The Psychic Life of Micro-organisms: A Study in Experimental Psychology

"The Leucophrys patula is a type distinctively carnivorous and possessed of an extremely voracious appetite, a fact which explains its power of multiplication, one of the greatest I have studied. With a temperature of 25° in my laboratory I have recently seen it separate by fission seven times in twenty-four hours, that is to say, a single individual produces from itself just one hundred and twenty eight others in that time. In constant pursuit of its prey, it seizes its victims by the two stout vibratile lips with which its mouth is armed, and swallows them alive and whole. The victims may be seen struggling and tossing about

for a time in the interior of the Leucophrys's body and afterwards to expire slowly under the action of the digestive juices of the vacuole in which they have been enclosed. Placed in a medium well-stocked with small Ciliates, the Leucophrys have their bodies constantly crammed with victims swallowed in the manner above described. Like the other hunter Ciliates the Leucophrys does not espy its victims from a distance and does not guide itself towards them. It simply darts about from right to left, every moment changing its direction. It thus increases its chances of coming in collision with its prey and every time that one of its unfortunate victims falls in contact with its vibratile lips, it is seized, irresistibly drawn towards the mouth and swallowed within less than a tenth of a minute."

Certain hunter Infusoria have methods of pursuit and capture which deserve to be examined separately. Claparede and Lachman in their excellent work upon Infusoria and Rhizoppds, have minutely described the manner in which a large Infusory, the Amphileptus Meleagris, attacks the Epistylis plicatilis. The Epistylis are colonizing vorticels of which certain individual members attain a size of not less than 0-21 mm. The Epistylis form aborescent groups, the ramifications of which are quite regularly dichotomous. These ramifications all grow at exactly the same rate and the individual branches all rise to the same height, representing what is called, in botany, a corymbous inflorescence. "We were observing one day," says Claparede, "in the hope of seeing what would come of the manosuvre, an Amphiltptus> which was slowly creeping upon a colony of Epistylis. The way in which it approached the Vorticels, feeling them, so to

speak, and partly enclosing them in its pliable body, already seemed suspicious. At last, it made a direct attack upon one of them by fastening itself upon the upper part of its body. It opened its huge mouth, which is never to be seen except when the animal is eating, and slipped over the Epistylis like the finger of a glove being drawn upon a finger of the hand. We saw the sides of the buccal aperture (which are capable of being dilated in a truly astonishing manner) slip slowly over the peristome and upon the body of its prey, and then draw together about the point where it was made fast to the pedicle. The cilia covering the body of the Ampfiileptusbeg&n to shake with that peculiar motion which is always noticed when a ciliated Jnfusory secretes a cyst. At the expiration of a moment or so, a fine line was seen to appear around the whole body which continued to spread so as soon to form the cyst." " (This might be called a cyst of digestion.) "The phenomenon as a whole is quite simple. An Amphileptus approaches an Epistylis devours it and encysts itself upon the spot, the victim being still attached to its pedicle. It then endeavors to wrench the Epistylis from its point of attachment by twisting; it turns on its axis from left to right and then from right to left, successively; when it has succeeded, it continues its work of digestion, and occasionally divides in two within the cyst itself. During the last stage of digestion, it rests for a while, when it commences again to turn abput in the cyst, evidently seeking to disengage itself. At the close of a certain number of hours, the cyst breaks. The Amphileptus issues forth and starts in quest of another victim."*

The hunter Infusoria are frequently armed with tricho cysts. Trichocysts are urtical filaments which serve the animalcula provided with them to disable or wound other micro-organisms. A large unmber of Infusoria, the Paramecia, the Ophryoglena, etc., use their trichocysts as organs of defense. With other species, of which we shall speak more at length, the trichocysts are organs of offense. They are located either in the sides of the mouth or in parts adjacent thereto; this is the case with the Lacrymaria, the Didinium, the Enchelys, the Lagynus, the Loxophyllum, and the Amphileptus.

These latter animalcula attack the live prey that constitutes their food, in the following manner. They dash upon their victim and bury the trichocysts with which they are armed, into its body. The Victim is immediately brought to a halt, whereupon the hunter seizes it and swallows it. So, when the Lagynus Elongatus intends to seize a victim that has fallen into its vortex and has thus been drawn into the neighborhood of its mouth, it throws itself swiftly forward. At the moment of contact the hunted Infusory becomes suddenly paralyzed and remains perfectly motionless. This paralysis is evidently caused by the trichocysts which line the aesophagus of the Lagynus and with which the latter has transpierced its prey at the moment it came in contact by its anterior extremity.*

In a higher stage of organization, the Microzoon possessing a mouth changes its position in order to intercept its prey, and give it chase. The Didinium Nasutum (Stein), a carnivorous In- fusory and one of the most voracious of our fresh stagnant waters, operates in a more complicated manner: The Didinium (fig. 7), as regards the general shape of the body, may be compared to a diminutive cask, rounded off at one of the ends and terminated at the opposite extremity by an almost level surface from the mids.t of which rises a conical projection quite strongly

Fig. 7.—D.idinium nasutum, enlarged marked. This projection two hundred diameters. The figure rep- . resents a Didinium overpowering a Pa- IS an Organ of deglutition ramtecium aurelia. The nettle-like filaments discharged by the Didinium are (swallowing) : a lonSfitU- seen on all sides of the Paramczcium; while the latter, already seized by the dinal StriatlOn IS noticed tongue-shaped organ of the Didinium, is being gradually drawn towards the buchere formed of minute cal orifice (after Balbiani).

solid rods, of extreme tenuity and independent of the sides. These organs are the weapons used by the Didinium in attacking the live prey which constitutes its sole nourishment. Not only does it attack and devour animalcula almost as large as itself, but frequently it even seizes individuals of its own kind. In such cases it is always Infusoria, and never the Rotatoria, although the latter often abound in waters which the Didinium inhabits. It appears, moreover, to have a marked predilection for certain species; and so it happens that the huge and inoffensive Pafamcecium aurelia is almost always its choice by preference among the animalcula that inhabit the same liquid.*

* The Didinium, Balbiani tells us, never attacks the Parma-cium bursaria, which is distinguishable from the P. aurelia by its green coloration. The prehension of food by the Didinium exhibits interesting aspects, which have not as yet been observed in any other Infusory. M. Balbiani, in his first observations, had often been surprised at seeing animalcula that the Didinium had passed by without touching, suddenly stop as if violently paralyzed; whereupon our carnivorous specimen straightway approached and seized them with seeming facility. More careful examination of the Didinium's actions soon furnished the key to this enigma. If, while swiftly turning in the water, the Didinium happens into the neighborhood of an animalculum, say a Paramecium, which it is going to capture, it begins by casting at it a quantity of bacillary corpuscules which constitute its pharyngeal armature. The Parmecium immediately stops swimming, and shows no other sign of vitality than feebly to beat the water with its vibratile cilia; on every side of it the darts lie scattered that were used to strike it. Its enemy then approaches and quickly thrusts forth from its mouth an organ shaped like a tongue, relatively long and resembling a transparent cylindrical rod; the free, extended extremity of this rod it fastens upon some part of the Paramecium's body. The latter is then gradually brought near by the recession of this tongueshaped organ towards the buccal aperture of the Didinium, which opens wide, assuming the shape of a vast funnel in which the prey is swallowed up.*

Up to this point we have paid little attention to movements of defence and of flight. Upon this subject a few words will suffice. When vorticels are alarmed, they are seen to contract forcibly their pedi- * Archives de zoologie experimental, 1873, Vol. II, p. 363. Observations sur le Didinium nasutum, by E. G. Balbiani. cle, which in a state of rest stays extended. Infusoria placed in a preparation where they are at their ease, swim quietly about; if any sharp excitation disturb them, they accelerate their pace; those armed with a rigid bristle at the posterior extremity, rush precipitately onward whenever another Infusory chances to touch that tactile appendage. The unaggressive Parmecia, when attacked, endeavor to escape, but are also able to defend themselves by means of the trichocysts with which their ectosarc is armed.

Unicellular organisms do not all live in a detached state; a. large number of species are found grouped together in colonies; the initial basis of these agglomerations is always a mother cell, the offspring of which instead of dispersing to live at large, remain agglutinated to one another. Ehrenberg had believed that in certain species (especially in the case of the Anthophysa vegetans, an aggregation of minute monads growing as a sort of bush) the colony was created by the union of minute organisms that originally lived at large; but observation has shown that his theory was incorrect. It may be laid down as a general rule that every colony of monocellular animals or vegetables spring from the divisions of a single cellule. The cellules of one and the same colony, therefore, are always sister cellules, and the colony represents a family in miniature.

A leading instance of a colony wholly temporary, is found in those organisms the cuticle of which does not take part in the phenomena attending the division of the protoplasm. In this case, the protoplasm beneath the envelope .alone divides; the segments resulting therefrom are often numerous, and it is not until the plasma has finished dividing that the maternal cuticle is destroyed and that the segments separate to live abroad in a detached state. Up to that time they remain bound together.

It is thus seen that the existence of this minute colony is a transient phenomenon, which lasts only during the time necessary for the division of the maternal body. These phenomena have been noticed among many of the Flagellates. What appears surprising is, that the maternal cellule, although continuing to divide beneath the envelope, keeps on moving about in the water by means of its own flagellum as if still constituting only a single animal. The reason of this is that one of the segments into which the plasm is divided and which is situated in the anterior part of the mother-cellule, remains connected with the flagellum and takes charge of its movements. This segment (like an individual distinct in itself) alone guides the bark that carries its sisters. And so, although this diminutive colony is as a rule but short-lived, a division of labor has 'been effected among its members; the anterior segment is alone entrusted with the office of locomotion.

The colony has a duration less ephemeral in the case of the Gonium pectorale, a Volvocine known in our fresh waters. It is formed by the aggregation of sixteen individuals which remain detached but adhere laterally to one another. The colony is developed in one way only: it is in the form of a minute rectangular plate of a beautiful green color. In the case of the Pandorina, the colony assumes the form of a minute sphere; it is composed of sixteen, or as many as thirty-two individuals, joined together beneath a

stout envelope; each member remains free in action, and projects its two flagella through the cuticle. With the Eudoryna elegans, the colony is modeled upon nearly the same plan excepting that it is composed of thirty-two individuals and that the latter, placed beneath the same cuticle at equal distances apart, do not touch one another. In the genus Volvox, colonies are found of which the structure is very complicated. Such are the great green balls formed by the aggregation of diminutive organisms, which form the surface of the sphere, and are joined together by their envelopes; they have each two flagella, - which pass through the enclosing membrane and swing unimpeded on the outside; the envelopes, each tightly holding the other, form hexagonal figures exactly like the cells of a honeycomb. Each Volvox is at liberty within its own envelope; but it projects protoplasmic extensions which pass through its cuticle and place it in communication with its neighbor. It is probable that these protoplasmic filaments act like so many telegraphic threads to establish a network of communication among all the individuals of the same colony; it is necessary, in fact, that these diminutive organisms be in communication with each other in order that their flagella may move in unison and that the entire colony may act as a unit and in obedience to a single impulse. The number of micro-organisms constituting a Volvox colony is quite considerable: as many as 12,000 have been counted.

It was upon analogous phenomena that Gruber based the existence of a diffused nervous system in the Stentors. The same line of reasoning may be followed in the case of the Volvox. Since unanimity of movement is demonstrable among twelve^ thousand micro-organisms constituting a colony, it must be inferred that their movements are regulated by the action of a diffused nervous system present in the protoplasm. This conclusion is all the more interesting from the fact that these Volvox are vegetable micro-organisms.

In the dicecian Volvox, the female cellules and the male cellules are joined together by themselves in separate colonies. When the time of fecundation arrives, the male cellules or antherozoids scatter and proceed to conjugate with the female cellules. The colony which bears the female cellules also contains neutral cellules which are not designed for fecundation; the latter simply perform a locomotive function; equipped with one eye and two flagella, they are intended to move the great colonial ball: they are the oarsmen of the colony. The Volvox, male, female, and neutral, all seek the light, whether solar or artificial, and settle near the surface of the water. As soon as the female colonies have been fecundated, the oospores change their color: they turn -from green to an orange yellow. At this point, the colony is seen to draw away from the light and to disappear from the surface of the water. This change of position is effected by means of the vibratile cilia with which each neutral cell is furnished and which project beyond the gelatinous sphere; now, as no change of color or form is noticed in the neutral cells after fecundation, it may be asked from" what cause they flee from the light which they formerly sought.

Colonies of Proto-organisms formed by the division of a mother cell of which the segments remain united, are not entirely without analogy with a pluricellular organism which likewise springs from a single cell called the egg, and the resultant divisions of which do not separate. The colony constitutes in a way a first step towards the physiological constitution of a pluricellular organism; it serves to fix a stage of transition in the animal kingdom, between Protozoa and Metazoa. A fact which strengthens this analogy is, that certain colonies, as the Synura uvella and the Uroglena volvox, can divide into two other colonies; strangulation acts upon the mass just as if upon a pluricellular organism. This curious observation was made by Stein and Biitschli.

Nevertheless, an essential difference still separates the Metazoa and the Protozoan colonies, even when in these colonies a division of function has been established among several individual groups. The physiological differentiation brought about in these Protozoan colonies is the result of a mechanism which differs in every respect from that by which it is effected in the case of the Metazoans. In the latter instance the differentiation results from the division of the embryo into germinative folia each of which is the origin of a separate group of organs. At a certain stage of development, the superposition of these folia gives rise to the formation of a gastrula; the gastrula is formed by two folia joined together, representing a pouch open to the outside; it is characteristic of Metazoans, the Protozoan never reaching this stage. Certain colonies observed by Haeckel, the Magosphcera planit la for example, and the volvox, of which we have before spoken, appear in the form of a sphere; they suggest an anterior stage of development to which the name of morula or of bias tula has been given; but they do not get beyond this stage.

We have now considered assemblages of organisms which live joined together like the Gonium and sometimes united by a material band like the Volvox, where the individuals are grouped together under one and the same cuticle. Voluntary and free combinations are much more rarely met with; nevertheless cases occur. There exist organisms which lead a life of habitual isolation but which understand how to unite for the purpose of attacking prey at the desired time, thus profiting by the superiority which numbers give.

The Bodo caudatus is a voracious Flagellate possessed of extraordinary audacity; it combines in troops to attack animalcula one hundred times as large as itself, as the Colpods for instance, which are veritable giants when placed alongside of the Bodo. Like a horse attacked by a pack of wolves, the Colpod is soon rendered powerless; twenty, thirty, forty Bodos throw themselves upon him, eviscerate and devour him completely (Stein). All these facts are of primary importance and interest, but it is plain that their interpretation presents difficulties. It may be asked whether the Bodos combine designedly in groups of ten or twenty, understanding that they are more powerful when united than when divided. But it is more probable that voluntary combinations for purposes of attack do not take place among these organisms; that would be to grant them a high mental capacity. We may more readily admit that the meeting of a number of Bodos happens by chance; when one of them begins an attack upon a Colpod, the other animalcula lurking in the vicinity dash into tne combat to profit by a favorable opportunity.

It is difficult in the extreme to mark out the lines of a psychology of Proto-organisms from data so incomplete as those we have just collected. We shall content ourselves with a few brief considerations. The apparent result of our investigations up to this point is, that the greater number of movements and actions observed in Micro-organisms are direct responses to excitations emanating from the medium in which they live. It is the condition of the medium that, to all appearance, rigidly determines the character and manner of their activity; in a word, they exhibit no marks of pre-adaptation.

But it will not do to let the* matter rest with this general survey of the subject; we shall have to examine more closely each detail of these reflex actions of adaptation, beginning with the sensory phase and ending with the motory phase. Analysis discloses that several determining elements may be distinguished in these phenomena; they are: 4. Movements calculated, either to approach the body and seize it, or to flee from it. We are not in a position to determine whether these various acts are accompanied by consciousness or whether they follow as simple physiological processes. This question we are obliged, for the present, to forego.

i. The perception of an external body. Among the lowest forms, it appears that perception is always the result of a direct irritation produced by contact of the external body with the protoplasm of the animalcule. This is what takes place, to all appearance, among the Amoebae; for these organisms, the condition necessary to the perception of a solid particle is contact with it. A step forward has been effected in those organisms that are able to perceive external objects by contact from a distance, as is observed for instance in the Actinophrys, which perceives all bodies that chance to touch its long filamentous pseudopods; yet, in this instance, the pseudopod merely acts the part of an extended tactile organ. The vibratile cilia, and still more the long lash of the Mastigophores, enable the animal to discern the presence of contiguous particles at a certain distance from its body, by the pressure exerted upon their appendages. It is not known whether there are many animalcula that perceive the presence of nutriment from a distance and without coming in direct contact with it; it appears, however, that this is the case with the Didinium which shatters its prey from a distance and without touching it.

2. Choice. We have seen that Micro-organisms do not absorb indiscriminately every solid particle they meet. They exercise -a choice. Among the lower species, the choice is in -the lowest degree rudimentary; the organism restricts itself to a discrimination of mineral particles, sand for example, from organic substances; it rejects the former and absorbs the latter. Among the higher animalcula. the choice is more intelligent. There are InfusoriaAhat feed only upon plants and animals. There are also those which feed exclusively upon one species.

This exercise of choice is one of the most incomprehensible of phenomena; it is exceedingly difficult to explain it without resort to anthropomorphism. If we hold to what observation directly teaches us, the choice may be said to consist in the following acts: when the animalcule perceives certain kinds of substances and particularly those substances which serve it as customary food, it invariably goes through the same movement, which consists of an act of prehension; when the substance touched, seen, or collided with, as the case may be, is of another kind, the Micro-organism does not go through this act. Such is the phenomenon; as to the explanation of the same, we are unable to give one.

According to M. E. Maupas, if certain Infusoria feed exclusively upon a certain species, it is because their buccal apparatus, or organ of prehension, makes it impossible for them to feed upon different species which possess different tegumentary envelopes. The question is to ascertain whether this explanation is applicable only in certain cases, as appears very probable to us, or whether, on the other hand, it is of complete and universal applicability. We confess that the hypothesis of M. Maupas does not explain to us why a hunter Inftisory that throws trichocysts, like the Didinium, attacks the Paramcecium aurelia and not the Paramacium bursaria.

It is possible that certain species attract the organisms which feed upon them, by means of a physical or chemical excitation. The researches of Prof. Pfeffer, of the Tubingen Botanical Institute, lend a certain confirmation to this hypothesis. 3. Calculation of the position occupied by the external body. It is a universal fact that Micro-organisms not only perceive external bodies, but that they also indicate, by their movements, an exact knowledge of the position occupied by these bodies. It might be said that they invariably possess a sense of position in _

space. The possession of this sense is absolutely indispensable to them, for it does not suffice them to know of the presence of an exterior body in order to approach it and seize it; they must furthermore know its position, so as to direct their movements accordingly. The simplest form of a sense of localization is met with in the Amoeba, which, when it closes about a nutritive particle, always emits its pseudopods at precisely that part of its body where the foreign substance caused the irritation. The most complicated instance of localization is met with in the Didinium, which we have so often cited; the Didinium knows precisely the position of the prey it follows, for it takes aim at the object of its pursuit like a marksman, and transpierces it with its nettle-like darts. Between these two species, we find all the intermediate instances of a localization of perceptions.