The change from positive to negative can also be brought about in other ways. One of the most striking cases of this sort is that described by Towle in one of the small crustaceans, _Cypridopsis vidua_. It was found that after an animal had been picked up in a pipette its response was always positive; that is, it swam toward the light, no matter what its previous condition had been. The disturbance caused by picking the animal up induced always a positive response towards light. If the light were moved, the Cypridopsis followed the light. In this way it could be kept positive for some time, but if it came to rest, or if it came into contact with the sides or end of the trough, it became, after a short time, negatively heliotropic, and remained negative as long as it could be kept in motion, without being disturbed, or coming into contact with a solid object. If when positive it were allowed to reach the glass at the end of the trough, it would swim about there, knocking against the glass, and then soon turn and swim away from the light. If the light were shifted while the negative animal was in the middle of the trough, it would turn and swim directly away, as before, from the source of light. It could be kept in this negative state as long as it did not come into contact with the ends. It appears that the positive condition in Cypridopsis is of short duration, and ceases after a while either as a response to contact or without any observable external factor causing the change. This crustacean lives at the bottom of pools, amongst water-plants, and here also, no doubt, the same change from one to the other reaction takes place. What possible advantage it may be to the animal to be kept continually changing in this way is not at all obvious, nor, in fact, are we obliged to assume that this reaction may be of any special use to it. Indeed, it is far from obvious how the change that causes the animal to swim toward the light when it is disturbed could be of the least advantage to it.