STIMULI AND THEIR ACTIONS 



383 



finally, the latter is wholly drawn in, and its mass mingles with the 

 rest of the body-protoplasm. Among the marine PolytJialamia also 

 there are many forms that possess very great irritability ; these draw 



I H 



FIG. 174. Contraction of a pseudopodium of Diffliiffia lobostoma after vigorous shaking Seven 

 successive stages of retraction. 



in their whole richly-branched net- work of pseudopodia upon a single j 

 shock. 



In the same manner there can be observed upon the slide 

 under the microscope the effects of a shock upon the motion of 

 flagella and cilia. E.g., Peranema, by means of the regular lash- 

 ings of the flagellum at its anterior end, moves through the water 

 quietly and in a straight line (Fig. 175). But, if the slide be 

 given a brief shock, there follows at once an 

 energetic lashing of the whole flagellum, 

 which gives the cell another direction. It 

 then continues its way quietly as before, with 

 only the end of its flagellum vibrating. The 

 mechanical stimulus, therefore, has had the 

 result of intensifying the stroke of the flagel- 

 lum. The same thing can be observed in the 

 ciliary motion of the ciliate Infusoria. If a 

 Paramcecium be observed in quiet and not too 

 rapid locomotion, moving through the water 

 by the play of its cilia as by innumerable 

 small and rapidly moving oars, it is seen that 

 upon being jarred it suddenly accelerates 

 its motion, returning immediately, however, Fl <?- ITS. Pemnema, a 



., . ' mi P . flagellate-infusorian cell. 



tO its preVlOUS rate. IhlS tact may be a, Swimming quietly; b, 



observed much more distinctly in Pleuronema 



chrysalis, a small bean-shaped infusorian, 



which usually lies in the water for a long time absolutely still and 



keeps its long, ray-like cilia completely quiet (Fig. 176). At the 



moment when it is slightly shaken, it suddenly makes a few, very 



