ic filament (_c_), by means of which the
cell moves. There is also to be seen near this end a red speck (_e_)
which is probably sensitive to light. A nucleus can usually be seen
if the cell is first killed with an iodine solution, which often
will render the flagellum (_c_) more evident, this being invisible
while the cell is in motion. The cells multiply by division.
Previous to this the flagellum is withdrawn, and a firm cell wall is
formed about the cell (Fig. 9, _B_). The contents then divide into
two or more parts, which afterwards escape as new individuals.

Of the forms that are united in colonies[2] one of the best known is
_Volvox_ (Fig. 10). This plant is sometimes found in quiet water,
where it floats on or near the surface as a dark green ball, just
large enough to be seen with the naked eye. They may be kept for some
time in aquaria, and will sometimes multiply rapidly, but are very
susceptible to extremes of temperature, especially of heat.

[2] The term "colony" is, perhaps, inappropriate, as the whole mass of
cells arises from a single one, and may properly be looked upon as an
individual plant.

[Illustration: FIG. 10.--_Volvox._ _A_, mature colony, containing
several smaller ones (_x_), x 50. _B_, Two cells showing the cilia,
x 300.]

The colony (Fig. 10, _A_) is a hollow sphere, the numerous green
cells of which it is composed forming a single layer on the outside.
By killing with iodine, and using a strong lens, each cell is seen
to be somewhat pear-shaped (Fig. _B_), with the pointed end out.
Attached to this end are two vibratile filaments (cilia or
_flagella_), and the united movements of these cause the rolling
motion of the whole colony. Usually a number of young colonies
(Fig. _x_) are found within the mother colony. These arise by the
repeated bipartition of a single cell, and escape finally, forming
independent colonies.

Another (sexual) form of reproduction occurs, similar to that found
in many higher plants; but as it only occurs at certain seasons, it
is not likely to be met with by the student.

Other forms related to _Volvox_, and sometimes met with, are
_Gonium_, in which there are sixteen cells, forming a flat square;
_Pandorina_ and _Eudorina_, with sixteen cells, forming an oval or
globular colony like _Volvox_, but much smaller. In all of these the
structure of the cells is essentially as in _Volvox_.

CHAPTER IV.

SUB-KING