work will no longer be equal; as a consequence the direction in which the animal moves will change. This change will take place in one of two ways, according as the animal is either positively or negatively heliotropic; in the positively heliotropic animal the resulting motion will be toward, in the negatively heliotropic from, the light. Where we have no central nervous system, as in plants or lower animals, the tension of the contractile or turgid organs is influenced in a different way, which we need not discuss here. The reader will perceive that according to the writer’s theory two agencies are to be considered in these reactions: first, the symmetrical arrangement of the photosensitive and the contractile organs, and second, the relative masses of the photochemical reaction products produced in both retinæ or photosensitive organs at the same time. If a positively heliotropic animal is struck by light from one side, the effect on tension or energy production of muscles connected with this eye will be such that an automatic turning of the head and the whole animal towards the source of light takes place; as soon as both eyes are illuminated equally the photochemical reaction velocity will be the same in both eyes, the symmetrical muscles of the body will work equally, and the animal will continue to move in this direction. In the case of the negatively heliotropic animal the picture is the same except that if only one eye is illuminated the muscles connected with this eye will work less energetically. The theory can be nicely tested for negatively heliotropic animals in the larvæ of the blowfly when they are fully grown, and for positively heliotropic animals on the larvæ of _Balanus_, and many other organisms. One of the difficulties in identifying the motions of animals to or from the light with the positive and negative heliotropism of plants consisted in the fact that plants are mostly sessile (and respond to a one-sided illumination with heliotropic curvatures to or from the light), while most animals are free moving and respond to the one-sided illumination by being turned and compelled to move to or from the light. This difficulty was overcome by the observation that sessile animals like the hydroid _Eudendrium_ (Fig. 43) or the tube worm _Spirographis_ (Fig. 44) react to a one-sided illumination also with heliotropic curvatures like sessile plants.[220] On the other hand, it had been found before by Strassburger that free-swimming plant organisms like the swarmspores of algæ move to or from the source of light as do free-swimming animals. [220] Loeb, J., _Arch. f. d. ges. Physiol._, 1890, xlvii., 391; 1896, lxiii., 273. [Illustration: FIG. 43] [Illustration: FIG. 44]