=Algæ.=--The algals have a special charm for microscopists. I am free to confess my interest in these organisms, and for several reasons. In this humid climate of ours they are accessible during the greater part of the year; they can be found in any damp soil, in bog, moss, and in water--indeed, wherever the conditions for their existence seem to be at all favourable for development. Should the soil dry up for a time, when the rain returns algæ are seen to spring into life and give forth their dormant spores, which once more resume the circle of formation and propagation. In the earliest stage of development the spore or spore cell is so very small when in a desiccated state, that any number may be carried about by the slightest breath of air and borne away to a great distance. To all such organisms I originally gave the name of Ærozoa; now recognised as ærobic and anærobic organisms (Fig. 281). [Illustration: Fig. 281.--Ærobic Spores × 200. 1. Ærobic fungi caught over a sewer; 2. Fragments of Penicillium spores; 3. Ærobic fungi taken in the time of the cholera visitation, 1854.] With reference to the ærobic bacteria I have only to add that in addition to the simple mode of taking them on glass slides smeared over with glycerine, special forms of æroscopes are now in use for the purpose, consisting of a small cylinder in which a current of air is produced by an aspirator and diffused through a glass vessel containing a sterilised fluid. These are in constant use in all bacteriological laboratories. The results obtained are transferred to sterilised flasks or tubes as those shown in a former chapter. Miquel, who has given considerable attention to the subject of ærobic and anærobic bacteria, reckons that the number of spores that find their way into the human system by respiration, even should health be perfectly sound, may be estimated at 300,000 a day. One of the most commonly met with forms of micro-organisms is _Leptothrix buccalis_. It chiefly finds its nutritive material in the interstices of the teeth, and is composed of short rods and tufted stems of vigorous growth, to which the name of _Bacillus subtilis_ has been given (Fig. 282). Among numerous other fungoid bodies discovered in the mouth, Sarcinæ have been found. See Plate IX., No. 7. [Illustration: Fig. 282.--Section of the Mucous Membrane of the Mouth, × 350. Showing: _a._ The denser connective tissue; _b._ Teased out tissue; _c._ Muscular fibre; _d._ _Leptothrix buccalis_, together with minute forms of bacteria and micrococci; _e._ Ascomycetes and starch granules.]