ON THE NATURE OF PHARYNGEAL MUSCLE-CELLS IN THE TARDIGRADA

As part of a transmission electron microscopic study of the embryologi cal development in tardigrades, the ultrastructure of the pharynx was examined. The intent was to establish whether the pharyngeal muscle ce lls constitute an ectodermal myoepithelium (as in many aschelminth pha rynges) or whether they are mesodermal (as in certain other aschelmint h groups and among articulates). In the latter case the cuticle would be produced solely by specialized epithelial cells. The eutardigrade s pecies Halobiotus crispae Kristensen, 1982, was investigated in four e mbryological stages, as a newly hatched juvenile, in the active adult stage, and in the hibernation stage pseudosimplex 1. A comparison was made with the arthrotardigrade Actinarctus doryphorus Schulz, 1935, in the active adult stage and in the simplex stage. The results indicate that the tardigrade pharynx is an ectodermal myoepithelium. The muscl es appear to be truly cross-striated and monosarcomerial. The phylogen etic implications of these findings are discussed briefly. (C) 1996 Th e Linnean Society of London