Cellulose is used as a food source by many organisms. Plant fibers are major components of ruminant diets, and the anaerobic microorganisms, including bacteria, fungi and protozoa, inhabiting the rumen ecosystem play a vital role in their digestion. Thus cattle and sheep have symbiotic microorganisms in their rumen that produce various cellulases and hemicellulases. Cellulolytic enzymes have been isolated from rumen fungi and some of them are well characterized. Previous dogma has maintained that cellulose, ingested by xylophagous or herbivorous animals, is digested by cellulolytic symbiotes. The first evidence in conflict with this contention involved the demonstration of cellulolytic activities in symbiote-free secreting organs (e.g., the salivary glands of termites) or defaunated guts. It is now clear that cellulases are produced endogenously in a number of invertebrate taxa that includes insects, crustaceans, annelids, molluscs and nematodes, as well as by micro-organisms such as bacteria, fungi and protozoans . The ability of animals to hydrolyze cellulose in the absence of microorganisms has been the subject of various studies.