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Enigmatic Gnathostomulida (Gnathifera, Spiralia): about monociliated pharyngeal receptors and the pharyngeal nervous system

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Abstract

It was lately suggested that cilia inside the pharynx of Gnathostomulida are a novel finding and that Herlyn and Ehlers (Zoomorphology 117:135–145, 1997) did not observe anything corresponding to a buccal ganglion in the pharynx of Gnathostomula paradoxa (Gnathostomulida). However, monociliated pharyngeal receptors were repeatedly reported for Gnathostomulida since the 1980th, and every statement of absent ciliation in the cited study referred to the pharyngeal epithelium excluding the imbedded receptors. Moreover, Herlyn and Ehlers stated that they did not regard the questionable nervous differentiation in the hind pharynx of Gnathostomulida as a buccal ganglion due to lacking evidence of an extra basal lamina sheath. The present study shows that it is also uncertain if the structure fulfills another frequently applied criterion of a ganglion, i.e., an arrangement of perikarya and neuropil in such a manner that the first enclose the latter. Notwithstanding the alternative definitions used, the transmission electron micrographs presented herein also confirm previous findings. Thus, the nervous differentiation in the posterior pharynx (buccal ganglion) gives rise to one weaker dorso-median nerve and two stronger lateral nerves which connect with the distal ciliated portions of the pharyngeal receptors. The single cilium of each receptor extends from the bottom of a pit into the pharyngeal cavity, whereby its proximal section is surrounded by eight microvilli. A survey on the considerable discrepancies between the descriptions of buccal/mastax/pharyngeal ganglia in Gnathostomulida, Micrognathozoa, Monogononta, Bdelloidea, Seisonidea, and Acanthocephala (present/absent, inside/outside the pharynx, ventrally/dorsally in the pharynx, ganglion/plexus, etc.) concludes this study.

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Fig. 1
Fig. 2
Fig. 3
Fig. 4

All transmission electron micrographs modified after Lammert (1986, his Figs. 22A, 22B, 49D, 54A, 63A, 61A, 48C)

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Notes

  1. To the best of my knowledge, Sterrer et al. (1985) referred to Lammert (1981).

  2. According to the common definition (e.g., Lammert 1986), a myoepitheliocyte contains myofilaments and resides distally of the basal lamina.

  3. A single neurite crossing an agglomeration of perikarya in Fig. 9 of Kristensen and Nørrevang (1977) is not to be concealed; but a neuropil is actually defined as a cluster of neurites (see Richter et al. 2010).

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Acknowledgements

The beginning of the title is borrowed from Sterrer et al. (1985), and picks up the title of Ax’ (1956) seminal paper “Die Gnasthostomulida, eine rätselhafte Wurmgruppe aus dem Meeressand.” Lammert could not be asked for permission of using his TEM images in present Fig. 4 as he passed away. I would like to thank Bernd Baumgart, University of Göttingen, Germany, whose archival inclination has proved to be highly advantageous for the present study. My thanks go further to two reviewers, whose comments improved the manuscript.

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  1. Institut für Organismische und Molekulare Evolutionsbiologie, Johannes Gutenberg University Mainz, 55128, Mainz, Germany

    Holger Herlyn

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Correspondence to Holger Herlyn.

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Herlyn, H. Enigmatic Gnathostomulida (Gnathifera, Spiralia): about monociliated pharyngeal receptors and the pharyngeal nervous system. Zoomorphology 136, 425–434 (2017). https://doi.org/10.1007/s00435-017-0369-4

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