188 ZOOLOGY 



of which are ciKated and obtain food for the colony; a few, however, have 

 no cilia. These are the reproductive cells, which later break away, and give 

 rise to new colonies when the old one dies. 



Protozoa and Metazoa. — Thus there have come to exist in the 

 animal world two types of life : the Protozoa, or one-celled indi- 

 viduals, and the Metazoa, or many-celled animals. 



In the Protozoa the life processes of growth, waste and repair, 

 and reproduction, are carried on by a single cell. In the Metazoa 

 each of these functions is performed by collections of cells. In 

 the Metazoa, too, division of labor becomes increasingly more per- 

 fect as we ascend the scale of complexity in form and structure 

 toward the highest type of all, man. 



Tissues and Organs. — As we have seen in plants, this results 

 in a large number of collections of cells in the body, each collection 

 alike in structure and performing the same function. Such a col- 

 lection of cells we call a tissue. 



Frequently several tissues have certain functions to perform in 

 conjunction with one another. The arm or leg of the human body 

 performs movement. To do this, several tissues, as muscles, 

 nerves, and bones, must act together. A collection of tissues per- 

 forming certain work is called an organ. 



Tissues in the Human Body. — 'Every animal body above the 

 protozoan is composed of a certain number 'of tissues. The cells 

 making up these tissues have certain well-defined characteristics. 

 Let us see what these cells may be, what their structure is, and, 

 in a general way, what function each has in the human body. 



(1) Muscle Cells. — A large part of our body is made up of 

 muscle. Muscle cells are elongated in shape, and have great 

 contractile power. In man they may be of two kinds, voluntary 

 (under control of the will) and involuntary. 



(2) Epithelial Cells. — Such cells cover the outside of a body 

 or line the inside of the cavities in the body. The shape of such 

 cells varies from flat plates to little cubes or columns. Some 

 epithelial cells bear cilia. 



(3) Connective Tissue Cells. — Such cells form the connection 

 between tissues in the body. They are characterized by possess- 

 ing numerous long processes. They also secrete, as do many 



