28 PROTOPLASM AND THE CELL 



All cells have the same general structure ; all are composed of 

 protoplasm, of which a part, called the nucleus, is different from 

 the remainder of the cell body, or cytoplasm. In the higher 

 types of living things, where innumerable cells make up the 

 body of the individual, the cells are specialized to perform 

 different functions. Groups or sheets of similar cells, perform- 

 ing a like function or functions, are called tissues, and aggregates 

 of different tissues for the performance of some one function 

 are called organs, whence the term organism. In both animal 

 and plant kingdoms there are individuals consisting of one single 

 cell; these are known as the unicellular organisms and may 

 be unicellular animals or unicellular plants; if animals they are 

 called Protozoa, if plants, Protophyta. Higher in the scale we 

 find animals on the one hand and plants on the other, consisting 

 of tissues only the sponges and coelenterates among animals, 

 and some types of Thallophytes among plants. Still higher 

 in the scale, finally, are organisms consisting of organs, the 

 highest types of living things. In the following pages we will 

 consider first the organisms of one cell, then organisms of 

 tissues, and finally organisms of organs. 



The fundamental vital functions are performed by all living 

 things but there is a great difference in the complexity of organs 

 for the performance of such vital activities. We speak of organ- 

 isms as generalized when all of the physiological activities are 

 performed by a relatively few organs, and as specialized when 

 each of the necessary activities is distributed among a number of 

 organs, each organ contributing a part. With man and the 

 mammals, specialization has gone the farthest; special organs 

 composed of many tissues, each tissue of a congeries of similar 

 cells, perform the vital activities. Each organ contributes its 

 activity or product to the aggregate or individual, and all or- 

 gans act in harmony for the good of the whole. This phenome- 

 non of dividing the necessary activities among many parts is 

 analogous to division of labor in human communities, and is 

 called the division of physiological labor. With animals at 



the other extreme of the animal scale from man, all of the vital 



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processes are performed by the protoplasm comprising only one 



