The best studied cellulolytic environment is the rumenwhich is essentially a plant cell wall degrading chemostatwhere the animal pretreats biomass by grinding it intosmall particles which then are digested by a very dense andcomplex mixture of anaerobic microorganisms. There areabout bacteria per ml in the rumen but only about 10%of them are cellulolytic even though cellulose is themajor carbon source available to rumen microorganisms.There are also some cellulolytic rumen fungi and protozoabut it is thought that bacteria are the major cellulosedegraders. The microorganisms in the rumen that arebound to feed particles are quite different from those thatare not attached and most cellulolytic microorganisms areattached to the particles However many attachedmicroorganisms do not degrade cellulose. It is interestingthat a metagenomic sequence of a leaf cutter ant colonygarden contained many bacteria whose genes coding forcarbohydrate degrading enzymes were most similar tothose in the bovine rumen although there were cleardifferences between the two sets of enzymes