56 THE CELL 



D. MECHANICAL STIMULATION 



In a great many different kinds of cells the production of energy may be 

 aroused by shocks of a purely mechanical nature. Agencies of this kind may 

 exercise also an important influence upon the locomotion of many organisms. 



In so far as they are due to gravitation these agencies are designated 

 as geotactic. 



The collection of Infusoria at the central end of a centrifuge (Jensen) ; the 

 movement of Paramcecium downward in its medium in condition of hunger and 

 of low temperature, but upward under opposite circumstances; the vertical 

 climbing of Cucumaria, Actinia, Asterina, Peripleneta, etc.; the orientation of 

 fishes; and the behavior of a decereb rated frog are instances of geotaxis. 

 Changes of position, changes of attitude, etc., taking place among the Metazoa 

 are to be regarded as reflexes of a complex order, analogous to those initiated 

 by chemical stimulation (cf. page 54). It is probable that they are brought 

 about according to the attitude of the animal by the stimulation of the end 

 organs of different nerves (of the skin, joints, etc.) through the pressure of the 

 body or through the pull which is exerted by unsupported parts. Finally, we 

 should mention the otolith apparatus as a seat of peripheral stimulation (cf. 

 Chapter XVIII). 



A second form of movement produced by mechanical influence is rheo taxis 

 i. e., changes of position induced by flowing water or currents .of air. 



In microscopical preparations it may be shown that spermatozoa move 

 against the current (Roth), and it has long been known that once these ele- 

 mentary organisms enter the oviduct they strive to reach the very end of the 

 tube, forcing their way against the current produced by the ciliated epithelium. 

 Wheeler has directed attention to the fact that air in motion influences the 

 movements of insects in a similar way. 



Another group of phenomena conditioned by mechanical stimulation is the 

 following. Frog's spermatozoa when mounted on a slide bore into all the little 

 scratches and crevices of the glass surface. These cells have therefore a decided 

 inclination to be in contact with solid bodies (thigmotaxis}. In line with this 

 Jennings has found that Paramoecium aurelia will attach itself to solid particles 

 in the preparation, and Putter has demonstrated that thigmotaxis represents 

 probably a quite general phenomenon widely distributed among all classes of 

 the Protista. 



Thigmotaxis is exhibited also by many higher animals. There are animals 

 such as the ants, which always seek out the concave corners and edges of cavities, 

 while other animals as constantly establish themselves on the convex edg'es and 

 corners of bodies. 



E. STIMULATION BY MEANS OF LIGHT 



Light, if we include only the so-called illuminating rays, stimulates directly 

 only a few kinds of cells. In higher animals it acts only upon the visual cells 

 of the retina, on the musculature of the iris, and, if it be concentrated 

 enough, upon the end organs of the heat nerves. Likewise the skin (of 

 certain invertebrates at least) is sensitive to light rays. In some unicellular 

 organisms movements have been observed which are undoubtedly induced by 



