Protozoa. 301 



Protoplasm is killed by very high temperature, killed or 

 its activity checked by low temperature, and, in general, 

 requires the conditions that we usually recognize as neces- 

 sary for life. 



In its chemical composition protoplasm is exceedingly 

 complex. Protoplasm is a living substance, and any 

 attempt to analyze it kills it; hence its exact composition 

 cannot be known. But the dead material left when it has 

 been killed can be analyzed, and consists largely of a sub- 

 stance called proteid. This consists of carbon, hydrogen, 

 oxygen, and nitrogen, with some sulphur, traces of iron, 

 and compounds of phosphorus, potassium, calcium, and 

 magnesium. Protoplasm seems to be a very unstable 

 compound, as we should naturally expect from its com- 

 plexity. Then, too, in its life and growth, it is constantly 

 changing, and, undoubtedly, changes more or less in its 

 composition from time to time. 



The Cell. Sometimes protoplasm occurs in a consider- 

 able mass, without any separation into distinct parts. But 

 usually it is found in more or less distinct particles, .and 

 these distinct particles of protoplasm are called " cells." 

 There may be a cell wall distinct from the mass of proto- 

 plasm, but this is not essential to a cell. Within the cell is 

 a more dense appearing portion called the "nucleus." A 

 cell living independently tends to be spherical, though since 

 it has the power of changing its shape, it often departs 

 from the typical form. When an amoeba goes into the 

 resting stage it assumes the spherical form. 



Protozoa and Metazoa. The protozoans are typically 

 one-celled animals. The other animals are many-celled 

 and are called metazoans. Their greater size is not due 

 to their having larger cells, but to the increased number 

 of cells. 



