STIMULI AND THEIR ACTIONS 



385 



Every one who has been at the sea-shore upon a quiet summer 

 evening has observed the production of light by mechanical 

 stimuli. The curious, transparent animal life that in quiet 

 weather exists at the surface of the water has the wonderful 

 power, irrespective of the classes to which the animals severally 

 belong, of glowing brilliantly at every movement of the boat or 

 the oars, or every dash of the waves. Where the water contains 

 much plankton, such as Noctilucce, Radiolaria, eggs of Ctenophora, 

 etc., the effect of a shower of sparks in the water can be produced 

 as well indoors. Every time sea-water is stirred in a glass the 

 unicellular organisms, at the moment of the touch with a glass 

 rod, flash up brightly, and then immediately sink again into 



Fig. 177. — Carchesiuin polypinum, a branched colony of VoHicellince. A, Unstimulated ; B, stimu- 

 lated by jarring. Tbe single individuals jerk together by the contraction of the-myoids of 

 their stalks. 



darkness. The experiment can be repeated innumerable times, 

 and the spectacle that is presented is of wondrous beauty. 



Before leaving the excitation-effects of mechanical stimuli, one 

 more group deserves attention, viz., the results of rhythmically 

 repeated shocks. Phenomena that are produced only incompletely 

 by single shocks are expressed much more strongly by summation, 

 providing that each succeeding impulse follows before the stimulus 

 of the preceding one has passed away. This fact is demonstrated 

 most distinctly in contraction-movements, where one con- 

 traction is superimposed upon another so that there is no time 

 for expansion to develop between them ; a genuine cramp 

 then appears, which is termed mechaniccd tetanus. The 

 peculiarity of tetanus lies in the fact that, although composed of 

 many single contractions, on account of their rapid succession it 

 gives the impression of a continual process. The simplest method 

 of producing rhythmic shocks is either to shake the objects in a 

 shallow basin by means of a rotating toothed wheel that has wide 



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