their communication with the medium they
subserve nutrition. Nutrition is of course holozoic or saprophytic in
the colourless forms, holophytic in the coloured; but these divergent
methods are exhibited by different species of the same genus, or even by
individuals of one and the same species under different conditions.
Binary fission has been widely observed, both in the active condition or
after loss of the flagella: it differs from that of true Flagellates in
not being longitudinal, but transverse or oblique (fig, 2, 2). Repeated
fission (brood-formation) within a cyst has also been observed, as in
_Pyrocystis_ and _Ceratium_; and possibly the chains of _Ceratium_ and
other (fig. 2, 5 and 6) genera are due to the non-separation of the
brood-cells. Conjugation of adults has been observed in several species,
the most complete account being that of Zederbauer on _Ceratium
hirundinella_ (marine): either mate puts forth a tube which meets and
opens into that of the other (as in some species of _Chlamydomonas_ and
Desmids); the two cell-bodies fuse in this tube, and encyst to form a
resting zygospore. The Dinoflagellates are relatively large for
Mastigophora, many attaining 50 [mu] (1/500") in length. The majority are
marine; but some genera (_Ceratium_, _Peridinium_) include fresh-water
species. Many are highly phosphorescent and some by their abundance
colour the water of the sea or pool which they dwell in. Like so many
coloured Protista, they frequently possess a pigmented "eye-spot" in
which may be sunk a spheroidal refractive body ("lens").

[Illustration: After F. Schutt in Engler and Prantl's
_Pflanzenfamilien_, by permission of Wm Engelmann.

FIG. 1.--_Peridinium divergens_ showing longitudinal and transverse
grooves in which lie the respective flagella l.f., t.f.; s.p., large
"sack pusule" discharging through a tube by pore o'; c.p., "collective
pusule discharging at o, and surrounded by a ring of formative" or
"daughter pusules"; n, nucleus.]

[Illustration: FIG. 2.

From Delage and Herouard's _Traite de zoologie concrete_, by permission
of Schleicher Freres.

1. Modified from Schutt, _Ornithoceras_.
2. Diagram of transverse fission of a Dinoflagellate.
3. After Schutt, _Exuviaeella_.
4. After Stein, _Prorocentrum_.
5, 6. _Ceratium_, single and series.
7. _Pouchetia fusus_ (Schutt).
8. _Citharistes_.
9. After Butschli, _Polykrikos_.]

The affinities of the Dinoflagellata are certainly w