[Illustration: FIG. 52.--_Ulothrix zonata_: a portion of a filament with zoospores, which are formed two in each cell (zoosporangium); the dark spots are the red “eye-spots”; 1, 2, 3, 4, denote successive stages in the development of the zoospores; _b_ a single zoospore, _v_ the pulsating vacuole; _c_ portion of a filament with gametes, sixteen are produced in each gametangium; _d_ free gametes, solitary or in the act of conjugation; _e_ the conjugation is completed, and the formed zygote has assumed the resting-stage.] Order 2. =Ulothricaceæ.= The thallus consists normally of a simple unbranched filament (sometimes a small expansion consisting of one layer of cells is formed, as in _Schizomeris_ and _Prasiola_ which were formerly described as separate genera). Asexual reproduction takes place by means of zoospores (with 1, 2, or 4 cilia), akinetes or aplanospores; the last named may germinate immediately, or only after a period of rest. Sexual reproduction takes place by the conjugation of gametes of about the same size, each having two cilia (Fig. 52 _d_). The zygote of _Ulothrix_, on germination, produces a brood of zoospores which swarm for a time and then elongate to become _Ulothrix_-filaments (alternation of generations). The gametes may also germinate without conjugation in the same manner as the zoospores. The principal genera are: _Ulothrix_, _Hormidium_, _Conferva_, _Microspora_.--_Ulothrix zonata_ is very common in running fresh water. Nearly all the species of _Hormidium_ occur on damp soil, tree-stems and stones. Order 3. =Chætophoraceæ.= The thallus consists of a single, branched, erect or creeping filament of cells, often surrounded by mucilage. The cells have only one nucleus. Asexual reproduction by zoospores with 2 or 4 cilia, by akinetes, or aplanospores. In many, conjugation between gametes with 2 cilia may be found. They approach on one side, Ulothricaceæ, and on the other, Mycoideaceæ. The principal genera are: _Stigeoclonium_, _Draparnaldia_, _Chætophora_, _Entoderma_, _Aphanochæte_, _Herposteiron_, _Phæothamnion_, _Chlorotylium_, _Trichophilus_, _Gongrosira_, _Trentepohlia_. Most of the species of _Trentepohlia_ are coloured red by the presence of a red colouring material, which occurs in addition to the chlorophyll. They are aerial Algæ which live on stones (_T. jolithus_, “violet stone,” so named on account of its violet-like odour in rainy weather), on bark and old wood (_T. umbrina_), or on damp rocks (_T. aurea_). _Trichophilus welckeri_ lives in the hair of Bradypus.