324 Our Surroundings 



Harmful Protozoa. Some forms of protozoa cause disease 

 in man. Among these is the malarial parasite, which is intro- 

 duced into the blood of man by the bite of a certain kind of 

 mosquito. Other forms, taken into the system in water, produce 

 intestinal trouble, and one form affects the jaws, causing the 

 teeth to loosen from their sockets. In South Africa thousands 

 of natives perish from a disease known as the sleeping sick- 

 ness, caused by protozoa introduced into the blood by the bite of 

 an insect. 



Domestic animals, such as horses, cattle, and sheep, often 

 suffer from diseases caused by protozoa which secure a foothold 

 in their blood by the bites of ticks and flies. 



Metazoa. In striking comparison with protozoa are metazoa. 

 Meta means after and zoa means animals, so metazoa means 

 animals that come after protozoa. Metazoa consist of groups 

 of cells organized into tissues and organs which carry on the 

 various functions that in the protozoa are performed by a single 

 cell. For example, certain groups of cells carry on absorption, 

 while others do the work of excretion. All higher animals are 

 metazoa. Man is a metazoan. 



The bird is a good example. In order to carry on the func- 

 tions necessary for life, it has separate groups of cells for sensa- 

 tion, for motion, for food-taking, for digestion, and for every 

 other life process. 



It should be borne in mind, however, that each cell of a 

 group performs in a limited way each of these functions, but its 

 individuality and its work are merged into the general life of the 

 organism of which it forms a part. The cell is thus seen to be 

 the unit of both structure and function in higher animals. 



Physiological Division of Labor. Intestinal cells absorb, 

 liver cells secrete, nerve cells carry sensation. This doing of 

 special work by different cell groups of a body is called the 

 physiological division of labor. In a single-celled animal, such as 

 an amoeba, there is no such division. Although such an animal 

 has no mouth, it secures food; no lungs, yet it breathes; no 

 stomach and intestines, yet it digests its food. In short, it has no 

 organs, yet it performs all the essential life processes. 



