PELOSPHAERA ROTANS LAUTERBORN
67
MOVEMENT
A matter of much interest in connection with the colonies we
are considering is their movement through the water. This is
obviously due to the lashing action of the flagella (except, of
course, in the case of Conochilus), but the question arises as to
whether this lashing is a co-ordinated or an unco-ordinated
process, i.e. whether the flagella of all the cells do or do not
beat in harmony with one another. At first sight the little
spheres seem to tumble about at random, and so one gets the
impression of unco-ordinated action of the flagella. It has long
been recognised, however, that, in some forms at any rate, one
particular part of the colony always takes the lead in forward
movement, thus constituting an anterior pole. This pole is often
marked by some morphological peculiarity of the cells such as
smaller size (e.g. Eudorina illinoisensis, Pleodorina californica) or
by the possession of larger eye-spots (e.g. Eudorina and Volvox).
In Conochilus I have noticed that the anterior pole is indicated
by a small clear space in the jelly free from individual rotifers.
Whether the forms showing no apparent morphological differ-
entiation of the cells also possess polarity of movement is uncer-
tain, but my own impression is that they do.
In addition to forward movement there is, seemingly in all
cases, a rotation of the whole colony. My own observations
lead me to believe that this rotation may, in some cases at least,
be constant in direction, for example, in Volvox always to the
left, in Conochilus always to the right, thus producing respec-
tively a left-handed and a right-handed spiral path through the
water. Whether constancy of direction of rotation occurs in all
cases is at present uncertain, but I think it is not improbable.
Very careful focusing under high powers, taking great care that
only the upper surface of the colony is in focus all the time, is
necessary to be certain about the direction of rotation. Other-
wise the lower surface may come into view and thus the real
direction of rotation be apparently reversed.
From the foregoing remarks about the constancy of forward
direction and rotation it follows that, in the flagellate forms,
there is very probably co-ordinated action of the flagella of all
the cells and this implies, of course, some physiological unity of
the whole colony.
In Conochilus the constancy is brought about in a different
way, the forward movement being the result of the absence of
individuals from a small section of the colony and the rotation,
I believe, of the oblique arrangement of the coronas in relation
to the surface of the sphere.