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 effect 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. The book is divided into the following themes: - Occurrence - Formation and Structure - Degradation - Chemical Composition - Functions. It is the first comprehensive compilation of all data concerning peritrophic membranes and will provide a basis