Electron microscope studies of the digestive tract ofBrachionus plicatilis (Rotifera)

Abstract

The rotiferBrachionus plicatilis was immobilized in an extended state by treatment with acrylamide. Longitudinal sections through whole animals were studied by transmission electron microscopy, and in addition animals were cut in different planes and studied by scanning electron microscopy. By these techniques several structures were characterized which seem to be relevant to the processes of enzymatic digestion and resorption in this species. The inner surfaces of both the “stomach” and the “intestine” are provided with rootless cilia, which show different densities, alignments, structures and properties in different compartments. The syncytial wall of the anterior part of the stomach is flat; its plasma membrane toward the stomach lumen is smooth and bears relatively few cilia. The posterior part of the stomach consists of voluminous cells. Their plasma membranes lining the stomach cavity show pits and infoldings that are continuous with a system of vacuoles within the cells containing membrane material probably derived from food organisms. The density of cilia in the posterior part of the stomach is about three times that in the anterior part. The “stomach glands” contain ribosomes, glycogen granules and secretory bodies, which are accumulated toward the region where these glands drain into the stomach cavity. The orifice of the stomach glands occurs in a permanent intercellular gap between the syncytial anterior stomach and three cells of the posterior part of the stomach. The cells of the intestinum are relatively flat and are characterized by mitochondria, narrow intercellular spaces between shingle-like overlapping wall cells and cilia differing in structure and properties from those of the stomach. The observations confirm the existence of functional differences between stomach and intestine. Digestion of nutrients, supported by secretions of the stomach glands, seems to be performed mainly within the stomach, and resorption of particulate food seems to occur almost exclusively in this part of the alimentary tract. The structural features of the intestine, on the other hand, indicate that small molecules may be transported through its wall and that here, perhaps, processes may take place which demand active transport.