242 



ANIMAL BIOLOGY 



of the cell body. In a certain sense, cells may be regarded 

 as little organisms, capable of growth, and multiplication; 

 the body being, as it were, a society of cells. As cells have 

 such varied things to do in the work of the body it is very 

 natural that they have different shapes and structures, 

 and the several varieties of tissues owe their differences 

 largely to the different kinds of cells of which they are 

 composed. The more common tissues fall into the follow- 

 ing classes: 



Epithelium. Epithelial tissues occur commonly in the 

 form of layers, the cells of which fit very closely together. 



FIG. 181. Forms of epithelium. 

 A, flattened or squamous; B, cili- 

 ated; C, columnar. 



FIG. 182. Fibrous 

 connective tissue show- 

 ing fibers and a few cells. 



Such tissue is found covering the outer surface of the body 

 and lining its various cavities such as the alimentary canal, 

 the ccelom, or body cavity, the interior of blood vessels, 

 etc. Sometimes the cells are very thin and flattened 

 (squamous epithelium), or sometimes nearly cubical 

 (cuboid epithelium), and very often elongated (columnar 

 epithelium). In the variety known as ciliated epithleium 

 the free edges of the cells are covered with cilia or short 

 hair-like processes which beat to and fro creating a current 

 in the liquid bathing the cell. 



Connective tissue is composed usually of scattered cells 



