CHAPTER I. 



THE CELL AND SIMPLE ANIMAL TISSUES. THE 

 GLASSIl ICATION OF ANIMALS. 



The foundation of all living bodies is a structure called a cell. 

 The cell 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 

 incalculable figures. 



In animals the cells lose their original form, whereas in plants 

 their true symmetry is more or less retained. All the parts, 

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

 and joined together in masses, forming the various groups or 

 tissues that constitute the bodies of animals. 



All animals originate from either a single cell (asexual repro- 

 duction) or the unition of two cells (sexual reproduction). 



The Cell-Steuctuee. 



The essential part of a cell is the jjrotoplasm. 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 partially 

 enclosed by means of a constricting membrane, the cell-wall. 

 The protoplasm of each cell is connected with that of the 

 surrounding cells by minute strands passing through pores in 

 the cell-walls. 



