CHAPTER XIII

THE WAVY PATH OF THE AMEBA AND THE SPIRAL PATHS OF CILIATES AND OTHER ORGANISMS

The most interesting feature of the path of the ameba is of course the waves. The path of an ameba closely resembles the projection of a helical spiral on a plane surface, and this at once calls to mind the spiral swimming of flagellates, ciliates, rotifers, larvae or various groups of animals, swarm spores and zoöspores of various algae and fungi. But before we take up the general subject of spiral movement, it will be worth while to see what other evidence there is beside the wavy path, that indicates that the “spiral urge” is present in the ameba.

It is well known that in a number of the small amebas, especially the soil amebas, there are two trophic stages, an ameboid stage and a free swimming flagellate stage. The change from one stage to the other is a matter of a few minutes only. In the flagellate stage (Figure 43) the amebas resemble a small flagellate like chilomonas, very closely. Their manner of swimming is very similar. And it is especially noteworthy in this respect that they revolve on their long axis and describe a well marked, regular spiral path, just as do the flagellates and ciliates. Unfortunately no records have yet been made of the paths these amebas describe when in the true ameboid stage. Since, therefore, as we shall see later, the slightly unsymmetrical shape of the flagellate stage is not the cause of the spiral path, it is probable that the mechanism controlling the activity of the flagellum can produce orderly locomotion only when the organism follows a spiral path.

Much has been written about the fundamental similarity or identity between flagella and pseudopods. All writers who have expressed themselves on this point incline to think that there is such similarity, that flagella are really very slender and very agile pseudopods. I am not going to record here the evidence for this conclusion, for I have recently had the good fortune to make some very convincing observations on a hitherto undescribed ameba

(which for the sake of reference will here be called flagellipodia, Figure 44) whose pseudopodia stand about midway between typical flagella and typical pseudopods in their activity. In its general characteristics it stands near A. radiosa, but quite unlike the stiff, static pseudopods which radiosa very frequently forms, this ameba has usually five or more slender pseudopods of which one or two or more are in slow flagellate motion. The distal third or half of the pseudopod is in the shape of a corkscrew. The free end of the pseudopod travels around in a circle (anti-clockwise in all instances observed), making one revolution in about three seconds. If this motion were very rapid it would act like a propeller and the ameba would swim through the water. The part of the pseudopod back of the mobile portion is usually also thrown into a spiral of gradually decreasing diameter until the spirality disappears. This portion of the pseudopod is not mobile in the same way that the distal portion is. Sometimes the whole of a pseudopod is thrown into a spiral, all of the turns being of equal size and only slightly motile. More than half of all the pseudopods formed become spiralized at one time or another of their existence, the greater number of these being however relatively immotile. Pseudopods frequently fall into spirals while they are being extended.

A better transition form between pseudopods and such flagella as are found, for example, in the peranemas, could hardly be imagined. The difference between crawling and swimming would seem to be merely a matter of speed of movement of the pseudopod. But important as such a transition form is for theoretical purposes in understanding the nature of both flagella and pseudopods, it is of special importance for our present purpose because it shows a strong tendency for pseudopods to fall into spirals and to move in spirals. This tendency is found not only in this species of ameba but is observed also occasionally in radiosa (Figure 7, p. 30) and in several other species. In these latter species the pseudopods are stiff and not capable of waving about in the water, as are those of flagellipodia, whether in the spiral shape or not. In radiosa the pseudopods may become spiralized only as a preliminary to withdrawal. It is evident therefore that the spiral urge can express itself best in a plastic pseudopod.

Taking all these observations together, the tendency of pseudopods to move in a spiral manner, the tendency of the ameba as a whole to move in a spiral path when in the flagellate stage, and the wavy path of amebas which is smoothest when in the clavate stage, all these observations seem to confirm the supposition that the wavy path is in reality a flattened spiral, and that the spiral urge in ameba is a very fundamental factor in the process of locomotion. In other words, there is present in ameba an automatic regulating mechanism controlling the direction of movement so that when free from stimulation a spiral path is followed.

Where can such a mechanism be located? In organisms of fixed form, such as vertebrates, the mechanism controlling and coördinating locomotion is in the central nervous system. Even in some protozoa (Euplotes) a motorium has been found whose function apparently is that of coördinating the action of at least some of the motile organs (Sharp, ’13, Yocom, ’18). But in ameba there is no fixed form. The ameba is continually mixing itself up. No two masses of protoplasm ever occupy the same space relations to each other for more than a moment, excepting perhaps within the nucleus. But the nucleus as a whole is continually changing its position with regard to the rest of the ameba, and almost certainly its position at any given moment in the ameba is the result, not of its own activity, but of the endoplasm and the ectoplasm. A formed nucleus, moreover, is not necessary to concerted movement, for Protamoeba, in which no granules of chromatin have been found, and there certainly is no formed nucleus present, moves in a concerted manner, though I am unable to state definitely whether it moves in a wavy path. (I have seen this organism only a few times, and on none of these occasions was I able to make the test). It seems therefore possible that the agency responsible for the movement of amebas in flattened spiral paths can be located at any particular point within the ameba. It seems more likely that this mechanism is a spatial aspect of the intimate colloidal activity occurring in such changes of phase as are associated with the phenomenon of contractility and streaming.

Seeing then that movement in spiral paths is possible in animals not possessed of fixed morphology, it becomes of great interest to see whether the spiral paths of free swimming ciliates, flagellates, etc., are similar to those observed in amebas.

Although the spiral paths of flagellates and swarm spores were first studied by Naegli in 1860, and subsequently discussed by numerous botanists and zoölogists, it was not until Jennings in a number of papers (’98-’04) on the spiral paths of numerous species of one-celled organisms and rotifers, described the essential facts underlying spiral movement, that the significance of this method of locomotion began to be realized. His work marked the beginning of a healthy reaction against the conception of ridiculous simplicity of structure and function which had for several decades been settling upon these organisms. He showed that the spiral path is not a purposeless, senseless reaction on the part of these small organisms, but that it is fraught with meaning, and that it may be regarded as one of the most important of their many activities.

In a paper “On the significance of the spiral swimming of organisms” Jennings (’01) develops the thesis that spiral swimming is an acquired habit, an adaptation which has become fixed in these organisms so that they would not be condemned to swim in circles, which would necessarily follow from their asymmetrical form. The organism, in other words, swims in a spiral in order to be able to swim in a generally straight course. This explanation involves of course the supposition that the unsymmetrical shape of the body was developed first, and then, since this led to circular paths, revolution on the long axis became necessary in order that a straight course might be maintained.

But in the explanation of body form in one of the rotifers he (l. c., p. 376) says: “In some of these primitively bilateral animals this spiral method of swimming has resulted in the production of an unsymmetrical form analogous to that of the infusoria.”

It is of course quite possible theoretically, that some of the unsymmetrical structures on an organism that habitually swims in spirals, are the result of its spiral swimming, and that other structures which go to make the organism unsymmetrical, are the cause of the spiral swimming. This hypothesis is not an attractive one, however, for, because of the endless variety of asymmetrical differentiation in spiral swimming organisms, it would be impossible to tell for the large majority of organs or organelles whether they were the cause or the effect of spiral swimming.

Before taking up the hypothesis that all moving organisms are subject to the tendency to move in spiral paths, a hypothesis which accords with all the known pertinent facts, it may be well to examine the thesis that rotation on the long axis is an adaptation which has been developed to compensate for the effect of an unsymmetrical shape of the body.

It will be noted first that this question cannot be decided by direct observation or experiment. The entire body of real evidence is written in phylogeny, and that is for this purpose a closed book. It is only the interpretations of observations that bear on this problem, and it is these interpretations that it is of interest to examine.

Referring now only to the ciliates, all of which have numerous motile organs, it has been observed by numerous writers that cilia are not confined to one or two methods of contraction, but that there is great latitude in the extent and direction of their activity. This is very well illustrated by a paramecium or a stentor whose ciliary systems enable these animals to execute a great variety of maneuvers depending upon the character of stimulation, the amount of food in the body, etc. (Jennings, ’06, Schaeffer, ’10). The cilia are under the control of the animal in the same way as the legs and arms of a man are under his control. Now supposing that the bodies of these organisms became unsymmetrical during the phylogenetic history and as a result became unable to continue to swim in a straight path, the pertinent question to ask is: Was it easier for these organisms to learn to revolve on their long axis than to learn to beat their cilia a little harder on the side toward which they swerved? Observation of the forms before us does not afford any evidence that rotation was the easiest solution. Moreover, if it was an acquired habit, is it not strange that it should have been easier to acquire the rotating habit for every single species of the six or seven thousand unicellulars which now obey the spiral urge, as well as the swarm spores and zoöspores, than to change the beat of the cilia in some other way, in at least a few species? This explanation also makes inevitable the assumption that the ancestors of our present unsymmetrical protozoans were symmetrical and swam in straight courses without revolving, a condition of affairs which contrasts strongly with present conditions, for none of the most nearly symmetrical unicellulars and swarm spores now swims without revolving on the long axis. It is therefore exceedingly improbable that spiral swimming is the result of an acquired habit.

Now what evidence is there in support of the hypothesis that the spiral path is a necessary accompaniment of locomotion, except as it may be broken by the effect of stimulation?

As a problem in engineering, it is clear that the shape of the body is not responsible for the spiral course, for almost every conceivable shape is met with in organisms swimming in spiral paths. The frequent spiral turns in the path of stylonychia cannot be the result of the shape of the body, which is almost, if not quite, as well adapted for swimming through the water as is that of a euglena or a fish, but for revolution on its long axis it is not nearly so well adapted. Moreover, some of the euglenas turn the ventral or smaller lip out in the spiral turns, while others turn the dorsal or larger lip out (Mast, ’10). Since there is no other asymmetry of shape in these euglenas, it is clear that the shape of the body has nothing to do with causing the spiral path. The immediate cause of spirality must therefore be the work of the motile organ, and not the shape of the body.

Similar observations on paramecium have shown that it is the special action of the cilia of a paramecium that causes it to rotate and not the shape of the body. Again the shape of a Stentor caeruleus is subject to very great variation due to varying amounts of food eaten, and to surgical operation, but a spiral path is nevertheless maintained while the body shape undergoes marked changes.

Although all free-swimming unicellular organisms revolve on their long (antero-posterior) axis, an occasional one does not move in spirals. This is observed in the large colonial flagellate Volvox occasionally, but not always (Mast, ’10). Since it is more frequently seen in the larger individuals, it is probable that the formation of spirals is prevented because of the increased physical inertia of the colony; for the older and larger colonies are much more unsymmetrical than the younger and smaller, owing to the unequal distribution of the reproductive elements. Spondylomorum and several other colonial forms describe smaller spirals than smaller solitary organisms. These colonial organisms consisting of from four to twenty thousand cells, each of which may be possessed of cilia, are marvels of locomotory coördination, but it is not at all clear how this coördination is brought about. Since the colonies are symmetrical however, the spirality of the path is clearly due to the special action of the cilia.

Some organisms possess body shapes that seem to be due to the habit of spiral swimming. Jennings (’01) describes a species of rotifer whose body forms a segment of a spiral. When swimming a spiral path is described, “of which its own twisted body forms a part” (p. 376). Elsewhere he has pointed out that the oral groove of a paramecium likewise coincides with its own spiral path. Indications of such correspondence between the axis of a structure and the spiral path the organism possessing it, describes, are numerous among free swimming animals. But such correspondence (with an imaginary spiral path) is also found in organisms that do not swim freely. One of the most interesting of such cases is found in the Oscillatoriaceae. In a previous chapter it was seen that many of these organisms are capable of moving about by means of a film of what is probably protoplasm, which moves spirally around the filament. A particle attached to this film describes a spiral path like that of a flagellate or a ciliate. Most of the Oscillatoriaceae that are capable of movement, consist of straight filaments; but two of the genera, Arthrospira and Spirulina, are spirally twisted in such a way that the spiral axis of the filament corresponds approximately to the spiral path of a particle attached to the surface film of an Oscillatoria filament, except, of course, in size. (The movement of the surface film of neither Arthrospira nor Spirulina has been studied).

That the spiral shape of a rotifer, for example, may be caused by swimming in a spiral path might perhaps be regarded as a plausible explanation, but it seems to me that it would be more satisfactory to explain the spiral shape of rotifers and Arthrospira, the direction of the oral groove of paramecium and similar structures in other organisms, as due to the same fundamental process that causes the spiral path in locomotion. This explanation is purely mechanistic and avoids the teleological element on which the other explanation ultimately depends.

Most of the asymmetrical shapes of the flagellates, ciliates, rotifers, etc., have originated in phylogeny without regard to swimming in spiral paths, and indeed in spite of it. In spindle-shaped organisms like euglena or paramecium the amount of energy required to revolve on the long axis, as compared with that required for forward movement, is small. But in stylonychia, a dorso-ventrally flattened ciliate, much more energy is required to revolve the animal, proportionally, than is needed for forward movement. It is of course perfectly evident that as a problem in engineering it requires much more energy to revolve a flat plate on its long axis than a spindle-shaped solid, in a dense medium like water. But in spite of all the obstacles to revolution which asymmetry of body form presents, none of them are serious enough to prevent revolution from occurring, unless the keeled rotifer Euchlanis (Jennings, ’01) presents such a case. Observation would lead one to believe, however, that the compressed body forms of some of the hypotrichans and some of the flagellates such as phacus, have made revolution on the long axis very difficult; but not difficult enough to destroy the tendency to revolve and describe spirals. In short, these organisms spiralize in spite of asymmetry, not because of it.

A simple but decisive experiment by Jennings (’06) showed that the revolution and the forward movement of a paramecium is due to the oblique stroke of the cilia, for the severed posterior portion of a paramecium, which is symmetrical, nevertheless still revolves during progression. The question now arises whether this oblique stroke is analyzable into components in another way than by local stimulation; for example, can one increase or decrease the amount of revolution faster than the amount of progression? Observation of paramecium and euglena in different temperatures answers this question affirmatively. Organisms from the same culture were subjected to two temperatures, the culture temperature of 21° C. and 8° C. At temperatures lower than 8° C. the paramecia quickly precipitated to the bottom of the dish.

In 21° C. paramecia revolve once while swimming 5.5 body lengths.

In 21° C. euglenas revolve once while swimming 4.2 body lengths.

In 8° C. paramecia revolve once while swimming 3.6 body lengths.

In 8° C. euglenas revolve once while swimming ¼ to 2 body lengths.

The effect of decreased temperature is therefore to retard forward movement and to increase proportionally the number of spiral turns, for a revolution of the body on the long axis is the equivalent of one turn in the spiral path. It will be recalled that a similar result was obtained with amebas; in the lower temperature the rate of forward movement was reduced and the tendency to deepen the waves increased. In both these classes of organisms, differences in temperature enable one to separate the forward movement component from the spiral component, in the same way and in general to the same extent.

In clear water of optimum temperature or somewhere near it, paramecia and euglena (Euglena gracilis, which does not readily react to light) often swim for long stretches without change of direction. When the temperature is lowered, however, the stretches of straight paths become much shorter. In a temperature of 8° C. changes of direction become very frequent. In paramecium some of these changes are probably due to shock of some sort, judging from mere appearance; but in many cases the change of direction is preceded by a slowing up of forward movement and the swinging of the anterior end in a wide circle one or more times around. Occasionally one observes slow forward movement with wide swinging of the anterior end, for considerable distances. In euglena this condition is more marked than in paramecium; frequently the anterior end spins around with the posterior end as a pivot for several minutes at a time, in low temperatures.

These observations are strikingly analogous to the circles formed in the paths of amebas in low temperatures, and geometrically they bear the same relation to the spiral paths of ciliates and flagellates as the circles do to the wavy path of the ameba.

Besides the effect of temperature on paramecium and euglena, effects which are continuous and automatic, it is of course well known that the spiral path may be readily broken into by appropriate stimulation of the sense organs. The automatic locomotory mechanism is then for the time being controlled with reference to the character of the stimulus and the experience of the organism. But as soon as the effect of the stimulus has disappeared, the automatic mechanism again controls locomotion.

Sense organs of orientation, including organs of equilibration, break in upon the spiral mechanism controlling direction of movement, and eliminate its effect. It thus happens that no animals with image-forming eyes or equilibrating organs move in spirals in three-dimensional space when these organs are functional. Conversely, animals without image-forming eyes or equilibrating organs move in spiral paths. In addition to the ciliates, flagellates, protophyta, swarm spores and zoöspores of algae and fungi, Oscillatoriaceae and rotifers, may also be mentioned the larvae of many worms, echinoderms and molluscs. All these are within the grip of the spiral urge. The grip is indeed slight, as we have seen, but in the absence of stimulation it is none the less absolute.

The movements of none of the animals in the higher groups have been studied in any detail. Excepting the movements of some of the ciliates, flagellates, amebas, rotifers, a few scattered protophyta and swarm spores our knowledge of the movements of spiral swimming organisms is of the most casual and fragmentary sort. Nothing beyond the mere fact that these organisms describe some kind of a spiral swimming, is known.

That a spiralizing mechanism is probably also present in organisms with highly developed equilibrating and orienting senses would be the logical expectation from what has been said regarding the presence of such a mechanism in the lower forms of life; but the effect of such a mechanism would naturally be suppressed when the orienting senses are functioning. To test this point, man was selected for experiment. With eyes blindfolded and ears plugged (this latter precaution was subsequently found to be unnecessary) so as to render the orienting senses ineffective, a normal man was directed to walk straight ahead over a large field towards an object he had just looked at. Although a number of experiments were made with several individuals, none of them was able to walk a straight path. All of them walked true spirals or series of circles with remarkably smooth curves (Figures 45, 46). The spirals were right and left handed in the same individual, and sometimes in the same experiment. In these experiments the subject was totally unconscious of the direction in which he was walking. No effort of consciousness seemed capable of changing the degree of curvature of the spiral or circle and keep it smooth, though one could of course at any time break into the spiral or circle and walk off in another direction. (The writer himself walked in several experiments.) If one has one’s mind strongly on the direction of walking, thinking of each step, the curve of the path shows small “wabbles”; but if one recites something or counts his paces, the curves are quite smooth.

Considerable unevenness of the ground has no effect on the curvature of the spiral. Structural differences in the legs are also without effect, for a person with one artificial leg walks quite as smooth a spiral as one with two normal limbs.

From these experiments on man, it follows that there is a “centre” in the central nervous system which automatically coördinates and controls movement during locomotion and, particularly from the point of view of this discussion, the direction of locomotion when the orienting senses are not functioning. This center must be very deep seated and automatic, and in so far as its influencing the direction of locomotion is concerned, it is of no discoverable use to man. It may be presumed to have existed before the present orienting senses originated in man, for there is very good evidence that horses and perhaps dogs, too, possess this mechanism. For these animals, like man, tend to walk in circles when lost, a peculiarity of behavior undoubtedly due to the activity of this mechanism and not to stronger right or left legs, etc., as has often been suggested (e. g., Thompson, ’17, p. 498). According to the accounts of experienced hunters, rabbits also run in circles when hard pressed by hounds, which may possibly be due to the suppression of the functioning of the orienting senses by fear, thus allowing the automatic directing mechanism to operate.

The facts are therefore that all organisms without orienting senses or equilibrating organs, or animals possessing such organs which are rendered ineffective by some means, will not move in straight paths nor in any kind of irregular path, but in orderly paths, so that a given segment of the path serves as a basis for predicting the further direction of the path. And the degree of accuracy to which such prediction may attain is proportional to the extent to which the activity of the automatic regulating mechanism may be kept free from outside interference. The organisms of which this holds true include, as far as known, all the free-swimming unicellulars, swarm spores of algae and fungi, uni-and multinucleate zoöspores, rotifers, a large number of worms and worm larvae of all classes (excepting the nematodes) and the larvae of many molluscs, echinoderms and copepods as well as some adult copepods. Organisms restricted to two dimensions of space in their movements, in which orderly paths have been recorded, are ameba and man and perhaps we may include the horse and the dog. This is indeed only a small number of organisms compared with all that can move; but there are representatives in the list of all the large groups excepting the higher plants, and without doubt observation will greatly extend the list, for there are mentioned here only such organisms whose movements have been definitely recorded or personally observed. As far as now known, no organism lacking orienting organs moves in a straight line. Many spermatozoa with flagellate tails seem, however, to do so, but no careful studies of their paths have yet been made.

The orderliness of the paths of these organisms when moving under such conditions as described above, is itself orderly; that is, the path of all these organisms is a spiral of one kind or another: (1) a helical spiral, as in the free-swimming unicellulars; (2) a true spiral in one plane, as in man; (3) a helical spiral projected on a plane surface, as in ameba.

These facts point inevitably to the hypothesis that the movements of these and all other moving organisms are controlled by an automatic regulating mechanism, which is of essentially similar nature in all organisms, as is indicated by the tendency to spiralize the path. This mechanism, being automatic, absolutely controls the direction of the path so long as outside interferences permit; but when sensory stimulation occurs, or when changes in temperature, etc. occur, the mechanism is no longer able to operate automatically or smoothly. The direction of the path then depends upon the nature and direction from which stimulation was received, and upon the degree and direction of change of temperature, etc.

The importance of this conception of movement lies in the fact that it enables us to look at a large mass of otherwise unrelated data from a single point of view. Secondly, it permits of a mathematical treatment of the whole subject of movement in organisms. And third, it replaces a teleological explanation of spiral movement in unicellulars, swarm spores, rotifers, etc., with a purely mechanistic explanation.