motionless cells. _B_, a cell that has
escaped from its envelope and is ciliated, x 300.]

Owing to the absence of a definite membrane, a distinction between
fission and internal cell division can scarcely be made here. Often
the cells escape from the gelatinous envelope, and swim actively by
means of two cilia at the colorless end (Fig. 12, _B_). In this
stage they closely resemble the individuals of a _Volvox_ colony, or
other green _Flagellata_, to which there is little doubt that they
are related.

There are a number of curious forms common in fresh water that are
probably related to _Protococcus_, but differ in having the cells
united in colonies of definite form. Among the most striking are
the different species of _Pediastrum_ (Fig. 11, _D_, _E_), often met
with in company with other algae, and growing readily in aquaria when
once established. They are of very elegant shapes, and the number of
cells some multiple of four, usually sixteen.

The cells form a flat disc, the outer ones being generally provided
with a pair of spines.

New individuals arise by internal division of the cells, the
contents of each forming as many parts as there are cells in the
whole colony. The young cells now escape through a cleft in the wall
of the mother cell, but are still surrounded by a delicate membrane
(Fig. 11, _E_). Within this membrane the young cells arrange
themselves in the form of the original colony, and grow together,
forming a new colony.

A much larger but rarer form is the water net (Fig. 11, _G_), in
which the colony has the form of a hollow net, the spaces being
surrounded by long cylindrical cells placed end to end. Other common
forms belong to the genus _Scenedesmus_ (Fig. 11, _F_), of which
there are many species.

ORDER II.--_Confervaceae_.

Under this head are included a number of forms of which the simplest
ones approach closely, especially in their younger stages, the
_Protococcaceae_. Indeed, some of the so-called _Protococcaceae_ are
known to be only the early stages of these plants.

A common member of this order is _Cladophora_, a coarse-branching
alga, growing commonly in running water, where it forms tufts,
sometimes a metre or more in length. By floating out a little of it in
a saucer, it is easy to see that it is made up of branching filaments.

The microscope shows (Fig. 13, _A_) that these filaments are rows of
cylindrical c