86 ELEMENTARY ZOOLOGY. 



material serves both of these ends. In the internal 

 skeleton of the vertebrates it chiefly serves as a support 

 and an attachment for the muscles which move the 

 parts of the body. 



92. Practical Exercises for Library. Of what various substances 

 are the hard parts of animals composed ? Where do the animals get 

 the minerals that enter into them? Study out some of the various 

 ways in which animals arrange this hard material for protection; 

 compare the snail, the clam, the crab, the turtle, and others. Do 

 you find that the forms that develop a strong protective shell are as 

 active, on the whole, as those which do not? Describe briefly the 

 nature and the purposes of the human skeleton. 



93. Growth. Growth, as we have seen, is the natural 

 outcome of the use of food and is one of the most valuable 

 of organic powers, but there are no special organs or 

 tissues of growth. Just as every cell must finally assimi- 

 late its own food and use its own oxygen, so every cell 

 is its own unit of growth. Growth merely means an 

 increase of volume or rather of mass. There are three 

 forms of growth common in all living objects: (i) growth 

 by absorption of water as a dry seed grows on being 

 soaked;. (2) by the formation of living protoplasm and 

 the multiplication of cells; and (3) by the formation 

 and deposit, in or among the cells, of substances which are 

 not living matter, as starch, fats, bony materials, etc. 



Growth does not continue indefinitely, either in the 

 individual cell or in the whole body made up by the divi- 

 sion of these cells; but in each complex animal the time 

 comes when the cells no longer have the power of utilizing 

 their income in such a way as to grow and multiply. The 

 cells grow old. The growing old and the death of the 

 complex individual are due to the fact that the individual 

 cells tend to lose their powers. Yet individual cells may 

 die without the whole animal ceasing to live, and the 



