Chapter III 



THE BIOLOGY OF UNICELLULAR FORMS AND THEIR 

 RELATION TO THE BIOLOGY OF MULTICELLULAR 



ANIMALS 



Fields of Study. Since the cell is the unit of structure and of 

 function, the study of the fundamentals of animal life logically 

 begins with the cell. But it does not cease there, for all living ob- 

 jects are influenced to a greater or lesser degree by other living 

 objects. Consequently, the principles of Animal Biology compre- 

 hend something more than the lives of single cells, for they include 

 the interrelations between cells. And because animals exhibit so 

 many diflferent forms and so many aspects, they may properly be 

 studied from many different points of view. In any broad con- 

 sideration of animal life in general it is convenient to approach the 

 study of an animal, or of animals in general, with ten interrelated 

 groups of principles in mind, all of which are more or less plainly 

 illustrated by all animals. 



(i) Principles of procurement and utilization of energy and ma- 

 terials: the science of nutrition. 



(2) Principles of the release of energy by protoplasmic reactions 

 and its transformation in the functioning of muscles, nerves, glands, 

 and other tissues and in the work done by the organism as a whole; 

 important divisions of the science of physiology. 



(3) Principles of waste disposal; the physiology of excretion. 



(4) Principles of unification of the organism by internal regu- 

 lating mechanisms that control protoplasmic reactions and re- 

 sponses; the sciences of endocrinology, of neurology, and of 



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