298 



MOVEMENT 



before us, in the upper half of the left-hand side, 

 there is an irregular rectangle, and in this there are two 

 vorticellse of unequal size. The larger of them, or 

 that on the left, is situated at about the same level as the 

 other ; in the following illustrations it gradually sinks 

 lower down in the field of the photograph, while the 

 turns of the spiral appear more closely approximated. 





{ i n mM 



Fjg. 201. — Showing the movements of some vorticellse and the retraction of tlieir 

 spiral stalks. The series must be followed from left to right, commencing with 

 the upper illustrations. 



This example is but one of the many interesting ones 

 that could have been chosen, and, moreover, the way 

 in which the figure is reproduced (namely, simili- 

 gravure), necessitates, in the case of such small objects, 

 some alteration in the images. In typography, the 

 shaded parts have to be indicated by cross-hatching, 



