CHAPTER XI. The Surface Layer As a Locomotor Organ

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.