FINE-STRUCTURE OF THE MIDGUT AND THE HINDGUT IN ECHINISCUS-TESTUDO DOYERE (HETEROTARDIGRADA)

The fine structure of the midgut and the hindgut of Echiniscus testudo (Heterotardigrada) was examined in non-moulting specimens shortly aft er reviving from anhydrobiosis. Immediately in front of the anterior e nd of the midgut the triradiate esophagus is surrounded by a muscle. T he midgut itself consists of a relative flat epithelium without microv illi. Only a single cell type can be distinguished which shows, howeve r, some morphological variation. Regenerative cells seem to be absent. The cytoplasm of the cell contains mitochondria, rough ER, dictyosome s, lipid droplets, and numerous spherites. Spherites seem to be discha rged into the lumen of the gut. Thus, as in other invertebrates, the m idgut is involved in excretion. In all specimens investigated the shor t hindgut (rectum) consists of a solid cell cord lacking a real lumen. Cells do not show features of transporting cells such as ''rectal pad s'' as found in the hindgut of Eutardigrada. Furthermore, the anus is closed by a modified folded cuticle. Obviously, rectum and anus are op en only when defecation takes place, which occurs usually in Heterotar digrada during the moult.