8 THE ESSEX NATURALIST 
as the group of cells is called, has a definite form and its cells
remain constant in number, or at least, do not increase in number,
throughout the life of the coenobium.
It is difficult to classify the Coenobia on a purely morphological
basis ; we may distinguish two main types : (1) those in which
individual cells are held together by mucilage ; (2) those in which
the cell walls are contiguous and where there is no obvious
mucilage investment. But such a classification is purely one of
convenience and has little real meaning, for the mucilage is
merely the swollen, gelatinised outer layer of the cell wall.
Of the former type the coenobium of the well-known Gonium
is plate-like, that of G. socialis (Duj.) Warming composed of
four cells united by pegs of mucilage and enclosed in a common
mucilage optically different from that of the pegs, while that of
G. pectorale Muell. normally consists of 16 cells similarly
united by and enveloped in mucilage. In Pandorina and Masti-
gosphaera the eight, 16 or 32 cells of the coenobium form a
compact sphere surrounded by a broad zone of mucilage, while
in Eudorina the cells, numbering 16, 32 or 64, occupy a position
towards the periphery of the mucilage and lie separate from
one another. The spacing of the cells of the Eudorina coenobium
is often very regular ; for example, in the common E. elegans
Ehr. there are anterior and posterior rings of four cells each and
two median ones of eight cells each. In some species the cells
at the anterior end of the coenobium are smaller than the others
and may remain purely vegetative. The closely related Pleodorina
shows more marked differentiation, for up to half the cells at
the anterior end of the coenobium are smaller than the posterior
ones and are purely vegetative.
It is convenient to deal next with Volvox, but this is not
intended to imply that I regard Volvox as the highest development
of the Gonium, Pandorina, Eudorina series, apparently a com-
monly accepted view for which it is difficult to find any justifica-
tion.
In Volvox the coenobium consists of some thousands of cells
arranged as a hollow sphere ; without referring to the detailed
structure of the coenobium, we may note a much greater
differentiation than in Pleodorina ; the great majority of the
cells is vegetative and there is some morphological differentiation
between those at the anterior pole of the coenobium, and the
more posteriorly placed ones ; in each coenobium there eventually
develops from one to 10 cells, in the posterior region, which
become larger than their fellows, lose their flagella and gradually
sink in from the periphery : these are the cells which are concerned
with asexual reproduction. Further, only a limited number of
cells function as gametangia ; a single egg, much larger than a