light. So that, whether we have to do with the movement of a sunflower, or with the locomotions of minute living units, the essential mode of production of the movement is probably similar. Of the actual locomotions of minute living units under the influence of light many instances might be cited; it will suffice, however, to mention the fact that any green zoöspores which may have been uniformly diffused through the water are very apt, when the vessel containing them is placed near a window, to collect on the surface of the water at the part where most light fulls upon them. Minute animal organisms are, however, often affected quite differently by this agent. They are frequently caused to move away from, rather than toward, its source; so that the creatures thus impressed "seek" the shade rather than the glare of sunlight.
The action of such influences and the production of such movements form the beginnings or substrata, as it were, of other phenomena with which we are now more particularly concerned. The unilateral influence of light and the movements to or from its source to which it may give rise afford a connecting link between diffused causes like heat, which, by affecting the general activity of the vital processes in the organisms, may lead to purely random movements, and those more localized influences now to be considered, to which the various definite or responsive movements of organisms are attributable.
The first, because it is the simplest, of these localized influences to be considered is a shock or mechanical impact of some kind, falling upon the external surface of the organism. This is the primordial or most general of all the modes by which the surface of an organism is impressible, and its sensitivity to such stimuli is both in the stage of impression and the stage of reaction closely akin to the general organic irritability of protoplasm—which, indeed, unquestionably constitutes its starting-point. This mode of impression, moreover, is one which tends to establish a correspondence between the organism and the most common events or properties of the medium in which it lives and moves. It is consequently the mode of impressibility most extensively called into play among all the lower forms of animal life. And although the whole, surface of an organism, or the greater part of it, in one of the simple animals to which we are referring, may be more or less impressible to shocks or impacts from contact with surrounding bodies, it often happens that such impressions more frequently fall upon, and are more readily received by, certain appendages situated at the anterior extremity of the animal, in close proximity to the mouth. Such specialized parts or tactile appendages are known as papillæ, setæ, tentacles, antennæ, or palpi, according to the forms which they assume in different animals.
Why such organs are developed so frequently at the anterior extremity of the animal, and in the neighborhood of the mouth rather
