ONE-CELLED ANIMALS 35 



Noctiluca live near the surface, and when disturbed at night 

 their little bodies glow like coals of fire. This class also in- 

 cludes a number of so-called colonial Protozoa such as Volvox, 

 Proterospongia and others. These are more or less closely 

 associated groups of similar individuals or colonies in which the 

 individual members show some differences and have more or 

 less special functions to perform. 



The members of the class Mycetozoa (Gr. mykes, fingers; 

 zoon, animal) resemble fungi in many respects and are often 

 included with them under the name "slime moulds." They 

 are of no economic importance. 



Protoplasm and the Cell. All the Protozoa have the body 

 composed, for its whole life, of but a single cell. By cell is 

 meant not necessarily a little enclosed or box-like bit of animal 

 substance, but simply a small (usually microscopic) mass of 

 protoplasm which is composed of an inner, denser part called 

 nucleus and a surrounding less dense part called cytoplasm. 

 Protoplasm itself, " the physical basis of life," is a substance or 

 group of substances, usually viscous or jelly-like, which always 

 contains certain very complex albuminous chemical compounds 

 called proteins. These proteins are never found in inorganic 

 matter and are always fround in living tissues. Proteins 

 contain carbon, oxygen, hydrogen and nitrogen, and are 

 almost the only group of substances found in living matter of 

 which chemists have not yet been able to make representatives 

 in the laboratory. Besides the all-important proteins proto- 

 plasm usually includes certain other characteristic compounds 

 known as carbohydrates and fats (which contain no nitrogen), 

 and various salts and gases, and always water. The gases are 

 oxygen and carbon dioxide, and the salts are compounds of 

 chlorine as well as the carbonates, sulphates and phosphates of 

 the alkalies and alkali earths. Common salt (sodium chloride) 

 is almost always present. 



What a Single Cell Can Do. All the larger animals are com- 

 posed of many cells which are grouped together to form organs 

 or tissues each with its special function to perform, but the 

 minute one-celled mass or protoplasm forming the whole 

 body of each Protozoan is able to carry on all the necessary 



