New morphological structures of Priapulus caudatus, Lamarck 1816 (Priapulida) and analysis of homologous characters across macroscopic priapulids

The macroscopic Priapulus caudatus Lamarck, 1816 is one of the most investigated priapulids, yet, the adult morphology warrants detailed characterization and analyses of potential priapulid homologies. Using light microscopy, scanning electron microscopy and histology, we examine adult specimens of P. caudatus from three localities in the Northeast Atlantic and the Arctic Oceans (Gullmarsfjord, North Sea, White Sea). Our examinations show ring papillae on the posterior trunk of P. caudatus for the first time in detail and describe previously overlooked structures on the circumoral field (hemispherical papillae, ring 0 scalids and circular folds). In addition, we add new information on pharyngeal teeth, scalids, trunk papillae, posterior warts, and on spinulets of the caudal appendage. The new collected data of P. caudatus allows us to compare structures across species of the macroscopic Priapulidae (Acanthopriapulus, Priapulopsis, Priapulus, Halicryptus) in detail and to build hypotheses on their homology within this clade.