46 THE ANIMALS AND MAN 



animals," or Protozoa — is that it is composed, for the ani- 

 mal's whole lifetime, of a single cell. A cell is the structural 

 unit of the animal body. The bodies of all other animals ex- 

 cept the Protozoa, the simplest animals, are composed of 

 many cells. These cells are of many kinds, but the simplest 

 kind of animal cell is that shown by the body of an Amceha, a 

 tiny speck of viscous, nearly colorless protoplasm without 

 fixed form. The protoplasm composing the cell is differenti- 

 ated to form two parts or regions of the cell, an inner denser 

 part, called the nucleus, and an outer clearer part, called the 

 cytoplasm. Sometimes, as in the Paramaecium, the cell is en- 

 closed by a cell- wall which may be simply a denser outer layer 

 of the cytoplasm, or may be a thin membrane secreted by the 

 protoplasm. Thus the cell is not what its name might lead us 

 to expect, typically cellular in character; that is, it is not 

 (or only rarely is) a tiny sac or box of symmetrical shape. 

 While the cell is composed essentially of protoplasm, yet it 

 may contain certain so-called cell-products, small quantities 

 of various substances produced by the life-processes of the 

 protoplasm. These cell-products are held in the proto- 

 plasmic body-mass of the cell, and may consist of droplets 

 of water or oil or resin, or tiny particles of starch or pigment, 

 etc. The cell cannot be said to be composed of organs, be- 

 cause the word organ, as it is commonly used in the study 

 of an animal, is understood to mean a part of the animal 

 body which is composed of many cells. But the single cell 

 can be somewhat differentiated into parts or special regions, 

 each part or special region being especially associated with 

 some one of the life-processes. In Parammcium, for exam- 

 ple, the food is always taken in through the so-called mouth- 

 opening; the fine protoplasmic cilia enable the cell to swim 

 freely in the water, the waste products of the body are 

 always cast out through a certain part, and so on. But this 

 is a very simple sort of differentiation, and the whole body 

 is only one of those structural units, the cells, of which so 



