ANIMAL ORGANISMS 461 



is characterized by the maintenance of a generally uniform size, an 

 approximate balance between income and outgo, and the regular carrying 

 on of all activities, including reproduction. This period is usually not 

 set off sharply from either of the two others. For a time in the early part 

 of the period some growth still occurs, though it gradually ceases, and 

 toward the end some wasting occurs, but it does not become prominent. 

 In the period of senescence, however, the katabolic processes exceed the 

 anabolic, the body shrinks, the energy production falls, and the repro- 

 ductive powers lessen or are lost altogether. These senescent changes 

 appear gradually, become slowly accelerated, and culminate in the 

 natural death of the organism. 



Throughout the life cycle changes occur in the degree of activity 

 of the various parts of the body which modify the factors concerned in 

 the production of a balance exhibited by the body as a whole. Certain 

 endocrine glands are active during youth. During maturity the activity 

 of these glands gradually lessens and the hormones produced by them 

 exert a lessened effect, while other glands reach full activity during this 

 period. In the same way the period of senescence shows a change in the 

 relative activity of different parts of the body and in the hormone pro- 

 duction of various organs. Thus the successive periods in the normal 

 life cycle follow each other as the result of a perfectly natural succession 

 of changes, and the body as a whole participates in them. 



466. Other Life Cycles. — The hfe cycles of different types of animals 

 vary greatly. Those of the malarial organism (Sec. 115), the sheep liver 

 fluke (Sec. 202), the beef tapeworm (Sec. 203), several parasitic round- 

 worms (Sees. 212 to 217), the fresh-water mussel (Sec. 257), the crayfish 

 (Sec. 298), and the mosquito (Sec. 319) have been described at greater 

 or less length, and certain aspects of the life cycle of many other forms 

 have been mentioned. In protozoans there is a period of growth, but 

 in many cases the cycle ends when full size is attained, the animal divid- 

 ing by binary fission; under ideal conditions death does not occur. 

 Many invertebrates, such as the insects, come to maturity, reproduce, 

 and die. In other invertebrates and in many lower vertebrates growth 

 never ceases but merely slows up as the animal ages; there is no period of 

 senility. Generally speaking, few animals ever live to what could be 

 called an advanced age, since as their powers diminish, those which have 

 so far survived the struggle for existence fall prey to other more vigorous 

 animals. The fierceness of the struggle for existence stands as a con- 

 stant threat to the completion of a normal life cycle. 



467. Practical Considerations. — Certain practical considerations 

 applicable to man appear as a result of the study of the life cycle. Adoles- 

 cence is a period of plasticity, of rapid change and ready adjustment; 

 senescence is accompanied by stability, slow change, and the inability of 

 the organism to adapt itself easily or perfectly to changed conditions. 



