28 PROTOPLASM AND THE CELL 



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 performing 

 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 only, 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, 

 the algae (in part) and Thallophytes among plants. Still higher 

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

 est types of living things. In the following pages we will con- 

 sider first the organisms of one cell, then organisms of tissues 

 and finally organisms of organs. 



The manifestations of vitality are shown by all living things 

 but there is a great difference in organization for the perform- 

 ance of such vital activities. We speak of organisms as general- 

 ized when all of the physiological activities are performed by a 

 relatively few organs, and as specialized when each of the neces- 

 sary activities is divided up among a number of organs, each 

 contributing a part. With man and the mammals, specializa- 

 tion has gone the farthest; special organs composed of many 

 tissues, each tissue of a congeries of similar cells, perform the 

 vital activities. Each small part or organ contributes its share 

 or product to the aggregate or individual and all work in har- 

 mony for the good of the whole* This phenomenon of dividing 

 the necessary activities among many parts, is analogous to 

 economic conditions known as division of labor in human com- 

 munities and is called the division of physiological labor. At 

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

 processes are performed by organisms composed of only one 

 cell. Here organic structures are reduced to their lowest terms, 



