FACTORS INVOLVED IN THE PROCESS OF ORIENTATION OF LOWER ORGANISMS IN LIGHT
Authors: MAST S. O. 
Published: January 2008

Summary1. The factors involved in the process of orientation differ greatly in different groups of animals.2. In the rhizopods orientation is brought about by inhibition in the formation of pseudopods on the more highly illuminated side, due to the gelating effect of light. Symmetry is only superficially involved and light does not act continuously.3. In the flagellates and the colonial forms orientation depends upon changes in the direction of the stroke of the flagella, correlated with rapid changes in the intensity of light on the photosensitive substance in the eyespots, due to rotation on the longitudinal axis. Symmetry is not involved in the flagellates and only superficially in the colonial forms, for they still orient if it is destroyed. These organisms are not held on a direct course by continuous action of light after they are oriented. They remain oriented because they tend to take a direct course in a homogeneous environment and because the orienting stimulus acts immediately if they turn from this course, and induces responses which bring them back.4. The factors involved in the process of orientation in ciliates are essentially the same as in flagellates.5. In worms random movements correlated with rapid changes in the intensity of light on the photoreceptors and responses which depend in character on the location of the stimulus in the eyes or at the surface of the body, usually play prominent roles in the process of orientation. Symmetry is involved, but it usually is not essential, for orientation still occurs after it is destroyed.6. In insects orientation is brought about by a series of co‐ordinated reflexes in the legs or wings specifically correlated in reference to character, with the location of the stimulus in one eye modified by simultaneous stimulation in the other eye, through integration in the central nervous system. The extent and the nature of modification of the effect of stimulation in one eye by simultaneous stimulation in the other depend upon the relation in the location of the stimulus and in the intensity of the light in the two eyes. Symmetry and “balanced action” in receptors and appendages on opposite sides are not essential, for insects with legs on one side removed or photoreceptors on one side destroyed still orient. Insects are not held upon their course after orientation by continuous action of light. The orienting stimulus ceases to act after they are oriented. They remain oriented because in the absence of action of external agents, they tend to take a straight course and because turning from this course produces changes in the location of the regions illuminated in the eyes and responses which immediately bring the insect back into its course.7. The process of orientation usually directs organisms toward regions which are favourable in reference to environment. It is therefore useful and it doubtless originated and evolved in the same way as other useful phenomena.8. Ants, like higher animals, go toward any given point in space without axial orientation. In maintaining a given direction of movement they regulate the direction of movement of the legs so as to compensate for changes in the direction in which they face. They therefore possess something akin to memory.