Order 3. =Dinobryinaceæ.= The individuals are originally attached, uni- or multicellular; each individual cell is distinctly contractile, and fixed at the bottom of a cup-shaped, open envelope. Cilia 2, but of unequal length. Asexual reproduction by zoospores, which are formed by straight or oblique longitudinal division of the mother-cell, during a palmella-stage which is produced in the winter aplanospores. _Epipyxis_, _Dinobryon_. Order 4. =Hydruraceæ.= The individuals are attached, without cilia, multicellular, branched, and with apical growth. The cells are spherical, but in the final stage almost spindle-shaped, and embedded in large masses of mucilage. Asexual reproduction by zoospores which are tetrahedric, with 1 cilia, and by resting akinetes. _Hydrurus_ is most common in mountain brooks. Class 2. =Dinoflagellata.= The individuals are of a very variable form, but always unicellular, and floating about in free condition. The cell is _dorsiventral_, _bilateral_, _asymmetric_ and generally surrounded by a colourless membrane, which has _no silica_ embedded in it, but is formed of a substance allied to _cellulose_. The membrane, which externally is provided with pores and raised borders, easily breaks up into irregularly-shaped pieces. In the forms which have longitudinal and cross furrows, _two cilia_ are fixed where these cross each other, and project through a cleft in the membrane; one of these cilia _projects freely_ and is directed longitudinally to the front or to the rear, the other one _stretches crosswise_ and lies close to the cell, often in a furrow (cross furrow). The chromatophores are coloured brown or green and may either be two parallel (_Exuviella_), or several radially placed, discs, which sometimes may coalesce and become a star-shaped chromatophore. The coloring material (pyrrophyl) consists, in addition to a modification of chlorophyl, also of _phycopyrrin_ and _peridinin_; this colour is sometimes more or less masked by the products of assimilation which consist of yellow, red or colourless oil (?) and starch. Cell-nucleus one: in _Polydinida_ several nuclei are found; contractile vacuoles many, which partly open in the cilia-cleft (Fig. 13 _gs_). In some an eye-spot, coloured red by hæmatochrome, is found. Pyrenoids occur perhaps in _Exuviella_ and _Amphidinium_.