A TYPICAL LIFE CYCLE 105 



the substance of the macronucleus. At each end of the infusorian is 

 a bright spot which appears and disappears with considerable regular- 

 ity; these are the contractile vacuoles, their function being to throw to 

 the outside of the body the waste matters that are formed during the 

 physiological activities of the cell. Each vacuole is supplied by a series 

 of canals from various parts of the body, the waste matters in fluid 

 form collecting in them to be emptied into the contractile vacuole and 

 thence disposed of. The peripheral protoplasm of paramecium is 

 filled with minute thread-like structures, the trichocysts, which are 

 thrown out when the cell is irritated. On the outside of the body, 

 finally, is a dense covering of minute lash-like whips which are con- 

 stantly in action during life, and by means of which the organism moves 

 about freely in the water, turning the while on its long axis. These are 

 the cilia which are arranged in spirally wound lines around the body, 

 while a somewhat more powerful set are located in the asymmetrical 

 peristome and are used to direct a food current toward the mouth. 



FIG. 37 



Paramecium aurelia. Macronucleus normal; micronucleus abnormally large. 



The food consists of any proteid matter small enough to pass 

 through the mouth opening. The organism will take in bits of flesh, 

 or parts of vegetable matter, or bacteria or lifeless matter, such as 

 carmine or indigo granules, all with equal voracity. The process of 

 ingestion is hastened by the activity of an undulating membrane 

 situated in the small gullet, and the bacteria or other food matters are 

 collected in a vacuole which forms at the base of the gullet. Con- 

 siderable water is taken in with the food, and when the vacuole is 

 large enough it is caught up in the protoplasmic flow and carried away 

 from the mouth opening. Numerous gastric vacuoles are thus formed 

 and the food is digested in them. 



When the organism is fully grown it reproduces by dividing into 

 two cells, each cell having the characters of the former one cell, which 

 has disappeared, indeed, although it has not died. Its protoplasm is 

 still living in the two daughter cells; these repeat the processes of 

 digesting and growing, and finally, each of them reproduces by trans- 

 verse division. The metabolic processes leading to reproduction by 

 division are thus repeated generation after generation, and, having all 

 that is necessary in the form of cellular organs for an indefinitely 



