THE BEHAVIOUR OF THE PROTOZOA Reflexes, instincts and intelligent actions are bound up with the presence of a nervous system. In consequence of this, out of the various existing types of behaviour, tropisms alone would seem applicable to the Protozoa. But this in no way covers the facts. The behaviour of the Protozoa is to a certain extent of a spe cial nature and may be contrasted with that of the Metazoa. In spite of an outward simplicity of organs of locomotion, the be haviour of these unicellular animals can nevertheless be very varied. The cause of the movements of the pseudopodia of rhizo pods, e.g. Amoeba, has been ascribed to surface-tension. When, in spite of this, under apparently identical external conditions one individual behaves differently from another, the difference de pends upon the varying physiological states in which the two animals happen to be. The apparently arbitrary spontaneous movements are accounted for by inner causes, which depend upon the alteration in the chemical reactions going on inside the cell, caused chiefly by metabolic processes. Not only pseudopodia) movement, but also the intake of food by these animals, and the construction of the shells of shelled amoebae, have been imitated by artificial models. In this way it has been demon strated that all these phenomena can be explained by chemical and physical laws equally applicable in inorganic nature. Yet it has up to now been impossible to construct a model which unites in itself all the life processes. Many questions remain still to be answered. The living organism possesses such a complex organi zation, having a definite structure made up out of numerous spa tial parts, that we are still far removed from being able to unveil all its secrets.
The infusorians and flagellates, which, owing to the possession of cilia or flagella, are much more motile than the rhizopods, tend, so long as they are swimming freely in the water, to move for wards in a spiral path, rotating continuously on their long axis. The sessile forms produce currents with their motile organs, bringing food towards them from the neighbouring water. These animals almost invariably respond to external stimuli—touch by a solid object, chemical substances in solution, etc.—with one and the same definite reaction. This is the "flight movement" or avoiding reaction (Jennings). The cilia of a swimming infusorian, which, to take an example, has hit against a solid object, reverse the direction of their beat, so that the animal is driven back wards for a short distance. Then the animal slows down its rota tion on the long axis. The beating of the cilia of the mouth region diverts the front end towards the aboral side so that the body of the animal now describes a conical figure. Finally, the infusorian again swims forwards as in the beginning, but owing to the pre vious swinging round of the long axis, the movement forwards is now in a different direction. If the infusorian again hits against the solid object, the action is repeated until, in the end, the obstacle is avoided. It is noteworthy that it is immaterial from which direction the stimulus comes and on which part of the body it impinges. The infusorian invariably responds by the same
avoiding reaction, the different phases of which are merely carried out with different intensities according to the strength of the stimulus. Since such a flight-movement is the reaction of infu sorians and flagellates to almost all stimuli (indeed it occurs in corresponding circumstances in many of the smaller Metazoa), the response of these animals is fundamentally always negative. The final result may, nevertheless, often be positive. Thus Para maecia collect in a drop of water rich in oxygen because they swim into this drop from the surrounding water without any re action but on arriving at the boundary of the drop they are stimulated to execute the avoiding reaction so that they can no longer get out of the drop. Since they hardly possess any positive reactions these Protozoa do not pursue their prey but only happen by chance upon their food. On this behaviour Jennings based his theory of the "method of trial and error" which he supposes ap plies to the whole animal kingdom. In point of fact the behaviour of many Metazoa, right up to the mammals, frequently gives the impression that the animals attain their goal through methodless trials and final chance success. In spite of this, the theory has turned out not to be of general application. It is in contradiction to Loeb's tropism theory which assumes forced movements, since chance plays a considerale role in Jennings's theory. Our de scription of the different types of behaviour has shown that in reality more complex factors intervene.
Among the Protozoa, particularly among many of the larger infusorians, other reactions in addition to that just described are possible. Thus Stentor roeseli, attached inside its tube of mucus, first bends to the aboral side when carmine particles are dropped into the water which it whirls past its body. Since, in spite of this, the particles continue to reach its surface, it next reverses the direction of beat of its cilia and then once more moves them in the normal sense. After this has been repeated several times without result, the animal withdraws into its tube, at first for a short, then for a longer, period. If the carmine particles are still there, a fourth type of reaction eventually super venes. The Stentor contracts forcibly, loosens its foot and swims away. The series of reactions employed by Stentor is graduated according to degree of effectiveness. In a certain measure the animal makes a purposeful choice out of its different reaction possibilities. The term "action system" has been used to desig nate the totality of the possible reactions which an animal pos sesses in consequence of its bodily structure and which char acterize its various reactions under different circumstances. The action systems of the Protozoa only include a relatively small number of definite movements. Animals are able to respond to the stimuli they encounter only with one or other of these move ments, or with a combination of them.