300 



MOVEMENT IN LOWER ANIMALS 



In the slipper animalcule or paramcecium (see Fig. 131), 

 which is a higher type of protozoans than 

 the arcella, the body wall is covered with 

 permanent projections of protoplasm 

 called cilia from their resemblance to 

 hairs. These hairlike structures are 

 composed of protoplasm like the pseud- 

 opodia, but, unlike them, they cannot 

 be withdrawn, and in contracting and 

 expanding they move at a different 

 rate and in only two directions. As 

 it contracts the cilium is bent quickly 

 and sharply backward; relaxing, it re- 

 turns much more slowly to an upright 

 position. Many cilia, acting in concert 

 all over the body, push the body 

 through the water just as oars do a 

 boat. A peculiarity of ciliary action, 

 however, is that they do not all 

 contract at once, but the movement 

 of one follows that of the one preced- 

 ing in definite order, so that in action 



they resemble the waving of a field of grain in a wind, 



each cilimn corresponding to a single stalk of the grain. 



Very rapid movement is , 



made possible by these 



Pig. 131 — ParamoB- 

 cium ; c, cilia; cv, 

 contractile vac- 

 uole ; 71, micro-nu- 

 cleus ; m, macro- 

 nucleus; g, gullet ; 

 /, food vacuole; 2, 

 modified cilia (tri- 

 chocysts). 



■^^mM^M/IWM 



m 



^r 



Fig. 132 — Ciliary movement. 



structures, and their 

 permanent form permits the body to be completely 

 covered with a wall, and at the same time this rigid 

 wall ftirnishes a resistant structure which the cilia can 

 push through the water much more readily than would 

 be possible if the body were plastic. 



