The Beggiatoa, a sewage fungus, found by me in the river Lea water of 1884 growing in great profusion, consists chiefly of mycelial threads and a number of globular, highly refractive bodies, and may be regarded as evidence of the presence in the water of an abnormal amount of sulphates which set free a gas, sulphuretted hydrogen, of a dangerous and offensive character. Another curious body closely allied to _Beggiatoa alba_ is Cladothrix; this assumes a whitish pellicle on the surface of putrefying liquids. These saprophytes obtain nourishment from organic matter; nevertheless they are not true parasites in the first stage of their existence, during which they live freely in the water or in damp soil; they, however, become pathogenic parasites when they penetrate into the tissues of animals, and necessarily live at the expense of their host. [Illustration: FUNGI, ALGÆ, LICHENS, ETC. Tuffen West, del. Edmund Evans. PLATE I.] Bacteria, as I have said, were for a long time classed with fungi under the name of Schizomycetes. But the more recent researches into their organisation, and more especially into their mode of reproduction, show that they rather more resemble a group of algæ devoid of chlorophyll. Zopf asserts that the same species of algals may at one time be presented in the form of a plant living freely in water, or in damp ground, in association with chlorophyllaceous protoplasm, and at another in the form of a bacterium devoid of green colouring matter, and receiving nourishment from organic substances previously elaborated by plants or animals, thus accommodating itself, according to circumstances, to two very different modes of existence. That widely-distributed single-cell plant, the _Palmoglœa macrococca_ of Kützing, that spreads itself as a green slime over damp stones, walls, and other bodies, affords an example. If a small portion be scraped off and placed on a slip of glass, and examined with a half or a quarter-inch power, it will be seen to consist of a number of ovoid cells, having a transparent structureless envelope, nearly filled by granular matter of a greenish colour. At certain periods this mass divides into two parts, and ultimately the cell becomes two. Sometimes the cells are united end to end, just as we see them united in the actively-growing yeast plant; but in this case the growth is accelerated, apparently, by cold and damp. Another plant belonging to the same species, the _Protococcus pluvialis_, is found in every pool of water, the spores of which must be always floating in the air, since they appear after every shower of rain.