Immunohistochemical (cLSM) and ultrastructural analysis of the central nervous system and sense organs in Aeolosoma hemprichi (Annelida, Aeolosomatidae)
The Aeolosomatidae are very small limnetic or terrestrial annelids of apparently simple organisation and uncertain phylogenetic position. They have been placed either at the base of the Clitellata, as a highly derived taxon within the Clitellata closely related to the Naididae, or as their sister group within the „Polychaeta”. A combined immunohistochemical (cLSM) and ultrastructural investigation of the central nervous system and the sense organs in Aeolosoma hemprichi was undertaken to look for characters which might support one of these theories. The position of the brain within the prostomium and the organisation of the ventral nerve cord, with its intraepithelial paired longitudinal nerves lying far apart from each other and the presence of a median longitudinal nerve, are atypical for the Clitellata and clearly differ from the situation found in Naididae. Moreover, the circumoesophageal connectives are bifurcated and enter the brain as dorsal and ventral roots; this arrangement is unknown in Clitellata, in which these connectives are unbranched. An ultrastructural analysis of the ciliated pits located laterally in the furrow between prostomium and peristomium in A. hemprichi and other Aeolosomatidae show that they are in fact nuchal organs. Such presumed chemosensory organs are typical of „Polychaeta” and absent in all Clitellata. Two pairs of ciliary sense organs are present in the prostomium in front of the brain of A. hemprichi. Although similarly organised sensory structures occur in many species of the Clitellata, they differ in position and certain ultrastructural features and are known from other Annelida as well. These results clearly support the exclusion of the Aeolosomatidae from the Clitellata and do not provide any evidence for a sister-group relationship between these two taxa.