THE COLONIAL HABIT IN SOME LOWER ORGANISMS 7
Brief mention must also be made of another group of peculiar
organisms, the Myxobacteria, which are found on dung and
which are peculiar among the bacteria in being flexible and
having a slow creeping movement. Aggregation of thousands of
such cells produces the characteristic fruiting body, in which the
reproductive cells occur.
If our definition of a colony is to be wide enough to embrace
such groups as the slime moulds, it requires some modification,
for in some of the Acrasieae at least, we have division of labour,
a differentiation among the amoebae, some forming stalk cells,
others spores. Clearly, also, the fructifications are not the
result of the activity of a single cell, but of a group of individuals.
We might regard such Protista as colonial organisms of a type
rather different from those with which we have dealt in the
algae, but it might be preferable to regard them, not so much as
colonies but rather as multiple organisms.
To return to the algae—I have deliberately omitted to mention
the colonial habit in the Volvocales (Chlamydomonadineae).
It is referred to in most elementary text books of botany and
zoology, but in my opinion the accounts are often misleading
in that they attempt to show, either directly or by implication,
the evolution of the multicellular organism from the colony.
Colonies similar to those with which we have already dealt
are found in the Volvocales. Thus in Dangeardinella the elon-
gate, biflagellate cells are united by means of very tenuous
threads; after division the daughter cells remain united by
simple or branched protoplasmic strands, sometimes ephemeral,
more often persistent and by successive divisions colonies com-
posed of from four to 16 cells arise ; the colony may exhibit
polarity, all the cells pointing in one direction, but this is not
invariable. In Raciborskiella the biflagellate, pyramiform cells
are united by their tapered, posterior ends to form spherical
aggregates of from two to 15 cells, a type of colony similar
to that of Synura. In both genera individual cells may become
free and form new colonies.
In these two genera the cells are naked and in the naked
members of the Chlamydomonadineae (Polyblepharidaceae)
multiplication takes place by the division of the individual into
two halves while the organism is active. Hence the colonial
forms do not differ, essentially, from those which we have
already considered.
In the remaining families of the Chlamydomonadineae the
cells are walled and the usual means of multiplication is by the
formation of four or sometimes eight or 16 zoospores from
the mother cell, within the cell wall. The colonial organisation,
if such it be, is of a distinct and peculiar type. The coenobium,