The Life Cycle of the Single Cell 15 



which waft the food down to the esophagus and thence into the interior 

 of the animal in the form of food vacuoles. The vacuoles circulate until 

 digestion is complete. The waste products are eliminated through the 

 anal pore. The surface of the animal is astonishingly intricate. It is cov- 

 ered by longitudinal rows of hexagons whose edges form distinct ridges. 



Radiating cana 



Cil 



Trichocysts 



(defense organelles) 



Fig. 5. A schematic drawinq of Paramecium, showing the many organelles. 



Each hexagon contains a basal granule from which a cilium arises. There 

 are about 2,500 cilia and the ciliary pattern is inherited in a precise 

 manner. 



BINARY FISSION 



Well-fed Paramecia reproduce about five times per day. The cell 

 simply constricts at the middle and separates into anterior and posterior 

 daughter cells. At first these are readily distinguishable as the front and 

 rear ends, but they soon change into two identical and small but otherwise 

 normal animals. Prior to the constriction, the three nuclei divide. The 

 macronucleus elongates and the two halves pull apart. The micronuclei 

 divide by normal mitosis. One product of each of the three nuclei passes 

 to each daughter cell. A number of other cell structures duplicate during 

 cell division. Buds protrude from the mouth and gullet of the dividing 

 animal. The buds pass to the anterior daughter cell and produce a new 

 mouth and gullet. The posterior daughter retains the old ones. The ciliary 

 basal granules also bud off duplicates of themselves and these produce 

 new cilia. The granules, cilia, and hexagonal ridges are then rearranged 

 upon the daughter cells in their usual precise geometric pattern. 



