216 THE POPULAR SCIENCE MONTHLY. 



light. So that, whether we have to do with the movement of a sun- 

 flower, 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 zoospores 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 or- 

 ganisms 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 sucli move- 

 ments form the beginnings or substrata, as it were, of other phe- 

 nomena 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 attrib- 

 utable. 



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 eori-espondence 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 refer- 

 ring, may be more or less impressible to shocks or impacts from con- 

 tact with surrounding bodies, it often happens that snich 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 papillae, setae, tentacles, antennae, 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 



