322 ELEMENTS OF COMPARATIVE ZOOLOGY. 



and when growth reaches a certain limit the animal (cell) 

 divides, and we have now two individuals in the place of the 

 original one. 



In other Protozoa different regions in the cell may be 

 specialized in different directions. A single example must 

 suffice. In the form figured we have but a single cell, but 

 it is a cell of definite shape. Exter- 

 nally the body is covered with a 

 denser layer, comparable in position 

 and use to a skin. A little deeper 

 are developed longitudinal lines of 

 contractile material which act in the 

 same way as the muscles of the 

 Metazoa, moving one part on another. 

 Over the outer surface are minute 

 hair-like organs (cilia) which are in 

 constant motion, and when the ani- 

 mal casts itself loose these serve like 

 so many oars to propel it through the 

 water. At the larger end of the body 

 these hair-like organs become much 

 larger, and they are here arranged in 

 a spiral. The effect of their constant 

 motion is to create a minute whirlpool 

 in the water, the centre of which is 

 in an opening in the larger end. This 

 K mav kg com pared to a mouth. The 



FIG. 146. Diagram of a * . .., ... ,. , 



Protozoan based upon water brings with it minute particles 



Stentor. c, large cilia , .. .. * T n 



around the oral disc ; cv, suitable for food, and these pass 



contractile vacuole; gr, , , ., . , . , ., 



gullet; wi, mouth; nm, through the mouth into a cavity com- 



muscular bands; n, nu- ., . , . -, ,-, 



cieus; w, nerve-ring. parable to a gullet, from which they 

 pass into the central part of the cell, where they are 

 digested. Then the indigestible portions are at last passed 



