230 COMPARATIVE ZOOLOGY 



secures the flexibility of the organs so essential to ani- 

 mal life. Every part of the human body, for example, 

 is moist ; even the hairs, nails, and cuticle contain water. 

 The contents as well as the shape of the cells are usually 

 modified according to the tissue which they form : thus, 

 we find cells containing earthy matter, iron, fat, mucus, 

 etc. 



In plants, the cell generally retains its characters well 

 defined; but in animals (after the embryonic period) 

 the cell usually undergoes such modifications that its 

 structural features become altered. The cells are con- 

 nected together or enveloped by an intercellular sub- 

 stance (matrix), which may be watery, soft, and 

 gelatinous, firmer and tenacious, still more solid and 

 hyaline, or hard and opaque. In the fluids of the body, 

 as the blood, the cells are separate ; i.e., the matrix is 

 fluid. But in the solid tissues they are held together 

 by intercellular substance. 



In the lowest forms of life, and in all the higher 

 animals in their earliest embryonic state, the cells of 

 which they are composed are not transformed into 

 differentiated tissues : definite tissues make their first 

 appearance in the sponges, and they differ from one 

 another more and more widely as we ascend the scale 

 of being. In other words, the bodies of the lower and 

 the immature animals are more uniform in composition 

 than the higher or adult forms. In the vertebrates only 

 are all the following tissues found represented : 



(i) Epithelial Tissue. This is the simplest form of cel- 

 lular structure. It covers all the free surfaces of the 

 body, internal and external, so that an animal maybe 

 said to be contained between the walls of a double bag. 

 That which is internal, lining the mouth, windpipe, 

 lungs, blood vessels, gullet, stomach, intestines in fact, 

 every cavity and canal is called epithelium. It is a 



