4 THE ESSEX NATURALIST
in the sub-order Tetrasporineae of the Volvocales, where the
mucilage may be shapeless (Tetraspora spp.) or of definite form
(Apiocystis, Palmodictyon). The dominant palmelloid stage
is well seen in the Chrysophyceae (Chrysocapsineae) where it
reaches its highest development in Hydrurus, which forms a
plant-like growth with branches as much as. a foot long, com-
posed of numerous cells embedded in an evil-smelling mucilage.
Hydrurus illustrates the difficulty in defining a colony, for its
cells are not all alike; of the many cells which occur in each
branch only the apical one divides, one daughter cell passing
back and contributing to the length of the branch, the other
remaining at the apex and repeating the process of division—
acting in fact as the apical cell of certain multicellular plants.
Further, the resting cysts of Hydrurus are formed only by certain
cells, which are carried out on branches of mucilage beyond the
"plant body." It would be of great interest to know if certain
cells produce cysts or function as apical cells because they are
differentiated for these purposes, or whether their special be-
haviour is due to their position, with which might be associated
for example, differences in nutrition.
The branching, tree-like (dendroid) colony is common
amongst the flagellates. Its beginnings are probably seen when
a somewhat shapeless mass of mucilage becomes lobed. In
Spongomonas sacculus Kent each lobe contains several cells,
but in S. uvella Stein each lobe contains but a single cell and
it is apparent that when a cell divides the daughters each produce
a lobe of mucilage and growth is strictly dendroid, although
individual branches are so thickly clustered that the arborescent
form of the colony is not apparent. Phalansterium digitatum
Stein shows the dendroid habit more clearly, while in Cladomonas
fruticulosa Stein the tree-like form of the colony becomes more
marked by reason of the unequal growth of the several tubes.
By lateral cohesion of several tubes the large fan-shaped colonies
of Rhipiododendron splendidum Stein are built up and the under-
lying dendroid form is concealed.
In some colonial flagellates the daughter cells remain asso-
ciated by long thin stalks secreted by the protoplast. Such are
the colonies of Chrysodendron ramosum Pasch. There is con-
siderable variation in the shape of colonies of this type, depend-
ing on the relative length of the stalks of individual cells; thus
may be noted the corymbose colony of Dendromonas Virgaria
(Weise) Stein and the umbellate one of Codonocladium umbellatum
(Tat.) Stein. Here also mention may be made of the Euglenoid,
Colacium, where a cell, similar to Euglena, settles down by its
anterior end and secretes a mucilage stalk ; later, division occurs
and stalk formation goes on in each half; thus, in time, branched