THE MANIFESTATIONS OF LIFE 



77 



at a fair rate of speed through the fluids in which they 

 live. These are locomotory in function. It is interest- 

 ing to observe that they may vibrate synchronously 

 in a given direction, or vibrate on one side of the body 

 more rapidly than on the other 

 so that the direction may be 

 changed, or may reverse their 

 direction so that the organism 

 may move backwards. When 

 organisms like Paramoecium 

 conjoin, their cilia vibrate 

 synchronously so that they 

 easily swim along together. 

 As has been shown in speak- 

 ing of galvanotropism, the 

 direction and movement of 

 the cilia may be modified by 

 electric currents. 



Many organisms are pro- 

 vided with cilia about the oral 

 opening, vibration of which 

 causes currents of water to 

 flow toward the mouth so 

 that objects adapted for food 

 are readily caught. Cilia of 

 this kind are usually longer 

 than those used for locomo- 

 tion. Beautiful examples are 

 found in Rotifers. 



Cilia also occur upon the cells of higher animals where 

 they subserve different purposes in the economy of the 

 animal without being of particular benefit to the cells 

 themselves. Thus the ciliated cells of the gills of lamelli- 

 branchiata bring currents of fresh sea-water to the gills 

 of the animals and so facilitate the aeration of its 

 blood. 



The respiratory passages of vertebrates are lined with 

 ciliated epithelium whose rhythmical vibrations assist in 



FIG. 23. Stylonychia mytilus: 

 ws, Cilia about the mouth open- 

 ing; c, contractile vacuole; n, 

 nucleus; n', para-nucleus; a, anus. 

 (Clan's Zoology.) 



