78 GENERAL BIOLOGY 



flagella, inserted at the end or side of the cell. It is 

 supposed that both cilia and flagella are hollow 

 extensions of the cuticle, which by a sudden contrac- 

 tion on one side produce a resultant movement 

 like that of a fishing pole when a fly is cast. Fixed 

 ciliated cells are also found as components of many 

 tissues in Metazoa, as in the nasal passages of verte- 

 brates. By the force of the beating cilia currents 



FIG. 27. The electric ray (Torpedo) ; the skin partially cut away so 

 that the electric organ, a, is visible ; it consists of numerous polygonal 

 columns of modified muscular tissue. (From Verworn, after Ranvier.) 



of liquid are urged along over the surface of the 

 epithelium. 



The most highly developed tissue of locomotion 

 is, of course, striated muscle. When a muscle con- 

 tracts, it shortens and thickens without changing 

 its bulk. Movement is communicated through its 

 fixed tendons to whatever bones or other structures 

 it may be attached. When a muscle has made a 

 number of contractions, it shows a marked increase 

 in acidity. This is due to the appearance of sar- 



