Summary.A. Loeb's tropism theory cannot explain all phenomena of orientation of animals in space. The opponents of this theory have discovered numerous cases of orientation where different mechanisms are involved. In 1919 A. Kühn established a system of orientation reactions in which other mechanisms are co‐ordinated with those of Loeb. In this way a solution was found for the differences of opinion concerning the validity of Loeb's tropism theory which had lasted for several decades.B. Kühn's system distinguishes the following orientation mechanisms:I.Tropismsare curvature movements of fixed organisms, that is to say plants, certain coelenterates and polychaetes.Taxisare direction movements of motile organisms, such as Protista, swarm‐spores, and particularly animals.II.Phobotaxismeans orientation in an undirected path. The orientation takes place as a response to discontinuous alterations in the intensity of stimulation. If the organism arrives at a position in the field of stimulation where the stimulus intensity is unfavourable to it, being above or below the optimum, then an avoiding‐reaction takes place with a resulting change in the direction of motion. The reaction is repeated again and again until the organism moves away from the unfavourable stimulus. According as the avoiding‐reaction is a result of a stimulus above or below the optimal intensity the organism is eventually forced to move away from or towards the source of stimulation. If the organism is acted upon both by supra‐ and infra‐optimal stimuli it becomes trapped in a zone of optimal stimulation. Kühn's phobotaxis is approximately synonymous with the trial and error reaction of Jennings.III. Intopotaxisthe movement is directed in respect to the source of stimulation. The movement takes place in one of three directions. It may be straight towards the stimulus (positive reaction), straight away from it (negative reaction), or at a definite angle with the line joining the source of stimulation to the body (transverse reaction). The following varieties of directed orientation are to be distinguished:(i)Tropotaxis: orientation in an excitation equilibrium for symmetrically placed receptors and a tonus equilibrium for symmetrically placed effectors. Each sense‐organ is a director acting in only one way. Movement is straight towards or away from the source of stimulation. The tropotactic nature of a reaction can be demonstrated in one of the following ways:(a) If a sense‐organ is extirpated, circus‐movement results.(b) If there are several sources of stimulation, the organism assumes a position on the resultant between the directions of stimulation. Kühn's tropotaxis corresponds approximately with Loeb's tropisms.(ii)Telotaxis: the assumption of a position so that a certain region of the receptor apparatus is acted upon by the stimulus. Definite turning reflexes are linked up with the stimulation of each point of the receptor apparatus. These vary with the strength and direction of the stimuli. The turning movements always swing the animal into the orientated position. The sense‐organ is a director acting in more than one way. The resulting movement is either directly towards or away from the source of stimulation. Proofs of telotactic movement are furnished as follows: (a) absence of circus‐movements as a result of unilateral extirpation of a sense‐organ; (b) absence of orientation along the resultant to several directions of stimulation; (c) orientation not necessarily disturbed by unilateral injury to the effector apparatus.(iii)Menotaxis: maintaining a definite angle between the direction of motion and the line joining the source of stimulation with the sense‐organ. Orientation takes place in a position in which a definite point in the sensory apparatus receives the stimulus. This point may vary.(iv)Mnemotaxis: orientation by memory impressions. A mnemotactic reaction is a sequence of telotactic or menotactic reactions which occurs in a definite time sequence by virtue of the memory. For this reason it is illogical to co‐ordinate mnemotaxis with other types of orientation.C. In the Special Part of this article an investigation is made into the question of how the various orientation mechanisms corresponding to different types of stimuli fit into Kühn's system.I.Phototaxis.(i)Photophobotaxis: in this category come first of all most of the light reactions of Protozoa. Frequently phobotactic and topotactic reactions are linked together. Cases ofphotokinesis, in which an organism comes to rest or moves in certain light intensities, must be distinguished from photophobotaxis.(ii) The differentiation betweenphototropotaxisandphototelotaxisis often difficult and has given rise to voluminous discussions. The proof of a phototropotactic reaction can be supplied as follows:(a) The experiment of two light sources. Under the influence of two sources of light the animal takes up a position on the resultant. The direction of the resultant can be calculated and in many cases agrees with mathematical exactness with the experimental values. Discrepancies from the resultants are explainable on the one hand through asymmetries in the receptor or effector systems, on the other hand through a condition of adaptation. The theoretical requirement that an animal reacting by positive phototropotaxis must continue to move on into darkness along the resultant between two lights is not realized for the following reasons: (i) the orientating lights vanish out of the optical field during the course of the reaction; (ii) illumination from behind inhibits the motor apparatus in many cases; (iii) illumination from behind involves a labile state of equilibrium.(b) With unequal intensities of the two lights positive animals move along a smooth curve towards the stronger light, since the direction of the resultant is displaced towards the stronger light at each point on the path.(c) When blinded on one side, a positive animal describes circular paths towards the side of the body with the eye intact, while a negative animal circles towards the blinded side. The diameter of the circles is inversely proportional to the light intensity.(iii)Phototelotaxisis the mechanism in all cases in which an animal steers towards an objective. The distinction from phototropotaxis is established as follows:(a) In the experiment of two lights the animal moves directly to one of the lights, leaving the other unnoticed. Often a change of direction towards the second light takes place in the course of the reaction.(b) When an animal is blinded on one side, a single eye suffices for movement in a straight line towards a light.(c) When the effectors are injured on one side, orientation may take place as before.(iv) Inphotomenotaxisthe animal orientates itself at a definite angle to the rays of light. With parallel rays the path is always along a straight line. With radiating light rays the path is a circle or a spiral. In the experiment with two lights an animal orientates itself with respect to one of the two lights, since orientation at a definite angle to the rays of two sources of light is not possible. Menotactic orientation relative to the sun is of great importance in direction‐finding by bees and ants. Thecompensatory movementsof many animals on the turntable, as well as many cases ofrheotaxisandanemotaxis, are nothing but the maintenance of definite sight impressions on definite parts of the retina, that is they are cases of photomenotaxis.(v) The mechanism of thedorsal light reflexof swimming animals is in many cases tropotaxis. When one eye is put out of action, the orientation is destroyed and rolling over about the long axis results. In one case a single eye was found to be able to maintain the orientation, so that here telotaxis is involved.II.Geotaxis.(i) Most geotactic reactions are explainable as tropotaxis. When symmetrically placed receptors (statocysts) are unequally stimulated (oblique position) reflex compensatory movements turn the animal into a position giving equilibria of stimulation and of tonus. The theory of tropotaxis can be extended in a similar way to radially shores).(ii) A proof of geotropotaxis can be supplied by the same methods outlined for phototaxis. Unilateral exstirpation of statocysts destroys equilibrium, and rolling about the long axis then occurs. When two forces act onConvoluta roscoffensis(centrifugal force acting in a different direction to gravity) the animals place themselves in the resultant of the two forces. There is an exact correspondence between values of the resultant found experimentally and theoretically.(iii) In a few cases orientation is maintained after unilateral removal of statocysts, so that here the principle of telotaxis applies.III.Chemotaxis.(i) In most cases orientation to chemical stimuli takes place in undirected paths as a result of avoiding‐reactions in response to changes of stimuli intensity. Chemophobotaxis is apparent in optimum reactions of Protozoa and mites when avoiding‐reactions result from diminution and increase of stimulus intensity. The animals congregate in a ring in an optimum zone around a source of odour acting to all sides.(ii) In certain cases orientation is directed straight towards the centre of diffusion. Here the mechanism is tropotaxis. When the antennae of insects are amputated on one side circus‐movements frequently result. In the case of a positive reaction the movement is towards the side of the remaining antenna. An experiment with two sources of stimulation gave no very definite results, but in certain cases showed resultant positions.IV.Thermotaxis. In a temperative gradient aggregation in a certain temperature zone frequently occurs. But orientation reactions cannot be involved here: these reactions may take place through “kinesis.” Undoubted thermophobotaxis occurs, however, in those cases in which animals collect in a circular optimal zone around a central source of heat. Directed movements with respect to a source of heat have been described in a few cases. Sometimes it is the capacity to follow a source of heat, sometimes the assumption of a position at a definite angle to the source of heat. Nothing is known of the mechanism of these instances of thermotopotaxis.V. Most of the reactions described asrheotaxisandanemotaxisare really photomenotaxis or phototelotaxis. The mechanism of the rheotaxis of fishes has not yet been fully explained. Rheotaxis of planarians is to be understood as tropotaxis in which hyperthetical receptors on the right and left sides of the body are acted upon by water currents of equal strength.VI. The mechanism ofphonotaxis(reaction to sound stimuli) is tropotaxis in the only case studied from the point of view of the orientation problem, namely that ofLiogryllus. When the tympanal organ of the female is extirpated on one side, the chirping male is found with greater difficulty.VII.Galvanotaxisis an ideal example of a topotactic reaction the mechanism of which is indisputably tropotaxis. If an animal lies across the current, then cathode and anode sides are exposed to different stimulation. Characteristic compensatory movements result, turning the animal into the direction of the current so that stimulation equilibrium results.VIII.Stereotaxisis not a true orientation reaction. Phenomena described under this name take place because kinesis or akinesis results from the absence or presence of certain contact stimuli. In this way the animal is led to definite situations in space according to the principle