The two camps on the tardigrade brain have been looking at different animals: a structured reading of the head-segment debate

Whether the tardigrade brain is one segmental unit homologous to the arthropod protocerebrum, or three lobes homologous to the proto-, deuto- and tritocerebrum, has been argued for forty years and is still argued in reviews published this decade. We did not try to settle it. We put every claim we could find on either side into a knowledge graph with its source, its locator, the kind of evidence it rests on and — this is the part that turns out to matter — the species it was actually observed in, and then looked at the shape of the disagreement rather than its content. The two hypotheses are supported by claims about almost disjoint sets of animals. The unipartite case rests on Hypsibius exemplaris (6 of its 10 claims) with Mesobiotus cf. harmsworthi, Macrobiotus hufelandi and two undescribed species, and three of its claims are recorded as direct observation. The tripartite case rests on Halobiotus crispae and Actinarctus doryphorus, and none of the four claims bearing on it is recorded as a direct observation in our reading — they are reports and inferences, largely from comparative neuroanatomy of marine and brackish species that no developmental or molecular study has ever examined. No tardigrade has been put through both camps' methods. That is a testable statement about the literature rather than about the animal, and it suggests the decisive experiment is not another argument but applying one camp's method to the other camp's species. We also report a smaller thing found the same way: a widely cited table mis-transcribes Kristensen (1982). Read at first hand, Kristensen puts the protocerebrum in the outer, eye-bearing lobes, but Persson et al.'s comparison table puts it on the inner lobes, contradicting their own text.