We have suggested[19] that one function of the contractile vacuole, in naked fresh-water Protists, is to afford a regular means of discharge of the water constantly taken up by the crystalloids in the protoplasm, and so to check the tendency to form irregular disruptive vacuoles and death by diffluence. This is supported by the fact that in the holophytic fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole is present in the young naked stage (zoospore), but disappears as soon as an elastic cell-wall is formed to counterbalance by its tension the internal osmotic pressure. DIGESTION is always essentially a catabolic process, both as regards the substance digested and the formation of the digesting substance by the protoplasm. The digesting substance is termed a "zymase" or "chemical ferment," and is conjectured to be produced by the partial breakdown of the protoplasm. In presence of suitable zymases, many substances are resolved into two or more new substances, often taking up the elements of water at the same time, and are said to be "dissociated" or "hydrolysed" as the case may be. Thus proteid substances are converted into the very soluble substances, "proteoses" and "peptones," often with the concurrent or ultimate formation of such relatively simple bodies as leucin, tyrosin, and other amines, etc. Starch and glycogen are converted into dextrins and sugars; fats are converted into fatty acids and glycerin. It is these products of digestion, and not the actual food-materials (save certain very simple sugars), that are really taken up by the protoplasm, {16}whether for assimilation, for accumulation, or for the direct liberation of energy for the vital processes of the organism. Not only food from without, but also reserves formed and stored by the protoplasm itself, must be digested by some zymase before they can be utilised by the cell. In all cases of the utilisation of reserve matter that have been investigated, it has been found that a zymase is formed by the cell itself (or sometimes, in complex organisms, by its neighbours); for, after killing the cell in which the process is going on by mechanical means or by alcohol, the process of digestion can be carried on in the laboratory.[20] The chief digestion of all the animal-feeding Protista is of the same type as in our own stomachs, known as "peptic" digestion: this involves the concurrent presence of an acid, and Le Dantec and Miss Greenwood have found the contents of food-vacuoles, in which digestion is going on, to contain acid liquid. The ferment-pepsin itself has been extracted by Krukenberg from the Myxomycete, "Flowers of tan" (_Fuligo varians_, p. 92), and by Professor Augustus Dixon and the author from the gigantic multinucleate Amoeba, _Pelomyxa palustris_ (p. 52).[21] The details of the prehension of food will be treated of under the several groups.