CHAPTEE I. 



THE CELL AND SIMPLE ANIMAL TISSUES. THE 

 CLASSIFICATION OF ANIMALS. 



The foundation of all livinj^- bodies is the cell. The ceil is 

 more or less the unit of life, and may even of itself constitute 

 a definite organism. Most organisms are nevertheless built up 

 of numbers of these cell units, numbers reaching into incalcul- 

 able figures. 



In animals the cells lose their original form, whereas in 

 plants their true symmetry is more or less retained. All the 

 tissues, then, of the animal (and plant) are composed of cells 

 collected and joined together in masses, forming the various 

 groups or tissues tliat constitute the bodies of animals. 



The Cell-Steucture. 



The essential part of (he cell is the protoplasm. This proto- 

 plasm is a clear gelatinous substance which is found in all cells, 

 both animal and vegetal. It has been described by Huxley 

 as the "physical basis of life.'' Generally protoplasm is par- 

 tially enclosed in the cell itself by means of a constricting 

 membrane, the cell-wall. The protoplasm of each cell is con- 

 nected with that of the surrounding cells by minute strands 

 passing through pores in the cell-walla. 



This living matter may also be observed in a naked or free 

 state. 



