From the researches of Van Sennis, in 1890, it seems pretty certain that the fermentation of cellulose is due to the symbiotic action of at least two different organisms The decomposition of the cellulose may be explained by considering that first a sugar-like carbohydrate is formed by hydrolysis, and that this is then split up into equal volumes of CO_{2} and CH_{4}. It may be noted that the fermentation is anaerobic, and no doubt, so far as bating is concerned, the chief products are the organic acids produced, principally butyric and acetic acids. Van Sennis nearly always found Clostridium butyricum associated with this fermentation.[86] [86] Proc. Roy. Soc., lxvii., 1900. Another group of organisms which have some influence in the bating process, are the class called by Beijerinck, Granulobacter. They produce butyric acid, and this acid, combining with the ammonia compounds of the dung, forms salts which undoubtedly exert an effect on the lime in the skins, though its action on the fibre is, perhaps, not so great as the compounds of lactic and propionic acids. The most common butyric ferment is the old Clostridium butyricum, now known as B. butyricus, (Prazmowsky), which is anaerobic. It forms spindle-shaped spores, hence the name Clostridium (from _κλωστηρ_κλωστηρ, a spindle). Another species (Fig. 23), found in milk by Hueppe (1884), is aerobic, and ferments lactic acid and its salts to butyric acid, CO_{2}, and hydrogen; it appears to correspond with Granulobacter polymyxa of Beijerinck. Oxalic acid is known to be produced by some bacteria and the moulds Penicillium and Sclerotinia, and in the white rot of the turnip it is produced by Pseudomonas; it is also produced by some saccharomycetes, such as B. Hansenii.[87] There is reason to believe that its production plays a part in the bate, as we have already mentioned in Chapter II., but the organisms producing it and their mode of action still remain to be investigated. [87] Reynolds Green, Fermentation, p. 350. There are, of course, a large number of putrefactive bacteria in the puer, among these B. putrificus (Fig. 19), isolated by Bienstock; it is a spore-bearing anaerobic bacillus, and is interesting as specially attacking fibrin. Now fibrin is extremely resistant to the action of most putrefactive bacteria, and it is very probable that specific organisms ferment the different albuminous compounds, in the same way that the different carbohydrates are each decomposed by specific ferments. Very interesting are the various forms of spirilla met with in dung; Figs. 24 and 25 show Spirillum volutans in the unstained condition, and also stained to show the flagellæ. It will be noted that the appearance is so different that, to an inexperienced observer, they might be taken for different species. The rôle played by these organisms still requires investigation. [Illustration: Fig. 23.--B. Butyricus. (Hueppe.)]