LEVELS OF ORGANIZATION 



CHAPTER 8 



Introduction to 

 the Metazoa 



Animals show different levels of organiza- 

 tion, and organization increases in complex- 

 ity from one level to another as they are 

 studied from the structurally simple to the 

 most complex. Our plan of study is to con- 

 sider animals in approximately the order in 

 which they have evolved. The patterns of 

 organization do not suddenly appear fully 

 formed, but are usually foreshadowed some- 

 where before becoming definitive. For ex- 

 ample, a hint of tissue, but not true tissue, 

 in the sponge, emerges as a definite tissue- 

 level in the jellyfish. 



Our studies of the Protozoa have given us 

 insights into the cell-level of organization, 

 and we will consider in increasing complex- 

 ity the following additional levels of organ- 

 ization which exist in all the other animal 

 phyla collectively called Metazoa. The 

 tissue-level of organization includes a group 

 of specialized cells similar in structure and 

 associated together to perform some definite 

 function; at the organ-level, there is co- 

 operative specialization of groups of tissues 

 to form an organ which performs one or 

 more special functions; at the organ-system- 

 level, there is close cooperation among sev- 

 eral organs to perform some general func- 

 tion such as digestion. 



This brief evolutionary history of levels of 

 organization with gradual increasing com- 

 plexity from one-celled protozoans to the 

 most complex metazoans covers a time span 

 of several hundred million years. 



THE METAZOA AND 

 PROTOZOA COMPARED 



78 



The somatic cells of metazoans are not all 

 alike as in the colonial protozoans, but differ 

 from one another both in structure and in 

 function. The body cells are not independ- 

 ent as in most of the protozoans, but are 

 dependent upon one another. This is the 



