50 PRINCIPLES OF ANIMAL BIOLOGY 



lie deeper. In larger organisms, however, a transport system is required, 

 as discussed in Chap. 11. 



Excretion. — Metabolism results in the formation of various gases, 

 water, and other compounds, which are of no value in the body or would 

 be harmful if allowed to accumulate. The process of their elimination 

 is called excretion. Gases resulting from metabolism are eliminated 

 along with carbon dioxide in respiration. Other waste substances pass 

 through the cell membranes to the exterior, or in some of the protozoa 

 they are collected by the contractile vacuoles, along with excess water, 

 and voided through the outlets of these organs. In higher animals 

 excretions are taken up by the blood and lymph, from which they are 

 then separated by special organs. 



Secret;;ion. — All cells produce certain chemical compounds which may 

 be used in the processes going on within the cell or in cavities adjoin- 

 ing the cells. Such products are called secretions. They differ from, 

 excretions in that they are used in performing some function. Many of 

 the secretions which are discharged from the cells are first stored in the 

 cells as granules, which finally break out of the cell and then become 

 gaseous or liquid. Other secretions produced as liquids within the cell 

 diffuse out and escape as rapidly as formed, are absorbed by other cells, 

 or are carried in the blood stream. Such secretions may perform their 

 functions at a considerable distance from the cells where thej' were 

 elaborated. Secretions are very diverse in their uses. Some aid in 

 digestion, others give protection because of their odor or because oi 

 poisonous properties, some serve as lubricating material, others oxidize 

 readily with the production of light as in fireflies. 



Growth. — Growth is caused by the conversion of foods into proto- 

 plasm at a more rapid rate than protoplasm is being broken down. 

 Increase in the size of cells may not be wholly due to increase in the 

 quantity of protoplasm. Fat cells increase in size because of the depo- 

 sition of globules of fat, a process which may be continued until there is 

 much more fat than protoplasm. In plant cells and certain animal cells 

 volume may be increased by the imbibition of water which may be 

 stored in vacuoles. In such extreme cases as those mentioned, the 

 quantity of protoplasm may be actually decreased, although the cell 

 may be larger. 



Reproduction. — Reproduction, or the formation of new individuals, 

 is likewise characteristic of living beings. In unicellular organisms, and 

 only in these, reproduction is equivalent to cell division. In higher 

 organisms, reproduction usually involves the formation of special cells, 

 the germ cells, which by their division, with rearrangement of the result- 

 ing cells, give rise to new organisms. Here reproduction involves cell 



