Peritrophic membranes are secretion products of the midgut. For long they have been related only to insects, but they occur widely in the animal kingdom. They affect a part of the gut lumen and function not only as a mechanical protection barrier, but also as an ultrafilter, a barrier to parasites, and as envelopes for fecal pellets. Such peritrophic envelopes are of great ecological importance in marine environments. This is a comprehensive compilation of all data concerning peritrophic membranes which aims to provide a basis for future researches in the field. Topics covered include the occurence, formation and structure, degradation, chemical composition, and functions of peritrophic membranes.