Behavior of the Lower Organisms

The function of this positive contact reaction is evidently, under ordinary conditions, to procure food for the animal. But Paramecium shows no precise discrimination, and often reacts in this way to objects that cannot furnish food. Thus, if we place a bit of torn filter paper in the water containing the animals, we often find that they come to rest upon this, gathering in a dense group on its surface, just as they do with bits of bacterial zooglcea (Fig. 47). The oral cilia drive a strong current of water to the mouth, as usual, but this bears no food. To bits of thread, ravellings of cloth, pieces of sponge, or masses of powdered carmine, Paramecium may react in the same way. In general it shows a tendency to come to rest against loose or fibrous material; in other words, it reacts thus to material with which it can come in contact at two or more parts of

the body at once. To Smooth, hard Fig. 47. — Paramecia gathered materials, such as glass, it is much less in a dense mass about a bit of filter clearly shows a certain discrimination in this behavior. These hard substances, it is evident, are less likely to furnish food than the soft fibrous material to which Paramecium reacts readily. But under certain conditions Paramecium comes to rest even against a smooth glass surface, or against the surface film of the water. Specimens are often found at rest in this manner in the angle between the surface film of a drop of water and the glass surface to which it is attached.

Paramecia often behave in the manner just described with reference to bodies of very minute size, — to small bits of bacterial zooglcea, or to a single grain of carmine. Such objects are of course too small to restrain the movements that naturally result from the activity of the oral cilia in the contact reaction. These cilia continue to beat in the same manner as when the object is a large one, producing currents similar to those shown in Fig. 46. This ciliary motion of course tends to drive the animal forward, and since all the active cilia are on the oral side, it tends also to move the animal toward the aboral side. The resultant of these two motions at right angles is movement in the circumference of a circle. The animal moves in the lines of the water currents shown

in Fig. 46, but in the opposite direction ; it is, as it were, whirled about in its own whirlpool. The resulting path is shown in Fig. 48. This circular movement, with the oral side directed toward the centre of the circle, is seen only in specimens showing the contact reaction to objects of minute size. The contact reaction modifies strongly the reactions to most other stimuli ; this is a matter which will be taken up later. Thus when Paramecium comes in contact with a solid object, it may react in three different ways. First, it may react either positively or negatively, this depending partly on Fig. 48. — Circular path followed by the intensity of the stimulus, partly

Paramecium in reacting to contact with a Qn h physiological condition of the minute particle. r J ° Paramecium. If it reacts negatively, this reaction may take either one of two forms. If the stimulation occurs at the anterior end, the animal gives the avoiding reaction; if it occurs elsewhere, the animal merely moves forward. The reactions to chemical stimuli occur through the avoiding reaction described in the preceding chapter. As we have seen, the avoiding reaction is produced as a rule by a change from one chemical to another. With regard to this relation, there are certain facts of importance.

In all cases a certain amount of change is necessary to produce reaction ; that is, the chemical must be present in a certain concentration before reaction is produced. The sensitiveness of different individuals varies greatly, and even that of given individuals changes much with changes in the conditions. It is therefore not possible to establish for any given chemical the weakest concentration that causes the avoiding reaction. But the animals when in ordinary water are very sensitive to the common inorganic chemicals, reacting to very weak solutions. Thus the weakest solutions causing reaction have been found to be for various chemicals about as follows : —

Sodium chloride, -^ to ^. per cent (-^ to yg-Q normal) ; potassium bromate, about -^ per cent ; sodium carbonate, about -^jTo to 3TT0 Per cent ; copper sulphate, about -g-^g- per cent ; potassium hydroxide, about 2^ per cent ; sodium hydroxide, 5-^y per cent ; sulphuric acid, -g-^-Q- per cent of an ordinary laboratory7 solution ; hydrochloric acid, g jj-q per cent of the usual solution ; alcohol, 1 per cent ; chloral hydrate, -§ per cent. For the inorganic chemicals, many of these solutions are so weak as not to affect at all the sense of taste in man.

Is the reaction of Paramecium to solutions due to the chemical properties of the dissolved substance, or to its osmotic pressure ? This question may be answered from the data which we possess (partly given above) as to the weakest solutions which cause reactions. If the reactions are due to osmotic pressure, then solutions having equal osmotic pressure must have equal stimulating power. The results of the experiments on the weakest solutions necessary to cause reaction show that this is not true. Thus, if the osmotic pressure of a solution of sodium chloride that will barely cause the reaction is taken as unity, the osmotic pressures of solutions of a number of other substances having the same stimulating effect are as follows : potassium bromate, ^ ; sodium carbonate, -^ 5 copper sulphate, 217-3 5 potassium hydroxide, -^-q ! sulphuric acid, -^ 5 ethyl alcohol, 8. The stimulating effect is not then proportional to the osmotic pressure, and must be due to the chemical properties of the substances in solution.

This is further shown by the fact that Paramecia will enter solutions of sugar and of glycerine having osmotic pressure many times as great as that of a solution of sodium chloride which they avoid. They swim into a 20 per cent solution of sugar or a 10 per cent solution of glycerine without reaction. The solutions are so concentrated that they cause plasmolysis; the Paramecia shrink into flattened plates. Just as the shrinking becomes evident to the eye of the observer, the Paramecia react in the usual way, by swimming backward and turning towards the aboral side. But this is as a rule too late to save them, and they die in the dense solution. Thus it is evident that osmotic pressure, acting by itself, produces the same "avoiding reactions" as do other stimuli, but the result is not produced till the Paramecia are already injured beyond help. The reactions to most solutions are then clearly due to their chemical properties.

Is the avoiding reaction that is produced by chemicals due directly to the injuriousness of the substance? This question may be answered by a series of experiments based on a method similar to that used in determining whether the reaction is due to osmotic pressure. If the reaction is due to the injuriousness of the chemicals, then two substances which are equally injurious must have equal powers of inducing reaction ; in other words, the repelling powers of any two substances must be proportional to their injurious effects.

An extensive series of experiments has shown that this is not true (Jennings, 1899 c; Barratt, 1905). We may compare, for example, the effects of chromic acid and of potassium bichromate. The weakest solution of the former which kills the Paramecia in one minute is ^-ifo per cent; the weakest solution of the latter having the same effect is 1 per cent. Hence the chromic acid is 150 times as injurious as the potassium bichromate. On the other hand, the weakest solution of chromic acid that sets in operation the avoiding reaction is still j^-q per cent, while potassium bichromate has the same effect in a -^ per cent solution. The repellent power is thus not proportional to the injurious effects; the potassium bichromate is repellent in a strength -^ that which is immediately injurious, which chromic acid does not repel until it has reached a strength that is already destructive. Similar relations are found for other pairs of substances. Thus the stimulating power of sodium chloride is ten times that of cane sugar, in proportion to its injuriousness.

Comparing a large number of chemicals from this point of view, it has been found that they may be divided into two classes. On the one hand are a number of substances which must be classified with potassium bichromate and sodium chloride, because their stimulating power is strong in proportion to their injurious effects. Paramecia avoid these substances markedly ; if a drop of a strong solution of one of them is introduced into a preparation of the infusoria, it remains empty, and none of the infusoria are killed by it. On the other hand, there is a large number of substances which, like chromic acid and sugar, produce stimulation only where they are strong enough to be immediately injurious. When a strong solution of one of these is brought into a preparation of Paramecia, it proves very destructive, for the animals as a rule do not react until they have been injured. The following table (from Jennings, 1899 c) shows the distribution of various chemicals from tills point of view : — -

r. Repellent power strong in proportion to injurious effects ; reaction protective. Ca(NO,),„ Sr(N03)2, Ba(NO,).„ Potassium bromate, Potassium permanganate, Potassium bichromate. Potassium rerricyanide, Ammonium bichromate. 2. Repellent power very weak in proportion to injurious effects ; reaction not completely protective. HF, HC1. HBr. HI. H,S04, HNO„ Acetic acid, Tannic acid. Picric acid. Chromic acid. Ammonia alum, Ammonioferric alum. Chrome alum. Potash alum, CuS04, CuCl,, ZnCl,, HgCl,, A1C1,, Copper acetate. Cane sugar, Lactose. Maltose, Dextrose, Mannite, Glycerine, Urea.

This table shows that the relative repellent power of different substances bear a somewhat definite relation to their chemical composition. All alkalies and compounds of the alkali and the earth alkali metals (save the alums, where the proportion of the metals is very small) have a relatively strong repellent effect ; most other compounds have not. While our general result is that the stimulating powers of different chemicals are not proportional to their injurious effects, yet one further fact of importance comes out clearly. All substances, whatever their nature, do produce, as soon as they become injurious, the avoiding reaction. With all the substances in the second column the avoiding reaction is produced when a strength sufficient to be injurious is reached and the reaction seems clearly due to the injuries produced. The significance of this fact will be discussed later.

In the chapter preceding the present one, we have seen that Paramecia collect in certain chemicals, owing to the fact that passage out of these causes the avoiding reaction. The two chief classes of chemicals in which the animals collect are acids and oxygen. Paramecia collect in all weakly acid solutions, no matter what acid substance is present. Sulphuric, hydrochloric, nitric, hydriodic, and many other inorganic acids; acetic, formic, carbonic, propionic, and other organic acids, have been tested, and the animals have been found to gather in all. The Paramecia collect even in solutions of poisonous acid salts, such as corrosive sublimate and copper sulphate, where they are quickly killed. In all these cases they swim into the solution without reaction, but give the avoiding reaction at passing out. They give the avoiding reaction also after the injurious chemical begins to act on them, but under the circumstances this does not save them from destruction.

It seems remarkable that the animals should thus tend to gather in acids, when, as is well known, the decaying vegetable infusions in which they live are usually alkaline in character. Specimens in water that is decidedly alkaline collect even more readily in acids than do those in a neutral fluid. A solution may contain both an acid and a repellent substance, as when ^g- per cent acetic acid is mixed with \ per cent fig. 49. - Collection of Para-

Sodium chloride. In this Case a CUrioUS mecia about the periphery of a effect is produced. The Paramecia gather mixture of salt and acidin a ring about the outer edge of the solution, as in Fig. 49. They are repelled both by the inner fluid and the surrounding water. The path of a Paramecium in such a ring is similar to that shown in Fig. 50. Strong acid solutions cause the avoiding reaction as do other chemicals. If a drop of strong acid solution is introduced into a preparation of Paramecia, the animals collect about its periphery, where the acid is diluted by the surrounding water, just as in Fig. 49. Individuals which swim against the inner strong acid respond by giving the avoiding reaction in a very pronounced way, — swimming far backward and turning toward the aboral side, for perhaps two or three or more complete turns. They react also at the outer boundary of the acid ring, so that within the ring the individual Paramecium follows such a path as is shown in

Often the reaction is not produced at the inner • ,Fl?" 51°-7Patho.an boundary of the ring, by the strong acid, until the in such a ring as is shown Paramecium has entered far enough to be injured, in Fig. 49. or even killed. A drop of strong acid introduced into a preparation is usually soon surrounded by a zone of dead animals. Acids, as we have seen (p. 64), belong with those substances which do not produce the avoiding reaction till they have become directly injurious.

Paramecia do not, under usual conditions, collect in oxygen. If we introduce an air bubble or a bubble of oxygen into a slide preparation of Paramecia, they do not as a rule collect about it. But if the outer air is excluded from this preparation by covering its edges with vaseline, and it is allowed to stand for a long time, the behavior changes. The oxygen has of course become nearly exhausted and now the Paramecia gather about the air or the oxygen. The collections are formed in exactly the same way as are those in acids.

Thus the experiments show that all reactions to chemicals take place through the avoiding reaction, and this reaction is produced by a change in the intensity of action of the chemical in question. With some chemicals, or under certain conditions, it is a change to a greater intensity that produces the avoiding reaction ; in other cases it is a change to a less intensity that produces the reaction. With acids both an increase and a decrease beyond a certain intensity produce reaction. We may express the facts for all chemicals in the following general way. For each chemical there is a certain optimum concentration in winch the Paramecia are not caused to react. Passage from this optimum to regions of either greater or less concentration causes the avoiding reaction, so that the animals tend to remain in the region of the optimum, and if this region is small, to form here a dense collection. For acids and for oxygen the optimum is a certain very low concentration. For

most other chemicals the optimum is zero ; an increase in intensity by any effective quantity produces the avoiding reaction, while decrease in intensity has no effect. Hence the Paramecia tend to collect where none of thechemical is present. The point needs to be brought out clearly that it is not merely passage from the absolute optimum that induces reaction, but passage in a direction leading away from the optimum. To constant conditions, even when not optimal, Paramecium becomes acclimatized ; it may live for example in a jJq per cent salt solution, though passage from water to this causes reaction. While in this salt solution, passage into conditions lying still farther from the optimum, as into \ per cent salt solution, causes the avoiding reaction, while passage to conditions lying nearer the optimum produces no reaction. Ac- climatization to non-optimal conditions is an ever present factor in the behavior of the organisms. This is another way of stating the fact that change is the chief factor inducing reactions.

Acids then take a peculiar position among chemicals merely in the fact that a certain positive concentration forms the optimum, passage to a lower concentration inducing reaction. The peculiar behavior of Paramecium with respect to acids plays a large part in its life under natural conditions. Paramecia produce carbon dioxide in their respiratory processes as do other organisms. This substance when dissolved in water produces an acid solution, the acidity being due to carbonic acid. In such a solution Paramecia gather as in other acids. This may be shown by introducing, by means of a capillary pipette attached to a rubber bag containing the gas, a small bubble of carbon dioxide into a slide preparation of Paramecia. The infusoria quickly gather in a dense collection about the bubble, at first pressing closely against it (Fig. 51, ,4). Later the Paramecia spread out with the diffusion of the carbon dioxide (B). After a time the animals are usually found chiefly about the margin of the area containing the carbon dioxide (C).

Fig. 51. — Collection of Paramecia about a bubble of CO-t. a is a bubble of air, b of CO2. A shows the preparation two minutes after the introduction of the CO2; B, two minutes later; C, eighteen minutes later. Now, the Paramecia gather in the solution of carbon dioxide produced by themselves, just as in that due to other causes. In this way dense spontaneous groups are formed, in which the phenomena seen in the collections about bubbles of carbon dioxide are reproduced. If a large number of Paramecia are mounted in water on a slide, they do not remain scattered, but soon gather in one or more regions (Fig. 52). Within such groups the individuals move about in all directions. On coming to an invisible outer boundary, they give the avoiding reaction in a mild form, so that they do not leave the group. The area covered by the group does not remain of the original size, but slowly enlarges, as shown in Fig. 52. It continues thus to increase in size until it covers the whole preparation.

By the use of proper indicators it can be shown that such spontaneous groups contain an acid, and this is beyond doubt due to the carbon dioxide known to be produced in respiration. The groups are formed in the following way. Two or three Paramecia by chance strike against some small, loose object, a roughening of the surface of the glass, or the like, and come to rest, in the way described in our account of the reaction to mechanical stimuli. They of course produce Q (•'.**". .".V5 /.'•' carbon dioxide, which diffuses into the

surrounding water. Other Paramecia that swim by chance across this area of carbon dioxide of course stop and remain. They too produce carbon dioxide, so that the area grows in size ; more Paramecia enter it, and finally a large and dense collection is formed. The area occupied by such a collection continually increases in size, because the Fig. 52.— Spontaneous groups formed Paramecia continue to produce carbon in the spreading out of such groups. dioxide, and this continues to diffuse

through the water. The tendency of Paramecia to gather in regions containing carbon dioxide plays a large part in their life under natural conditions, and this, together with the fact that they themselves produce carbon dioxide, explains many peculiar phenomena in their behavior. When placed in tubes or vessels of any kind, Paramecia usually show a tendency to collect into groups or clouds, having a definite boundary (Fig. 53). This is of course a result of their reaction to carbon dioxide produced by themselves. In all experimental work on the reactions of these organisms to stimuli it is necessary to take these |V__> facts into account. For example, in order to get clear results in such work, Param ecia must not be taken with a pipette directly from a dense collection in a culture jar, and at once mounted on a slide. Such collections contain carbon dioxide, which may become unequally distributed throughout the preparation, as a result of the fact that some of the water outside the collection is likely to be taken up with the pipette at the same time. The Paramecia quickly gather in the region containing most carbon dioxide, and their reactions to other substances are inconstant and irregular, owing to the interference due to the reaction to carbon dioxide (Fig. 53, B). For experimental work it is always necessary before each experiment to place a few drops of the water containing the Paramecia in the bottom of a shallow watch-glass, and to aerate it thoroughly by stirring it and bringing it into contact with the air by means of the pipette. Then this aerated water and its contained Paramecia must be used for the experiments. This aeration must be repeated before each experiment, and the test for the reaction to other chemicals must be made immediately after the Paramecia are mounted, before they have had time to produce an appreciable quantity of carbon dioxide. If these precautions are neglected, the reactions of the Paramecia are inconstant, and the results of experiments are likely to be very misleading. Paramecia in a solution of carbon dioxide react to other agents in a manner entirely different from the reaction of individuals in water not containing carbon dioxide.

The account of their reactions given in the present chapter assumes that the carbon dioxide has been in every Fig. 53. — Spontaneous collections of Paramecia, due to COo. A, Collections formed in an upright tube, after Jensen. B, Collection formed beneath a cover-glass, when water is taken directly from a dense culture of Paramecia. C, Collection in the bottom of a watch-glass. case removed from the water. The experimental results described cannot be verified unless this is done.

In general it cannot be too much emphasized that in all experimental studies on the behavior of Paramecium close attention to their reactions with reference to carbon dioxide is necessarv. When inconstant results are obtained, or results seeming to contradict those noted by other observers, it will often be found that inattention to the carbon dioxide produced by the animals is at the bottom of the difficulty. As we saw in the preceding chapter, a change to a temperature decidedly above or decidedly below the optimum causes Paramecia to give the avoiding reaction, while a change leading toward the optimum does not. As a result the animals collect in temperatures as near the optimum as possible.

The effects of heat and cold differ slightly, since heat increases the rapidity of movement, while cold reduces activity. Both produce the avoiding reaction in the same way, but in heated water the reaction is continued violently till the animals escape or are killed, while in ice water the animals after a time become benumbed and sink to the bottom. The reactions to heat and cold are seen in a striking way when the Paramecia are placed in a long tube or trough, one end of which is heated while the other is maintained at the normal temperature, or is cooled. The Paramecia then pass to the region that is nearest the optimum, forming here a collection. By changing the temperature of the ends or of the middle, the Paramecia may be driven from one end to the other or caused to gather in any part of the trough. Such experiments were devised by Mendelssohn (1895, 1902, 1902 a, b). He passed tubes beneath the middle and ends of the trough or slide bearing the animals, and through these tubes he conducted water of different temperatures. By changing the connections of the tubes, that end of the trough which is at first heated may later be cooled, etc., without disturbing the animals in any other way.1

If in this way we heat the water at one end of the trough to 38 degrees while we cool the opposite end to 10 degrees, the Paramecia collect in an intermediate region. By varying the temperatures at the two ends, the infusoria may be driven back and forth, as represented in Fig. 54, taken from Mendelssohn. By grading the temperatures properly, the sensitiveness of Paramecium to changes in temperature may be measured, and the optimum temperature determined very accurately.