DERIVED ORGANIZATION— TAX0N0M1C STRUCTURES 153 



layer containing trichocysts if the latter are present; (3) a contrac- 

 tile zone containing the basal bodies of cilia, myonemes and coordin- 

 ating fibers; (4) a denser zone which shades off into the endoplasin 

 and supplies an anchorage for nuclei and contractile vacuoles. 

 A single cilium is constructed on much the same plan as a flagel- 

 lum, consisting of a central axial filament or fiber, and an elastic 

 sheath of protoplasm. Movement is due to the active contraction 



Fig. 83. — Cyclidium glaucoma. Cilia with axial filaments protruding from plasmic 

 sheaths. Silver line technique. (After Klein, Archiv f. Protistenkunde, 1929, 

 courtesy of G. Fischer.) 



in one plane of the axial fiber and recovery to the elasticity of the 

 enveloping sheath. The contractile element originates from a basal 

 body in the contractile zone. In many organisms local thickenings 

 occur at intervals along the axial filaments. These are similar to 

 basal bodies and are clearly demonstrated by silver nitrate impreg- 

 nations for bringing out the silver line system (Figs. 82 and 83). 

 The arrangement of cilia on the surface of the body varies in 



