Abstract Tardigrades are microinvertebrates, whose body plan consists of a head region followed by four segments, each bearing a pair of lobe-like legs. Most tardigrades lack a posterior extension or tail beyond the last pair of legs. However, studies of the eutardigrade Hypsibius exemplaris Gąsiorek, Stec, Morek however, cad expression appears more anteriorly when the tail disappears at a later stage. A similar pattern was observed with the posterior end of fkh expression. These observations are consistent with the hypothesis that the posterior extension develops into the hindgut. We hypothesize that the anterior migration of this region might be associated with the repositioning of the last pair of legs, potentially contributing to their rotated phenotype. Comparisons with the other panarthropods suggest that their common ancestor had a tail-like posterior extension, and the tail of H. exemplaris embryos might represent a remnant structure of its deep evolutionary history.
Mapalo et al. (Sat,) studied this question.