The heliotropic responses of the planarians appear to be of use to them, causing them to hide away in the daytime, and to come out only after dark, when their motions will not discover them to possible enemies. But some of the planarians are protected in other ways, so that they will not be eaten by fish, probably owing to a bad taste; so that it is not so apparent that they are in real need of the protection that their heliotropic response brings to them. Their turning towards their food is, however, beyond question of great advantage to them, for in this way they can find food that they cannot detect in any other way. The unicellular plants were amongst the first organisms whose tropic responses were studied, and the classical work of Strasburger gave the impetus to much of the later work. In recent years the unicellular animals, the protozoans, have been carefully studied, more especially by Jennings. His results show that the reactions in these animals are different in some important respects from those met with in higher forms. For instance, most of the free-swimming infusoria are unsymmetrical, as are also many of the flagellate forms, and as they move forward they rotate freely on a longitudinal axis. It is therefore impossible that they could orientate themselves as do the higher animals that have been described above, and we should not expect these Protozoa to react in the same way. In fact, Jennings shows that they exhibit a different mode of response. Paramœcium offers a typical case. As it moves forward it rotates toward the aboral side of the body. As a result of the asymmetry of the body, the path followed, as it revolves on its own axis, is that of a spiral. Did the animal not rotate, as it swims forward, its asymmetrical form would cause it to move in a circle, but its rotation causes, as has been said, the course to be that of a spiral, and the general direction of movement is forward.[31] The rotation of a paramœcium on its axis is in turn caused by the oblique stroke of the cilia that cover the surface of the body. Their action when reversed causes the animal to rotate backward. Footnote 31: The same result is attained by a bullet that is caused by the rifling to rotate as it moves forward.