical sense organs, a quadrinucleate sensory support cell with four ducts and two apical sensory nerves presumably represent an auta pomorphic character of the Archiacanthocephala. Furthermore, it can be deri ved from data in the literature and the present study that the praesomal ho oks are totally covered by epidermis in the basal pattern of the Acanthocep hala, whereas the ontogenetic loss of the epidermal covering can be regarde d as an autapomorphy of the Archiacanthocephala.