METAZOA 157 



the more each part depends upon all the others, and the more dif- 

 ficult it is to replace or to do without a damaged tissue or organ. 

 A simple protozoan can be divided and each half perform all the 

 vital functions. Needless to say this cannot be done with higher 

 specialized forms like the insect or bird, in which the interdepen- 

 dence has developed to a considerable degree. 



By increase in numbers 

 Division of Labor is made possible, 



by which 



Workmen"* " ^ Col 1st 



gain. tfafn 



Adaptation 



for 



Business 



called called 



SPECIALIZATION 



FIG. 48. Chart showing evolution of specialization. 



Forms of Metazoans. The sponges have their division of labor 

 confined to specialization of separate cells for various functions. 

 The next higher group (ccelenterates) which includes the hydra, 

 coral polyps, sea anemone, and jellyfish, have cells performing 

 similar functions grouped together in true tissues. 



The next group (true worms), such as the earthworm, carry this 

 division of labor still farther, having special digestive, circulatory, 

 and excretory organs, of complicated structure, and a true nervous 

 system with perhaps the beginning of a brain. 



Still more complicated in structure and specialized in function 

 are the molluscs which include clams, oysters, snails, squids, and 

 devil fish. These have very complicated gills for breathing, heart 

 and circulatory system much more developed, muscular and 



