32 



MULTICELLULAR ORGANISMS 



that, as specialists, skillfully perform their particular 

 functions. In such cases the individual cell loses its in- 

 dependence to a large degree, but in cooperating with its 

 fellows produces an organization that is as much above 

 Hydra, for example, as modern human society is above 

 a savage community. 



Tissues, Organs and Systems. — In Hydra the divi- 

 sion of labor is about as complete as in the higher types 



Fia. 8. — Body Colls, a, from skin; h, from lining of intestine; 

 c, fat cell; d, muscle cell from stomach; c, white and red blood 

 cells; /, nerve cell; g, cells from salivary gland; h, l)one cell. 



of plants, but above this point in the animal kingdom 

 the great majority of species, amounting to fully half a 

 million, are of greater complexity. In the dog or man, 

 for example, the organization of the body involves a very 

 great number of cells of different types (Fig. 8), each of 

 which is usually grouped together to form a tissue. Thus 

 we have the highly contractile cells, those responsible for 

 the movements of the body, fashioned into muscular 

 tissue. This is bound together by cells and fibres form- 

 ing connective tissue, is attached in many cases to bony 

 tissue, is impelled to act by impulses sent out by nervous 

 tissue, and directly or indirectly is protected by 



