6. It may also be of interest to know that in heliotropism the motions of the legs are automatically controlled by the chemical changes taking place in symmetrical elements of the retina. In order to prove this point we will turn to the phenomenon of galvanotropism. The galvanic current forces certain animals to move in the direction of one of the two electrodes just as the light forces the heliotropic animals to move towards (or from) the source of light. The change in the concentration of the ions at the boundary of the various organs, especially the nerves, determines the galvanotropic reactions. When the shrimp _Palæmonetes_ is put into a trough with dilute salt solution through which a current of a certain intensity flows, the animal is compelled to move towards the anode.[232] It can walk forwards, backwards, or sidewise. Here we can observe directly that the effect of the current consists in altering the tension of the muscles of the legs in such a way as to make it easy for the animal to move toward the anode and difficult to move toward the cathode. Thus if the current be sent sidewise through the animal, say from left to right (Fig. 45), the legs of the left side assume the flexor position, those of the right the extensor position. With this position of its legs the animal can easily move to the left, _i. e._, the anode, and only with difficulty to the right, _i. e._, the cathode. This change in the position of the legs occurs when the animal is not moving at all, thus showing that the galvanotropic movements take place not because the animal intends to go to the anode, but that the animal goes to the anode because its legs are practically prevented by the galvanic current from working in any other way. This is exactly what happens in the heliotropic motions of animals.[233]
[232] Loeb, J., and Maxwell, S. S., _Arch. f. d. ges. Physiol._, 1896, lxiii., 121.
[233] That the mechanisms by which heliotropic and galvanotropic orientation is brought about are identical was shown by Bancroft in _Euglena_ (Bancroft, _loc. cit._).
To understand what happens when the current goes lengthwise through the body it should be stated that _Palæmonetes_ uses the third, fourth, and fifth pairs of legs for its locomotion. The third pair pulls in the forward movement, and the fifth pair pushes. The fourth pair generally acts like the fifth, and requires no further attention. If a current be sent through the animal longitudinally, from tail to head, and the strength be increased gradually, a change soon takes place in the position of the legs (Fig. 46). In the third pair the tension of the flexors predominates, in the fifth the tension of the extensors. The animal can thus move easily with the pulling of the third and the pushing of the fifth pairs of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is rendered easy, the backward motion is difficult. Hence it can easily move toward the anode, but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direction, namely, from head to tail, the third pair of legs is extended, the fifth pair bent; that is, the third pair can push, and the fifth pair pull. The animal will thus move backward easily and forward with difficulty, and it is thus driven to the anode again.