PRIMARY FUNCTIONS OF THE ORGANISM 37 



localization of the function of contractility to certain 

 regions of the cell. 



In Vorticella, a ciliate protozoan that is usually 

 rooted plantlike to a fixed base, the long stalk which 

 the cell-body develops 

 (see fig. 14) is very 

 contractile, extending 

 the vorticella-bell to 

 a considerable dis- 

 tance and then sud- 

 denly pulling it back. 

 During this move- 

 ment the stem coils 

 and uncoils like a 

 spring, owing to the 

 presence in it of a 

 very contractile fiber 

 of differentiated pro- 

 toplasm. The stem 

 contains little else 

 than this contractile 

 fiber (myoneme, of 

 authors), and we may 

 say that the sole func- 

 tion of the stem, aside 

 from that of support- 

 ing the "bell," is to withdraw it out of danger or ex- 

 tend it into an area of greater food supply. In return 

 for this the bell eats and digests and reacts for both. 

 Here is a division of labor that has proceeded so far 

 that a different kind of protoplasmic substance has 



FIG. 14. Vorticella: a, extended; 

 6, contracted ; c, the stalk more highly 

 magnified, showing the contractile fiber 

 which is not seen in a and b. 



