Irritability: A Physiological Analysis of the General Effect of Stimuli in Living Substance

by Max Verworn

Fig. 42.

Interference of galvanotaxis and thigmotaxis in Paramaecium aurelia. The individuals which are thigmotactically attached to slime particles remain at rest while the freely swimming individuals move toward the cathodic pole.

Fig. 43.

Hypotrichous infusoria. A—Stylonychia. B—Urostyla.

Still more complex and striking is finally the following case of interference between thigmotaxis and galvanotaxis. The hypotrichous infusoria as Stylonychia, Urostyla, Oxytricha, etc., have a marked functional and morphological differentiation of their cilia. They possess a bow-like row of perioral cilia, which sweep in the food; a number of cilia on the ventral surface used for locomotion by which they move about upon objects in the water; a row of border cilia on each side, which, during swimming, contribute the propelling force. The perioral cilia also form the elements which bring about a screw-like movement on the axis. They further possess several cilia, which permit a rebounding of the organism, and finally certain forms have anal cilia, which probably serve as breaks and to steer the organism. (Figure [43].) Their usual mode of locomotion is that of creeping, moving by means of the cilia on the ventral surface. These movements depend upon the positive thigmotaxis of the cilia of locomotion. At the same time there is inhibition of the cilia on the sides. When the infusoria are excitated by a new stimulus, the cilia used for rebounding become active, the body frees itself from its position of attachment and begins to swim, wherein the cilia on the sides, as well as the perioral cilia, act in the manner mentioned above. I have made the striking observation that the hypotrichous infusoria respond differently to the galvanic current, depending on whether they are swimming or in a fixed position. If one places a drop of water with numerous Urostyla on a slide between parallel pieces of fired clay which serve as electrodes, it will be seen, upon the closing of a current, that all of the individuals which are freely swimming and turning in a screw-like manner around their axis, steer immediately toward the cathode, exactly as in the case of the Paramecia. On the other hand, those which are fixed to the bottom of the slide as a result of thigmotaxis, upon closing of the current, make a short turn and assume a position wherein the long axis is at right angles to the direction of the current, and the perioral rim is directed toward the cathode. In this position they move through the field. (Figure [44].) When the current is broken the individuals draw backwards, distribute themselves and creep and swim in all directions in the water. If during the course of the passage of the current, an individual which has been swimming begins to creep, the axis immediately assumes the position above described in the case of the organisms which are in contact with the bottom and vice versa. The thigmotaxis, therefore, influences galvanotactically swimming organisms in a most characteristic manner. As a consequence of the interference of thigmotaxis and galvanotaxis, the organisms move in a direction transversely to the direction of the current. This most striking reaction has been cleared up by Pütter,[167] the explanation being based upon an accurate investigation of the mechanism of ciliary activity. The galvanotactic swimming toward the cathode is explained by the same principle as that applicable to all galvanotaxis.[168] As a result of the excitation produced by the anode, the cell body must assume a position wherein the border cilia, which are of greatest importance in swimming, are equally stimulated on both sides of that part of the body directed toward the anode. It is only in this position that forward swimming is possible, for as a result of unsymmetrical excitation of the border cilia a turning must at once occur, which automatically brings about a resumption of the position of the long axis. The perioral cilia bring about the screw-like movement around the axis during swimming. It follows that the freely swimming individuals must necessarily move towards the cathode. In the case of the thigmotactically moving individuals the activity of the border cilia is inhibited. The perioral and the locomotion cilia bring about the assumption of the position of the axis, above described. The perioral cilia during movement bring about a turning of the body on the vertical axis toward the side opposite that of the orifice and it follows that the body can occupy only that axial position wherein the perioral cilia are least excitated. This is, however, only the case when the long axis of the body is transverse to the direction of the current, and the perioral cilia are directed toward the cathode, for stimulation arises from the anode. The reason why the infusoria do not turn toward the anode from this transverse position of the axis is to be found in the fact that the anterior locomotion cilia are stimulated to a greater extent by the turning toward the anode, and bring about a movement in the contrary direction. The transverse position of the axis is thus the result of an antagonistic action between the perioral and the anterior locomotion cilia. It therefore follows that the characteristic position, which is necessarily assumed by the thigmotactically creeping individuals, is brought about by an interference action between tactile and galvanic stimulation.

Fig. 44.

Urostyla grandis. Interference of galvanotaxis and thigmotaxis. The freely swimming individuals move towards the cathode (left side). The creeping individuals move in transverse direction.

These, then, are a few examples of the interference action of various stimuli on the single cell. They show us in part fairly simple, and in part very complex states. It now behooves us to obtain a general understanding of interference action, to learn the fundamental laws in connection with these complex actions, to shell out, as it were, the general factors involved in the special conditions. In this connection the examples already referred to furnish all of the data necessary for our first orientation. In the simple instance in which the effect of galvanic stimulation was augmented by increase of temperature and again in the case where there was a diminution of excitation resulting from the alcohol, the interference of the two stimuli is consequent upon the fact that the location of attack is the same. The constant current acts upon a portion of the infusorium, which also responds to elevation of temperature. We have a real, or, as I may term it, “homotopic interference,” for it is an interference in which the general point of attack is the same for both stimuli.