THE PHYLUM PROTOZOA 



151 



V-CilicL 



Cuticlfz- 



Cecil Surface) 



Connecting 

 fibril ^ 



Basal body ^--Trichocyst 



Figure 8.3. Cilia. A small bit of the surface of a ciliate, showing the cuticle, pro- 

 jecting cilia, and underlying structures. The trichocysts are discussed later in the text. 



the movement of the flagellum. The flagellum may follow any of several 

 different patterns of movement at different times, but nothing is known 

 of the way in which this is controlled by the cell. The basal body is 

 often joined by a filament to the centriole, from which it may be pro- 

 duced during development. It is considered to be a modified centriole, 

 controlling the activity of the ffagellum just as the centriole of the sperm 

 dtiring spermiogenesis (p. 119) gives rise to the axial filament of the 

 sperm tail and presumably is important in controlling its movement. 



The ciliates also have a basal body (Fig. 8.'^) at the base of each 

 ciliimi. These have no connection with the centriole, but are connected 

 with each other by a network of slender fibrils, made visible with a silver 

 stain, the kind of stain used to demonstrate nerve fibers in the metazoa. 

 Each basal body activates its cilium, and coordination among the cilia 

 is accomplished through the fibrillar network. The basal bodies near 

 the mouth initiate a wave of activity that passes over the body of the 

 animal. Wave follows upon wave, as shown in the ciliary motion of 

 the Paramecium (Fig. 8.12), producing a smooth, rapid motion. Micro- 

 surgical incisions that cut across the connecting fibrils produce a local 

 asynchrony among the cilia, and may seriously disrupt locomotion. 



In many Hagellates a visual organelle (Fig. 8.2) is associated with 

 the conductile and locomotor organelles. The visual organelle ot Eiiglenn 

 has two parts, a patch of red pigment in the protoplasm beside the 

 Hagellum, and a tiny, light-sensitive photoreceptor on the flagellar base. 

 The shading of the photoreceptor by the pigment spot enables the 

 animal to determine the direction from which the light comes. In other 

 species the photoreceptor is seated in a pigment cup with the opening 

 anterior. In a few cases the thin cuticle covering the animal is swollen 

 over the cup to produce an optic lens. 



Confracf//e Vacuoles. A prominent structure in many protozoans 

 is an excretory organelle, the contractile vacuole (p. 93). It is found in 

 all fresh-water forms and in many marine species, but is uncommon 

 among the parasites. A fresh-water environment is hypotonic to the 

 protozoan and a method of removing water that enters through osmosis 

 is needed. In marine forms (which are always isotonic with sea water) 

 the contractile vacuole is used to excrete the water that accumulates in 



