Several questions, not two camps: species, methods and the tardigrade head

The tardigrade head debate is usually summarised as two hypotheses: a single head segment whose whole brain matches the arthropod protocerebrum, or three brain lobes matching the proto-, deuto- and tritocerebrum. Earlier versions of this paper used that summary. It is too simple. At least four published positions answer at least four separate questions (how many segments contributed to the brain; whether the brain or its lobes match arthropod brain regions; whether there is a ganglion below the mouth; and whether the gut's nerve system is tied to a trunk ganglion), and one author's claims can fall on different sides of different questions. We put every claim we could find into a knowledge graph with its source, its locator, its kind of evidence and the species it was actually observed in, and read the positions off the graph. The species behind them barely overlap. The one-segment case rests mainly on Hypsibius exemplaris and Mesobiotus cf. harmsworthi, studied by immunohistochemistry, development and gene expression; the three-lobe case on Halobiotus crispae and Actinarctus doryphorus, and none of its claims in our reading is a direct observation; a third, many-segment but all-protocerebral model rests on electron microscopy of one heterotardigrade, Echiniscus (Viridiscus) viridissimus (Dewel and Dewel 1996; Dewel et al. 1999). Where two positions have studied the same animal, Actinarctus, they disagree (Schulze, Neves, and Schmidt-Rhaesa 2014). We set the positions out question by question, note that the Dewels proposed the trunk-ganglion alignment usually credited to Mayer et al. 2013 some seventeen years earlier, and identify a direct conflict testable in H. exemplaris: whether its gut nerve system supplies the midgut and joins the second trunk ganglion, or supplies only the foregut and joins no trunk ganglion. We also report that a widely cited table mis-transcribes Kristensen (1982): Kristensen puts the protocerebrum in the outer, eye-bearing lobes, and Persson et al.'s comparison table puts it on the inner lobes, against their own text. Dewel, Ruth Ann, Graham E. Budd, Diana F. Castano, and William C. Dewel. 1999. "The Organization of the Subesophageal Nervous System in Tardigrades: Insights into the Evolution of the Arthropod Hypostome and Tritocerebrum." Zoologischer Anzeiger 238: 191–203. Dewel, Ruth Ann, and William C. Dewel. 1996. "The Brain of Echiniscus Viridissimus Peterfi, 1956 (Heterotardigrada): A Key to Understanding the Phylogenetic Position of Tardigrades and the Evolution of the Arthropod Head." Zoological Journal of the Linnean Society 116: 35–49. https://doi.org/10.1111/j.1096-3642.1996.tb02331.x. Schulze, Corinna, Ricardo C. Neves, and Andreas Schmidt-Rhaesa. 2014. "Comparative Immunohistochemical Investigation on the Nervous System of Two Species of Arthrotardigrada (Heterotardigrada, Tardigrada)." Zoologischer Anzeiger - A Journal of Comparative Zoology 253: 225–35. https://doi.org/10.1016/j.jcz.2013.11.001. Version 4 (7 October 2026). Versions 1–3 framed the debate as two camps. Georg Mayer pointed out that this misdescribes it, and sent Dewel & Dewel 1996 and Dewel et al. 1999, which hold a third published position (many segments, all protocerebral). Version 4 retitles the paper, sets four positions against four separate questions, adds the Dewels' species and a gut-nerve conflict testable in Hypsibius exemplaris, notes that the Dewels drew the trunk-ganglion alignment in 1996, and computes every count with a script (positions-v4.json).