CHAPTER VIII 



METAZOAN ORGANIZATION 



All animals whose bodies consist of few or many cells functioning 

 as a unit are called metazoans. In most respects the vital activities 

 of Metazoa are similar to those of Protozoa. Since Metazoa are 

 more or less like compound Protozoa with some degree of inter- 

 cellular differentiation, it is thought by many authorities that they 

 arose through organization of single-celled organisms. In some forms 

 of compound or colonial Protozoa, only two cells adhere together after 

 cell division, but in others the cells may remain attached after many 

 di\^sions. The size of different colonies may range from two to two 

 thousand similar cells. In the most complicated protozoan colonies 

 there may be several different types of cells. The representatives 

 of class Mastigophora are the most likely ancestral forerunners 

 of Metazoa. The colonial forms, such as Gonium, Pandorina, 

 Eudorina, Pleodorma, and Volvox, are rather plantlike in character- 

 istics, but a series of this type shows the possibility of the relative 

 complexity of different colonial forms. There are several genera of 

 animals which are intermediate between Protozoa and Metazoa, but 

 for the most part the two groups are fairly distinct. 



General Characteristics 



This group includes all of the strictly many-celled animals. The 

 cells are definitely organized and classified morphologically as well 

 as physiologically. There is a well-regulated division of labor. 

 Among the single-celled animals each cell, like primitive man, is 

 largely independent of its fellows, doing for itself all that is neces- 

 sary to carry on living processes. In the many-celled animal, as in 

 a highly developed society of men, certain individual cells become 

 more proficient in doing certain kinds of work, and as a result, a 

 special group is able to care for a particular function necessary to 

 the life of the entire organism. In return, other special groups care 

 for other functions. In this way each exchanges the products of 

 its labor for the products of the labors of the other groups. In 

 human society this becomes more and more complicated as civiliza- 



10.1 



