Some ultrastructural observations on the midgut epithelium of Isohypsibius augusti (Murray, 1907) (Eutardigrada).

The midgut epithelial cells of the eutardigrade Isophypsibius augusti are organized into a convoluted monolayer. Only a single cell type could be distinguished although this cell type displayed considerable morphological variation. The midgut begins with crescent-shaped cells. More distally the cells are of variable height depending on the stored amount of nutritional material. No regenerative cells are present. Adjoining cells are held together by apical zonulae continuae. All the cells are characterized by a striated border, some basal infoldings, cytosis vesicles, numerous mitochondria, and abundant rough endoplasmic reticulum. Dictyosomes occur in small numbers. Ovoid or spherical inclusions (spherites), often concentrically laminated, are common. The cells, especially those along the middle part of the gut, are filled with large amounts of nutritional storage which includes polysaccharide material. The possible functions of the midgut in Tardigrada are discussed.