78 



GENERAL PHYSIOLOGY 



face motile but permanent processes, of the shape of eye-lashes. 

 These ciliated cells are very wide-spread ; they occur not only in 

 tissues as ciliated epithelium-cells (Fig. 20, a), but also free-living 

 and constituting the great host of the Ciliata, or ciliate Infusoria, 



Fia. 20. a, Ciliated epithelium-cells. (After Schiefferdecker.) b, Stylonychiamytilus, a ciliate-in- 

 fusorian cell possessing variously differentiated cilia ; WZ, mouth-region; C, contractile vacuole ; 

 N, macronucleus ; N, micronucleus ; A, anal opening. (After Stein.) c, Euglena viridis, a 

 flagellate-infusoriaii cell possessing a single flagellum. n, Nucleus ; o, eye-spot ; c, vacuole. 

 (After Stein.) 



and the Flagellata, or flagellate Infusoria, according as the one- 

 celled body possesses many similar or variously differentiated cilia 

 (Fig. 20, 6), or only one flagellum or several (Fig. 20, c). Finally, 

 there are cells that deviate from the type by being enormously 

 extended in one direction, so that they appear as slender, band- 

 or thread-like forms. Extremes in this direction are smooth and 

 cross-striated muscles-cells (Fig. 21, a) and many spermatozoa 

 (Fig. 21, 6). 



In contrast to the astonishing variety of form, it is surprising 

 that the size of cells varies only within relatively narrow limits. 

 It is a very noteworthy fact that by far the majority of all cells are 

 microscopic. The size of organisms varies within very wide 

 limits, from the extreme minuteness of a bacterium, measuring 

 only a few thousandths of a millimetre, up to the enormous mass 

 of an elephant or the huge spread of an American mammoth - 

 tree. But large organisms are never found consisting of a single 



