Fig. 12.—Chrsopyxis bipes: m envelope, Ec chromatophore, cv contractile vacuole.
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.
The reproduction takes place as far as is known at present, only by division. This, in many salt water forms, may take place in the swarming condition, and, in that case, is always parallel to the longitudinal axis. The daughter-individuals, each of which retains half of the original shell, sometimes do not separate at once from each other, and thus chains (e.g. in Ceratium) of several connected individuals may be formed. In others, the division occurs after the cilia have been thrown off and the cell-contents rounded. The daughter-cells then adopt entirely new cell-walls. A palmella-stage (motionless division-stage) sometimes appears to take place, and also aplanospores (?) with one or two horn-like elongations (e.g. in Peridinium cinctum and P. tabulatum); at germination one, or after division, two or more, new individuals may be formed.
Sexual reproduction has not been observed with certainty.
The Dinoflagellata move forward or backward, turning round their longitudinal axes; in their motion they are influenced by the action of light. The motion possibly may be produced only by the transverse cilium, which vibrates rapidly; whilst the longitudinal cilium moves slowly, and is supposed to serve mainly as a steering apparatus. They live principally in salt water, but also in fresh.
Besides the coloured forms, which are able to make their own organic compounds by the splitting up of the carbonic acid contained in the water, there are a few colourless forms (e.g. Gymnodinium spirale), or such as do not possess chromatophores (Polykrikos); these appear to live saprophytically, and may be able to absorb solid bodies with which they come in contact.