HISTOCHEMICAL AND ULTRASTRUCTURAL-STUDY OF THE ALIMENTARY-TRACT OF THE FRESH-WATER SNAIL LYMNAEA-STAGNALIS

The alimentary tract of the freshwater snail Lymnaea stagnalis, except the salivary glands and the digestive gland, was studied with histochemical and ultrastructural methods. The main focus was on the epithelia. Previous studies have indicated that the digestive gland, where intracellular digestion occurs, plays the major role in the digestion process. The main cell types of the tract are ciliated cells and cells with microvilli. The presence of ciliated cells reflects transport of food particles and faeces. In particular areas, viz. the prooesophagus and the pylorus, the epithelium is almost exclusively composed of ciliated cells. Both cell types show signs of endocytosis, lysosomal breakdown of substances, and storage of reserve materials (glycogen, lipid, protein). These features are very prominent in the crop and in the first part of the prointestine and they would indicate that absorption of energy‐rich substances can occur in the whole tract. Ten types of gland cells were distinguished. Their possible functions are discussed on the basis of their histochemical properties and ultrastructure of the secretion products. The observations suggest that some of the cell types produce two different substances, one of which may have enzymatic properties. The epithelium of the gizzard consists of cells possessing stout microvilli, which extend into a special coating produced by these cells. The morphology of the epithelium reflects the function of the gizzard, which is comminution of food particles. In the mid‐ and postintestine occur areas composed of villi cells that possess an extensive basal labyrinth. The morphology of the cells indicates that they may be involved in absorption of water and ions from the faecal pellets. The whole tract is richly innervated. Numerous neurons occur in the connective tissue and muscle layers. Furthermore, in several areas numerous intraepithelial neurons were observed. Copyright © 1990 Wiley‐Liss, Inc.