STIMULI AND THEIR ACTIONS 



401 



light again acts upon it, its motion begins anew, and Engelmann 

 was able to determine by means of a spectral apparatus that it is 

 the rays of the orange and the ultra -red which especially exert this 

 stimulating effect upon its motion. 



Isolated examples of the excitation of ciliary motion by light 

 occur among the ciliate Infusoria, which in general thus far have 

 shown themselves not irritable to light. In another connection ^ 

 we have become acquainted with P^eMromema chryscdis (Fig. 186), 

 which in the undisturbed condition lies still in the water without 

 moving its long, leaping cilia, and only from time to time makes a 

 quick spring by a sudden stroke of the latter. If these small 



Fig. 186. — Pleuronema chrysalis. A, Unstimulated, lying quiet ; B, stimulated, in the act of 

 springing by the stroke of its cilia. 



Infusoria, which as a rule are observed in great quantity together, 

 are lying still upon the slide in one spot, in ordinary daylight a 

 leaping movement can be induced in them by removing the screen 

 over the mirror of the microscope, and the motion is repeated fre- 

 quently, when the screen is not shoved in again.^ The ciliated cells 

 jump about wildly like a crowd of excited fleas, until they are 

 again shaded. The motion of the cilia does not begin at the exact 

 moment at which the light strikes them but only after a latent 

 period, which continues for about 1 — 2 seconds. By the insertion 

 between the source of light and the stage of the microscope of 

 coloured glasses and liquids, the penetrability of which to waves of 

 definite wave-lengths has previously been established spectroscopic- 



1 Cf. p. 383. " Of. Vervvorn ('89, 1 ; appendix). 



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