10 THE ESSEX NATURALIST
like extrusion therefrom, come together to form the coenobium,
which, before liberation, has assumed its mature form. One
is' tempted to speculate whether such structures are, in fact,
true Coenobia or whether Hydrodictyon and its allies are not,
rather, multiple organisms, more comparable with say, the
plasmodium of a myxomycete.
It is, of course, true that individual cells of the coenobium
are often very similar to those of solitary flagellates (e.g., Chlamy-
domonas) and since this similarity of structure has, not un-
reasonably, been accepted as evidence of ancestry, the view
appears to have arisen that the coenobium represents a colony
of a special type. It is difficult to justify this point of view.
In Volvox, undoubtedly the most advanced coenobiate type, the
organism is composed of thousands of cells and shows marked
division of labour, with vegetative cells, cells concerned with
asexual reproduction, and gametangia; further the vegetative
cells differ somewhat according to their position in the coenobium.
The coenobium arises as a plate of cells which becomes more and
more spherical and which passes through a peculiar process of
inversion before its development is completed. Nor are the
daughter Coenobia so formed liberated immediately, for they
pass into the interior of the mother coenobium and are later
set free when the parent structure breaks up. It may well be
asked whether this vegetable blastula has not reached a com-
plexity of organisation perhaps as high as that exhibited by the
sponges and certainly much greater than that of such simple
filamentous algae as Spirogyra and Ulothrix.
To a lesser degree Pleodorina and species of Eudorina exhibit
division of labour. Were the cells of these Coenobia not so
similar to those of certain flagellates, it is difficult to believe
that these organisms would not, from the first, have been re-
garded as multicellular organisms and not as colonial flagellates,
for there seems to be little doubt that the coenobiate types must
be so regarded. Perhaps the simpler ones, like Gonium and
Pascheriella shed some light on the evolution of the multicellular
organism from a flagellate ancestor, albeit from flagellates of an
advanced type. If we persist in including these coenobiate
types among the flagellates, and the flagellates among the
Protozoa, it seems clear that our definition of the phylum is
due for a radical change, for there can scarcely be justification
for defining them as non-cellular—or unicellular—organisms.
In connection with the colonial habit it seems appropriate
to make brief mention of certain isolated examples of double
or multiple individuals, as well as the peculiar double forms
exhibited by the Distomataceae. In this latter order the cell
is obviously double, and even the nuclei are duplicated.