THE ORIGIN OF THE ANIMAL KINGDOM. 69 



organs, and each can live by itself perfectly well if 

 separated from the others. Such a group does not 

 form a multicellular animal, since the cells are each 

 complete individuals. It is rather a colony of uni- 

 cellular animals. Nevertheless, such a colony is the 

 first step toward the production of the multicellular 

 organisms. From embryological evidence also there 

 can be little doubt that such was the method by 

 which the multicellular animals and plants began to 

 develop in the early history of the world. We have 

 no means of determining to what extent this forma- 

 tion of such colonies of independent animals occurred. 

 It was indeed only a stepping-stone toward the next 

 stage in the history of living things, a stage of much 

 more importance, viz., the formation of the first true 

 multicellular organism. 



To form a multicellular organism it is not sufficient 

 that there should simply be a large number of cells 

 attached together. This occurs in many of the ani- 

 mals that are regarded as unicellular. In order that 

 there should be a true multicellular animal, a meta- 

 zoan in distinction from the protozoan, it is necessary 

 that there should be a certain amount of division of 

 labor among the cells. So long as each cell carries on 

 all the functions of life in itself the aggregation of 

 cells forms simply a colony, but when the different 

 cells in such a group begin to assume different func- 

 tions e.g., some of them capturing food and others 

 digesting it, then the different cells become strictly 

 dependent upon each other, and there arises a true 

 multicellular animal. A multicellular animal is one 

 in which there is an aggregation of cells, each per- 



