Studies on the intestine of Daphnidae (Crustacea, Cladocera) ultrastructure of the midgut of Daphnia magna and Daphnia obtusa.

The fine structure of midgut cells in two species of Cladocera (Crustacea: Branchiopoda) has unique features when it is compared to that in the midguts of other arthropods. Features which suggest that food is absorbed in digested form are: the presence of the peritrophic membrane, the glycocalyx, the almost complete absence of pinocytotic vesicles, and the large surface area of a great number of microvilli. Digested products presumably pass into the hemolymph through the network of extracellular canals and cisterns surrounding the basal ends of intestinal epithelial cells. Cells of the midgut in Cladocera differ from those of other arthropods in the simplicity of their basal plasma membranes, which are not highly folded. The small number of membrane invaginations suggests that water reabsorption is very slight, as is the usual condition in aquatic animals. The origin and evolution of peculiar structures we call "multivesicular-like bodies" have been investigated. These display a variety of different morphological features. Some contain acid phosphatase activity and are considered as specialized lysosomes.