38 ELEMENTARY ZOOLOGY 



will be learned in the next exercise, the bodies of all 

 other animals except 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 Amoeba, a tiny speck of viscous, nearly 

 colorless protoplasm without fixed form. The protoplasm 

 composing the cell is differentiated 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 Paramxcium, the cell is enclosed 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 essen- 

 tially of protoplasm, yet it may contain certain so-called 

 cell-products, small quantities of various substances pro- 

 duced by the life-processes of the protoplasm. These 

 cell-products are held in the protoplasmic 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, because 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 ^art or special region being especially 

 associated with some one of the life-processes. In 

 Paramoecium, for example, the food is always taken in 

 through the so-called mouth-opening; the fine proto- 

 plasmic 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 



