MUSCULAR AND SKELETAL SYSTEMS. 



239 



character and the saucerlike (Fig. 154) or bell-shaped 

 form and graceful movements of these beautiful crea- 

 tures are well known. Their locomotive apparatus 

 consists of fine muscular fibers, arranged in circular 

 and radiating manner on the interior of the saucer or 

 bell. When these fibers contract, the saucer or bell 

 is drawn together, the water expelled, and the animal 

 driven in the contrary direction. When the fibers relax, 

 the somewhat firm, gelatinous mass again expands by- 

 its own elasticity to its original form and size. 



PROTOZOA. 



Infusoria. — In these, again, we have a wholly differ 

 ent mode, viz., ciliary motion and locomotion. If the ani 

 mal be fixed, as a, then the incessant lashing of micro 

 scopic cilia, situated mainly 

 about the mouth, creates 

 whirling currents, which lead 

 down to the throat, and thus 

 contribute to alimentation. 

 Whatever is suitable as food 

 goes down ; what is not, is 

 rejected and carried away 

 by the same current. But 

 if the body is free, 6, the 

 same ciliary motion, react- 

 ing against the water, produces the most vivacious loco- 

 motion. Under the microscope it is seen to whirl and 

 dart about in every direction without visible means; 

 but with the higher power, especially when the motion 

 is slower, the incessant lashing of the cilia is visible. 



Rhizopods. — Here we have motion, as well as all 

 other functions, reduced to simplest terms, and find its 

 origin in general contractility of protoplasm. The struc- 

 tureless, or almost structureless, mass of living jelly con- 

 17 



Fig. 155. — Infusoria : a, an at- 

 tached form : d, a free form. 



