Order 2. =Dinifera.= With tranverse and generally longitudinal furrow. Many radially-placed, disc-formed chromatophores. The most common genera are--_Ceratium_ (Fig. 13), _Peridinium_, _Glenodinium_ (Fig. 13), _Gymnodinium_, _Dinophysis_. Order 3. =Polydinida.= With several transverse furrows, no chromatophores, and several cell-nuclei. Only one genus--_Polykrikos_. The order _Polydinida_ deviates in a high degree from the other Dinoflagellata, not only by its many tranverse furrows, each with its own transverse cilium, and by the absence of chromatophores, but also in having several cell-nuclei and a kind of stinging capsule, which otherwise does not occur within the whole class. It may therefore be questionable whether this order should really be placed in the vegetable kingdom. Class 3. =Diatomeæ.= The individuals--each known as a _frustule_--assume very various forms and may be unicellular or multicellular, but present no differentiation; many similar cells may be connected in chains, embedded in mucilaginous masses, or attached to mucilaginous stalks. The cells are bilateral or centric, often asymmetrical, slightly dorsiventral and have no cilia; those living in the free condition have the power of sliding upon a firm substratum. The cell contains 1 cell-nucleus and 1–2 plate-shaped or several disc-shaped chromatophores. The colouring material “_Melinophyl_” contains, in addition to a modification of chlorophyl, a brown colouring matter, _diatomin_. 1 or 2 pyrenoids sometimes occur. Starch is wanting and the first product of assimilation appears to be a kind of oil (?). [Illustration: FIG. 14.--_Pinnularia_: _B_, from the edge, shows the valves fitting together; _A_, a valve.] [Illustration: FIG. 15.--Various Diatomaceæ. A _Diatoma vulgare_. B _Tabellaria flocculosa_. C _Navicula tumida_ (lateral views). D _Gomphonema constrictum_ (lateral views). E _Navicula west[=i][=i]_ (lateral views).]