Unicellular animals react to other things besides differences in the chemical composition of different parts of a solution. In many cases they react to light, swimming toward or away from it according to whether they are positively or negatively heliotropic. If they are positively heliotropic, and while swimming run into a shadow, they react as they would on coming into contact with a drop of acid. Since they rotate as they swim forward, we cannot explain their orientation as in the case of other animals that hold a fixed vertical position. If we assume that the two ends of the body are differently affected by the light, for which there is some evidence, we can perhaps in this way account for their turning toward, or away from, the source of light. Changes in the osmotic pressure of the different parts of the fluid, mechanical stimulation produced by jarring, extremes of heat and of cold, all cause this same characteristic reaction in Paramœcium; and this accounts for their behavior toward these agents that are so different in other respects. Paramœcia, as well as other protozoans, show a contact response. They fix themselves to certain kinds of solid bodies. If, for example, a small bit of bacterial slime is put into the water, the paramœcia collect around it in crowds, and eat the bacteria; but they will collect in the same way around almost any solid. On coming in contact with bodies having a certain physical texture, the cilia covering the paramœcium stop moving, only those in the oral groove continuing to strike backward. The animal comes to rest, pressed against the solid body. If one or more paramœcia remain in the same place, they set free carbon dioxide, as a result of their respiratory processes. There is formed around them a region containing more of this acid than does the surrounding water. If other moving paramœcia swim, by chance, into this region, they are caught, and as a result an accumulation of individuals will take place. The more that collect the larger will the area become, and thus large numbers may be ultimately entrapped in a region where there is formed a substance that, from analogy with other animals, we should expect to be injurious. The question as to how far these responses of the unicellular forms are of advantage to them is difficult to decide, for while, as in the above case, the response appears to be injurious rather than useful, yet under other conditions the same response may be eminently advantageous. In other cases, as when the paramœcia back away, and then swim forward again, only to repeat the process, the act appears to be such a stupid way of avoiding an obstacle that the reaction hardly appears to us in the light of a very perfect adaptation. If we saw a higher animal trying to get around a wall by butting its head into it until the end was finally reached, we should probably not look upon that animal as well adapted for avoiding obstacles.