BAT´RACHOSPER´MEÆ.--The members of this family resemble to the naked eye the little masses of _Draparnaldia_, and they are found in the same localities. They are of various colours, being green, brown, purple, or red. They consist, as in _Bat´rachosper´mum monilifor´me_ (Pl. V. fig. 8), of branched filaments, which have a knotty appearance under a low power. The larger filaments are composed of cells arranged end to end, the knots consisting of numerous smaller whorled filaments, _i. e._ filaments arising from around them at the same level (fig. 7). The cells composing the whorled filaments are beaded or moniliform, and are prolonged into colourless hair-like points. The globules seen among the branches (fig. 7) consist of groups of spores. ZYGNEMA´CEÆ.--The members of this family resemble the Confervaceæ in consisting of simple cellular filaments (Pl. V. figs. 11, 13), but differ from them in the elegant arrangement of the endochrome: this forms beautiful spiral bands, as in _Spirog´yra quini´na_ (fig. 11), or star-shaped masses, as in _Zygne´ma crucia´ta_ (fig. 13). A remarkably curious phenomenon met with in them is the manner in which the spores are formed, and which is known as _conjugation_. In this process the opposite cells of two distinct filaments, lying near together, push out protrusions of the cell-walls, which meet and open into each other, forming cross tubes, as in _Spirog´yra nit´ida_ (Pl. V. fig. 12). The contents of the opposite cells of the filaments then unite, forming large spores, which remain either in the cells of one of the filaments or in the cross tubes. The three species figured are common in clear pools. DESMIDIA´CEÆ.--The Desmidiaceæ are truly microscopic, few of them being even perceptible to the naked eye without the very closest examination. They are very beautiful, on account of their bright green colour and often elegant forms. Many of them are very common, existing in every pond or ditch; but they abound most in clear open boggy pools on heaths. On placing some water containing them in a glass jar and exposing it to the light, they will often be found adhering to the glass, or forming a layer on the surface of the muddy sediment. The Desmidiaceæ consist mostly of single cells (Pl. V. figs. 9, 16); and these consist of two equal halves or segments, as indicated either by a paleness of the endochrome or a deep constriction at the line of junction, which is called the _suture_. The cells are often elegantly lobed and cut, or spiny; and in many the surface exhibits minute markings, consisting of little protrusions of the cell-wall outwards, or inflations, as they are called.