Order 1. =Desmidiaceæ.= The cells generally present markings on the outer wall, and are mostly divided into two symmetrical halves by a constriction in the middle, or there is at least a symmetrical division of the protoplasmic cell-contents. The cell-wall consists nearly always of two layers, the one overlapping the other (Fig. 35 _C_). The cells either live solitarily or are united into unbranched filaments. The mass of protoplasm formed by the fusion of the two conjugating cells becomes the zygote, which on germination produces one (or after division 2, 4 or 8) new vegetative individual. The chromatophores are either star-, plate-, or band-shaped, and regularly arranged round the long axis of the cell. [Illustration: FIG. 35.--A Cell of _Gymnozyga brebissonii_, external view showing the distribution of the pores. _B A_ portion of the membrane of _Staurastrum bicorne_ with pores containing protoplasmic projections. _C_ Cell-wall of _Hyalotheca mucosa_ during cell-division: the central part, being already formed, shows the connection with the divisional wall.] The Desmidiaceæ are not able to swim independently, many species, however, show movements of different kinds by rising and sliding forward on the substratum. These movements, which are partly dependent upon, and partly independent of light and the force of gravitation, are connected with the protrusion of a mucilaginous stalk. The mucilage, which sometimes surrounds the whole individual, may acquire a prismatic structure, it is secreted by the protoplasmic threads which project through certain pores definitely situated in the walls (Fig. 35 _A_, _B_). VEGETATIVE MULTIPLICATION takes places by division. A good example of this is found in _Cosmarium botrytis_ (Fig. 36 _A-D_). The nucleus and chromatophores divide, and simultaneously the central indentation becomes deeper, the outer wall is then ruptured making a circular aperture through which the inner wall protrudes forming a short, cylindrical canal between the two halves to which it is attached (Fig. 36 _C_). After elongation the canal is divided by a central transverse wall, which commences as a ring round its inner surface and gradually forms a complete septum. The dividing wall gradually splits, and the two individuals separate from each other, each one having an old and a new half. The two daughter-cells bulge out, receive a supply of contents from the parent-cells, and gradually attain their mature size and development (Fig. 36 _B-D_). Exceptions to this occur in some forms. [Illustration: FIG. 36.--_Cosmarium botrytis. A-D_ Different stages of cell-division.] [Illustration: FIG. 37.--_Cosmarium meneghinii_: _a-c_ same individual seen from the side, from the end, and from the edge; _d-f_ stages of conjugation; _g-i_ germination of the zygote.]