384 



GENERAL PHYSIOLOGY 



energetic strokes with its cilia, so that it springs through the water 

 like a flea, and immediately thereafter becomes quiet in another 

 spot. Similar cases exist in great abundance among the active 

 Infusoria. It is found everywhere that mechanical stimuli cause 

 energetic strokes of cilia. 



Infusorian life offers innumerable opportunities to observe 

 the effects of mechanical stimuli upon muscular motion. Smooth 

 muscle-fibres (myoids) are wide-spread among Infusoria ; and just 

 as everything in the life of these Protista, which are in endless, 

 activity, takes place with great rapidity, so their contractile fibres, 

 react upon the slightest jarring with a sudden, strong contraction. 

 There are few sights in the microscopic world so pleasing as the- 



Fig. 176. — PUxf.rone'ma chrysalis. A, Lying still. -B, In the actof springing, upon being stimulated 

 by shaking ; the cilia are just performing a stroke. 



contraction of a much-branched tree of Vorticelliiice upon very slight 

 jarring (Fig. 177). At the moment of the impulse all the stalk- 

 myoids contract suddenly and simultaneously, and the stalks are 

 coiled in delicate spirals (Fig. 177, B). Stentor also, which in rest 

 has its beautiful, trumpet-shaped body unfolded, at every jar 

 suddenly draws itself into a stalked ball by the contraction of the 

 many myoid-fibres lying in the external layer of the body (Fig. 168, 

 p. 376). The cross-striated muscles of the higher animals behave 

 similarly, without of course possessing the same high grade of 

 irritability. In order to cause contraction, by means of a mechanical 

 stimulus, in a frog's muscle, for example, a stronger shock to- 

 the muscle-substance is needed than in the case of an infusorian. 



