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. Dinoflagellata occur in the “Plankton” of the open sea, where they form together with Diatomaceæ the basis for the animal life. It is known with certainty that some salt water forms (like the _Noctiluca_, which belongs to the animal kingdom and to which they are perhaps related) produce light, known as phosphorescence. [Illustration: FIG. 13.--_A_ and _B Glenodinium cinctum_. _A_ seen from the ventral side, _B_ from behind; _fg_ transverse cilium; _g_ longitudinal cilium; _ch_ chromatophores; _a_ starch; _n_ cell-nucleus; _v_ vacuole; _oc_ eye-spot; _C Ceratium tetraceros_ from the ventral side; _r_ the right, _b_ the posterior horn; _lf_ longitudinal furrow; _gs_ cilium-cleft; _v_ vacuole; _g_ longitudinal cilium. (_A_ and _B_ mag. 450 times, _C_ 337 times.)] _Dinoflagellata_ (_Peridinea_, _Cilioflagellata_) are allied through their lowest form (_Exuviella_) to the Syngeneticæ and especially to the order Chrysomonadinaceæ. They may be divided into three orders. Order 1. =Adinida.= Without transverse or longitudinal furrows, but enclosed in two shells, and with two parallel chromatophores in each cell. _Exuviella_, _Prorocentrum_.