(vi.) Most active Protozoa tend to take up a definite position in respect to a current of ELECTRICITY passing through the medium, and in the majority of cases, including most Ciliates, _Amoeba_, and _Trachelomonas_, they orient their long diameters in the direction of the lines of force and swim along these to assemble behind the cathode. The phenomenon is called "galvanotaxy," and this particular form is "negative." _Opalina_ (Fig. 41, p. 123), however, and most Flagellates are "positively galvanotactic," and move towards the anode. H. H. Dale[32] has shown that the phenomenon may be possibly in reality a case of chemiotaxy, for the direction of motion varies with the nature and concentration of the medium. It would thus be a reaction to the "ion" liberated in contact with the one or other extremity of the being. Induction shocks, as we have seen, if slight, arrest the movements of Protozoa, or if a little stronger determine movements of contraction; if of sufficient intensity they kill them. No observation seems to have been made on the behaviour of Protista in an electric field. A magnetic field of the highest intensity appears to be indifferent to all Protista. (vii.) We have already referred to the effect of dissolved CHEMICAL SUBSTANCES present in the water. If the substance is in itself not harmful, and the effect varies with the concentration, we term the reaction one of "tonotaxy," which combines {23}with that of "chemiotaxy" for substances that in weak solution are attractive or repellent to the being. _Paramecium_, which feeds on bacteria, organisms of putrefaction, is positively chemiotactic to solutions of carbon dioxide, and as it gives this off in its own respiration, it is attracted to its fellows. The special case of reaction to gases in solution is termed "aerotaxy," or "pneumotaxy," according as the gas is oxygen or carbon dioxide. We find that in this respect there are degrees, so that a mixed culture of Flagellates in an organic infusion sorts itself out, under the cover of a microscopic preparation, into zones of distinct species, at different distances from the freely aerated edge, according to the demands of each species for oxygen and CO_{2} respectively. Finally, we must note that the apparently "spontaneous movements" of Protists can hardly be explained as other than due either to _external_ stimuli, such as we have just studied, or to _internal_ stimuli, the outcome of internal changes, such as fatigue, hunger, and the like. Of the latter kind are the movements that result in REPRODUCTION.