THE VITAL PROPERTIES OF THE CELL 



also he observed that the mean position through which the cilia 



oscillated was different for a time in one half 



of the rows of cilia from that seen on the 



other side. 

 III. The Contractile Vacuoles, or 



Vesicles, of Unicellular Organisms. 



Contractile vacuoles occur very frequently in 



Amoebae, Reticularia, Flagellata (Figs. 7, 43, 



44), and Ciliata (Fig. 50 cv). In the last, 



where they have been most accurately ex- 

 amined, there is generally only one single 



vacuole in the whole body ; occasionally two 



are present (Fig. 50), or rarely a few more ; 



they are always situated just below the sur- 

 face of the body, under the ectoplasm. They 



may be easily distinguished from the other 

 fluid vacuoles, of which 

 large numbers may be 

 distributed throughout 

 the body, by the fact, 

 that at regular intervals 

 they discharge their con- 

 tents to the exterior, and 

 then gradually fill up 

 again. Hence they tem- 

 porarily disappear (Fig. 

 50 cv) to reappear again 

 in a short time (cv 1 ). 

 The evacuation takes place through one or 

 more special pores, which can be observed 

 on the surface of the infusorian immediately 

 over the vacuole. " Each pore appears as a 

 rule as a minute circle, the border of which 

 is dark, but which is bright in the centre ; 

 this brightness of the centre is due to the re- 

 fractive power of the pellicular and alveolar 

 layer. Sometimes each pore is connected to 

 the vacuole by means of a fine excretory 

 tube. In addition, it is not uncommon to 

 find special conducting canals (1, 2, or more) 

 regularly arranged in its neighbourhood. In 



FIG. 49. Spirostomum 

 ambiguum. The con- 

 tinuity of the surface 

 which bears the peri- 

 stomatic cilia has been 

 interrupted by an in- 

 cision. ( After Verworn 

 (VI. 40), Fig. 25.) 



FIG. 50. ParamcBcium 

 caudatum semi-diagram- 

 matic (R. Hertwig, Zoo- 

 logie, Fig. 139): K nu. 

 cleus ; rik secondary nu- 

 cleus; o mouth aperture 

 (cytostome) ; na l food 

 vacuole in process of for- 

 mation ; na food vacuole ; 

 cv contractile vacuole, 

 contracted; cv 1 the same 

 contractile vacuole, dis- 

 tended j t trichocysts, 

 t 1 the same with their 

 threads ejected. 



