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.
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_.
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.