THE SURFACE LAYER AS A LOCOMOTOR ORGAN
The discussion of the surface film of ameba and its movements during
locomotion naturally led to a discussion of the various theories that
have been offered to explain ameboid movement and protoplasmic
streaming. Now the fact that the ameba possesses a traveling surface
film which can carry particles recalls similar behavior in Oscillatorias
and in the diatoms. No new observations have been made very recently,
but by comparison of the behavior of particles carried by an
Oscillatoria filament and by ameba, it is found that the nature of the
movement, the rate of movement, the degree of adhesion of the particles,
the sizes of the particles carried and so on, are similar in both
organisms. This indicates that there is a surface layer on Oscillatoria
threads that is similar to that which has been described in amebas, and
whose movement is probably also effected by changes in surface
extension; but just how this change is effected is not clear owing to
the spiral path the particles take as they travel along the Oscillatoria
filament. The spiral has an angle of about sixty degrees, which must be
related in some way to the finer structure of the cells of which the
filament is composed. The suggestion that movement is caused by the
rapid and forcible exudation of mucus is exceedingly improbable if not
physically impossible. It is difficult to see how the spiral direction
of the flow of mucus could be brought about, to say nothing of the
frequent change in direction of the flow. In a surface tension film,
however, the direction of movement is readily determined by the location
of the points where the tension is changed. Mucus secreting glands would
need special structural devices such as secretory tubes bent at an angle
to control the direction of flow, while no such structural devices are
necessary if the propelling force is surface energy. In short, it is
difficult to see how any movement at all could be produced by the act of
secretion of mucus, while from what we have seen in the ameba, surface
tension changes could easily produce movement in Oscillatoria. The
spiral feature of the movement has no explanation that is based on
observational data. It may be added here that the surface film in amebas
is powerful enough to enable them to move by means of it. One sometimes
sees _sphaeronucleosus_ or small individuals of _verrucosa_, that are
lying loose on the substratum, actively streaming, but moving slowly and
more or less irregularly backwards. This movement is due to the activity
of the surface film.
The suggestion that no extra-cellular protoplasmic layer has been
demonstrated in Oscillatoria is not a cogent argument against the
surface tension hypothesis, since the surface film would need to be but
a small fraction of a micron thick, too thin to be demonstrated by
histological methods now in vogue. It is also to be remembered that the
surface film in ameba can be demonstrated in no other way at present
than by its particle-carrying capacity.
The main features of the movements of diatoms are very similar to those
of Oscillatoria. Müller (’89, ’97, ’99) has shown that the gliding
movements of diatoms are not due to the ejection of water, but to the
streaming of protoplasm on the outside of the shells. Foreign particles
are carried by these shallow streams of protoplasm in quite the same
manner as by the surface film of the ameba. And there seems to be no
evidence against the assumption that these shallow streams, at least the
surface films over them, owe their movement to changes in surface
tension.
Desmids also glide about slowly, leaving a track of mucus behind. Only
one explanation for locomotion has been advanced, and that is that it is
due to the secretion of mucus (Klebs, ’85). This explanation is likely
to be as wide of the mark as the similar explanation in the case of
Oscillatoria. There is no question concerning the excretion of mucus,
but the source of the locomotive energy is probably here also surface
energy, though the observational data are too few to try to locate the
regions where the changes in tension occur.
It has been a matter of considerable surprise to me to find that the
so-called “crawling” euglenas, in addition to the diatoms, desmids,
Oscillatoria, Beggiatoa and perhaps other forms of life such as the
Gregarinidas, also possess extra-cellular films which carry particles
as do amebas and Oscillatoria, and move about through the agency of this
film. The film travels spirally around the euglena as it does in
Oscillatoria filaments. In at least two species the film moves parallel
to the spiral striations on the outer surface. In one species no spiral
striations could be detected, although the film moved spirally. The
species of euglenas in which these movements were observed, were not
identified.
The character of the movement of the euglenas is very similar to that of
the diatoms excepting that most of the diatoms do not revolve on their
longitudinal axes. The movement of particles on the surface film of
euglenas is quite like that in Oscillatoria, though it is only under
exceptional circumstances that one can see particles attached to the
surface film. The movement of the particles indicates that the surface
film moves from the anterior end toward the posterior end, but whether
the “spine” is to be included was not definitely determined. The degree
of cohesiveness of the film is high, for locomotion is rapid, even if
only a small part of the posterior end is in contact with the
substratum, as when moving over an Oscillatoria filament. To one who has
seen the movements of the surface films of amebas, diatoms and
Oscillatorial filaments, the most reasonable conclusion seems to be that
the cause of locomotion in crawling euglenas is the same as that in
Oscillatoria and diatoms.
Evidence contributing to this conclusion is found in the circumstance
that crawling euglenas, diatoms and Oscillatoria threads are much more
refractory to galvanic currents than flagellate euglenas or other
flagellates or ciliates: The electrical apparatus at my disposal was
rather crude, but I was unable to find that I could influence the
direction or character of movement of Oscillatoria filaments, diatoms or
crawling euglenas without injuring the organisms. Currents which had
produced a marked effect on ciliates or flagellates produced no effect
whatever on amebas, diatoms, Oscillatoria or crawling euglenas. Diatoms
are particularly resistant to the effect of electrical currents.
The general conclusion regarding the source of energy of the moving
surface films, whether found on amebas, diatoms, desmids, or crawling
euglenas, is that all derive their motive power from the energy in the
superficial films of these organisms; while ameboid streaming, if it is
a surface tension phenomenon as seems to be the case, depends upon the
surface energy of the interfaces of the emulsoid colloidal system in the
endoplasm. It has already been seen that those cases of locomotion due
in large measure to the power of contractility in the ectoplasm
(Difflugia, Foraminifera) are also explained as being due to a change of
phase in the colloidal system, which is in itself a surface tension
effect. It appears therefore that all the lower organisms that move,
excepting flagellated or ciliated organisms (of whose motor mechanism we
have no detailed knowledge), depend upon surface energy as the source of
the energy of movement.