44 



THE CELL 



of the cell body. The former arrangement is found in the Flagellata, in the 

 spermatozoids of plants, many Bacteria, and in the swarm spores of many Algse 

 and Fungi. In locomotion the flagella precede and pull the body after them. 



The latter arrangement is found in the spermatozoa 

 of most animals, where the flagellum drives the cell 

 body forward after the manner of a propeller. 



If the cilia should beat back and forth in both 

 directions with equal force, only a to-and-fro 

 movement would result, and no locomotion would 

 be possible. Careful investigation of both flagella 

 and cilia has shown that they always strike more 

 forcibly in one direction than in the other, a fact 

 which at once makes clear the mechanical re- 

 sults of their movements. 



In the freely motile cells the object accom- 

 plished by the ciliary movement is locomotion. In 

 addition to this, if the cell is swimming in a fluid 

 specifically lighter than itself, motion of the cilia 

 prevents it from sinking to the bottom. Again in 

 certain unicellular animals they participate in the 

 ingestioii of food, being arranged in a circle around 

 the mouth, and when in motion creating a vortex 

 in the water which sucks suspended particles into 

 the mouth (Fig. 28). 



The ciliated cells covering the surface of the 

 mucous membranes in Metazoa drive such particles 

 as may happen to be on the surface in a given 

 direction, and thus play a role in many ways very 

 important to the animal. 



Cilia are always fastened to a protoplasmic 

 substratum and are never outgrowths of a firm 

 cell membrane. Very often, however, they are 

 not immediate extensions of the protoplasm, but 

 rest directly upon a thin layer of transparent 

 substance which, though very closely resembling 

 the substance of the cilia, appears not to be con- 

 tractile but to lie as a kind of coat on the naked 

 surface of the cell protoplasm. The coats of the 

 adjacent cells are so closely connected that in 

 many cases a considerable patch of this layer can 

 be lifted up as one piece. 



Certain cilia continue to move even if they 

 be separated entirely from the protoplasm or the 



basal part of the cell. Since both cilia and flagella can of themselves perform 

 many complicated movements we must assume that they consist of a contractile 

 substance. If in addition we assume that they contain also a noncontractile 

 supporting substance, we should have a somewhat satisfactory explanation of 

 the different forms of movement of these structures (Putter). 



FIG. 28.^Vorticella. 



a group of individuals as 

 seen under the low power; 

 B, a single vorticella with 

 stalk extended and beside 

 it another with stalk con- 

 tracted; d, denotes disc; p, 

 peristome ; vc, contractile 

 vacuole ; vs, vestibule ; cf, 

 contractile fiber; nc, nucleus; 

 cl, cilia. 



