In another crustacean, one of the marine copepods, _Labidocera æstiva_, it has been shown by Parker that the male and female react in a somewhat different way both to light and to gravity. The females are strongly negatively geotropic, and this sends them up to the top of the water. The males are very slightly negatively geotropic. The females are strongly positively heliotropic toward light of low intensity; the males show the same response to a less degree. To strong light the females are negative and the males are indifferent. On the other hand, the males are attracted to the females, probably in response to some chemical substance diffusing from the females, since the males show the same reaction when the females are enclosed in an opaque tube through whose ends a diffusion of substances may take place. This crustacean frequents the surface of the ocean from sunset to sunrise. During the day it retires to deeper water. Its migrations can be explained as follows: The females come to the surface at night, because they are positively heliotropic to weak light, and also because they are negatively geotropic. They go down during the day, because they react to bright light more strongly than to gravity. The males follow the females, largely because they react positively chemotactically toward the females. Some other animals respond in a somewhat different way to light, as shown by the fresh-water planarians. These animals remain during the day under stones, where the amount of light is relatively less than outside. If they are placed in a dish in the light in front of a window, they crawl away from the light, but when they reach the back of the dish they do not come to rest, but continue to crawl around the sides of the dish even toward the light. The light makes the worms restless, and while they show a negative response as long as they are perfectly free to move away from the light, they will not come to rest when they come to the back of the dish if they are there still in the light, because the irritating action of the light on them is stronger than its directive action. If, however, in crawling about they come accidentally into a place less bright than that in which they have been, they stop, and will not leave this somewhat darker spot for a brighter one, although they might leave the newly found spot for one still less bright. At night the planarians come out and wander around, which increases their chance of finding food, although it would not be strictly correct to say that they come out in search of food. If, however, food is placed near them, a piece of a worm, for example, they will turn toward it, being directed apparently by a sense of smell, or rather of taste.