It is reported that planarians show thermotaxis, but the mechanism of it has not been clarified. Through our observations of their movements under a temperature gradient, we found that they may react not only to momentary temperature but also to temporal change of the temperature around them caused by their motions. They have been considered to have the most primitive concentrated nervous system. Our study suggested the possibility that they have short-term memory. Therefore, we can presume that short-term memory is the most fundamental functions of concentrated nervous systems.