rotoplasm in which the nucleus is embedded. The greater part of the
cavity of the cell is occupied by one or several fluid vacuoles. The
characteristic brown colour of diatoms is due to the presence of
chromatophores embedded in the lining layer of protoplasm. In number and
form these chromatophores are variable. They contain chlorophyll, but
the green colour is masked by the presence of diatomin, a brown pigment
which resembles that which occurs in the Brown Algae or Phaeophyceae.
The chromatophores contain a variable number of pyrenoids, colourless
proteid bodies of a crystalloidal character.

One of the first phenomena which comes under the notice of the observer
is the extraordinary power of motion with which the frustules are
endowed. Some species move slowly backwards and forwards in pretty much
the same line, but in the case of _Bacillaria paradoxa_ the motion is
very rapid, the frustules darting through the water in a zigzag course.
To account for this motion various theories have been suggested, none of
which appear to be altogether satisfactory. There is little doubt that
the movements are connected with the raphe, and in some diatoms there is
much evidence to prove that they are due to an exudation of mucilage.

_Classification._--The most natural system of classification of the
Bacillarieae is the one put forward by Schutt (1896), and since
generally followed by systematists. He separates them into two primary
divisions, the 'Centricae' and the 'Pennatae.' The former includes all
those diatoms which in the valve view possess a radial symmetry around a
central point, and which are destitute of a raphe (or a pseudoraphe).
The latter includes those which are zygomorphic or otherwise irregular,
and in which the valve view is generally boat-shaped or needle-shaped,
with the markings arranged in a sagittal manner on each side of a raphe
or pseudoraphe.

_Reproduction._--In the Diatomaceae, as well as in the Desmidieae, the
ordinary mode of increase is by simple cell-division. The cell-contents
within the enclosure of the siliceous case separate into two distinct
masses. As these two daughter-masses become more and more developed, the
valves of the mother-cell are pushed more and more widely apart. A new
siliceous valve is secreted by each of the two masses on the side
opposite to the original valve, the new valves being situated within the
girdle of the original frustule. When this process has been completed
the