ome
invested with a cell wall, and grow into a new filament. These cells
are called zooespores, from their animal-like movements. They are
provided with two cilia, closely resembling the motile cells of the
_Protococcaceae_ and _Volvocineae_.

There are very many examples of these simple _Confervaceae_, some like
_Conferva_ being simple rows of cells, others like _Stigeoclonium_
(Fig. 14, _A_), _Chaetophora_ and _Draparnaldia_ (Fig. 14, _B_, _C_),
very much branched. The two latter forms are surrounded by masses of
transparent jelly, which sometimes reach a length of several
centimetres.

[Illustration: FIG. 14.--_Confervaceae_. _A_, _Stigeoclonium_. _B_,
_Draparnaldia_, x 50. _C_, a piece of _Draparnaldia_, x 2. _D_, part
of a filament of _Conferva_, x 300.]

Among the marine forms related to these may be mentioned the sea
lettuce (_Ulva_), shown in Figure 15. The thin, bright-green,
leaf-like fronds of this plant are familiar to every seaside student.

[Illustration: FIG. 15.--A plant of sea lettuce (_Ulva_). One-half
natural size.]

Somewhat higher than _Cladophora_ and its allies, especially in the
differentiation of the reproductive parts, are the various species of
_OEdogonium_ and its relatives. There are numerous species of
_OEdogonium_ not uncommon in stagnant water growing in company with
other algae, but seldom forming masses by themselves of sufficient size
to be recognizable to the naked eye.

The plant is in structure much like _Cladophora_, except that it is
unbranched, and the cells have but a single nucleus (Fig. 16, _E_).
Even when not fruiting the filaments may usually be recognized by
peculiar cap-shaped structures at the top of some of the cells.
These arise as the result of certain peculiarities in the process of
cell division, which are too complicated to be explained here.

There are two forms of reproduction, non-sexual and sexual. In the
first the contents of certain cells escape in the form of large
zooespores (Fig. 16, _C_), of oval form, having the smaller end
colorless and surrounded by a crown of cilia. After a short period
of active motion, the zooespore comes to rest, secretes a cell wall
about itself, and the transparent end becomes flattened out into a
disc (_E_, _d_), by which it fastens itself to some object in the
water. The upper part now rapidly elongates, and dividing repeatedly
by cross walls, develops into a filament like the origi