Life of Plants and Animals 213 



All meta'zoa reproduce by means of eggs. Occa- 

 sionally the eggs develop without fertilization. They 

 are, in fact, buds instead of eggs in some instances, 

 being composed of a group of cells instead of a single 

 cell. 



In protozoa, the cell, of which the animal consists, 

 divides, each half becoming a separate individual; 

 while, in the metazoa, the cells resulting from cleavage 

 of the egg remain associated. 



Tissues. By a process of specialization, cells become 

 different, and similar ones are aggregated into groups 

 constituting the tissues of the animal. These may be 

 classified as (i) epithelial, (2) connective, (3) carti- 

 laginous, (4) osseous, (5) dental, (6) adipose, (7) mus- 

 cular, (8) nervous tissues. 



ORGANS, SYSTEMS, AND FUNCTIONS. The tissues 

 are arranged into organs. The latter are aggregated 

 into systems, carrying on the vital functions of the 

 animal. 



The functions may be classed as: (i) nutritive, 

 involving the physiological processes of (a) ingestion, 

 (b) digestion, (c) absorption, (d) circulation, (e) assimi- 

 lation, (/) secretion, (g) excretion, (h) respiration; 

 (2) sensation; (3) motion; (4) reproduction. 



Sensation and motion are not peculiar to animals 

 any more than the other functions; yet animal intelli- 

 gence seems to grow out of them, and consequently 

 they may be called animal as contrasted with vegeta- 

 tive functions. 



Nutrition in animals, as in plants, is necessary, 

 because living things waste. In animals, as in plants, 

 the loss is made good by food, which is both organic 

 and inorganic. All things taken into the body, serv- 

 ing to build up the body, and to restore wasted energy, 

 may be called a food, provided the organism is not 

 injured by it. 



Inorganic foods of animals are chiefly air, water, 



