THE VITAL PROPERTIES OF THE CELL 



83 



of their force components, is invariably of minor importance ' 

 (Hensen III. 11). 



<HTT 



Fig. 47. Explanation of the mechanism of the movements of spermatozoa (after 

 Hensen, Fig. 22). A The four phases of position assumed by the border of the flagellum 

 when an undulation passes over it. I to I 1 , the first ; II to II 1 to II 2 , the second ; III tj 

 If I 1 to III' 2 , the third; IT to IV 1 , the fourth stage of the bending of the border in a lon^i- 

 tidinal undulation. B Section of the thread-like tail and membrane, in its two positions 

 of greater elongation. C and D resolution of forces. E Movement of an ordinary sper- 

 matozoon ; abc various phases of this movement. 



b. Cells with numerous Cilia. The 

 Infusoria are chiefly to be distinguished 

 from other unicellular organisms by the 

 large number of cilia they possess, on which 

 account they are called Ciliata (Fig. 48). 

 Cilia are much smaller than flagella, be- 

 ing, as a rule, about 1 to 3 /x thick, and 

 about 15 fx long. They may number 

 many thousands. For example, it has 

 been calculated, that the Paramecium 

 aurelia possesses approximately 2,500. 

 As for the Balantidium elongatum, which 

 is parasitic in the Frog, and which is very 

 thickly ciliated, Butschli (III. 3) is of 

 opinion that it has nearly ten thousand 

 cilia ; these are generally arranged in 

 several longitudinal rows, which either 

 encircle the body in spirals, or are con- 

 fined to a certain portion of its sur- 

 face. 



In addition to the cilia, many Infusoria 

 possess special large organs of locomotion, 

 cirri, and undulating membranes. The 



Fig. 48. Stylonychia my- 

 til aft (after Stein; from Claus' 

 Zoology) seen from the ventral 

 surface. Wz Adoral zone of 

 cilia; C contractile vacuole; 

 N nucleus ; N l nucleolus ; A 

 anus. 



