METAZOA 



189 



A, blood cell; 5, a cili:ited cell; C, a nerve cell showing axis cylindei at C; D, bone-form- 

 ing cell ; E, a heart muscle cell ; F, fibrous connective cells. 



other cellS; a substance like jelly, called intercellular substance. 

 This stands in the same relation to the cells as does mortar 

 to the bricks in a wall. 



Several other types of cells might be mentioned, as blood cells, 

 cartilage cells, bone cells, and nerve cells. A glance at the figure 

 shows their great variety of shapes and sizes. 



Functions Common to all Animals. — The same functions per- 

 formed by a single cell are performed by a many-celled animal. 

 But in the Metazoa the various functions of the single cell are 

 taken up by the organs. In a complex organism, like man, the 

 organs and the functions they perform may be briefly given as 

 follows : 



(1) The organs of food taking: mouth and parts which place 

 food in the mouth. 



(2) The organs of digestion: the food tube and the glands 

 connected with it. The fluids secreted by the latter change the 

 foods from a solid form (usually insoluble) to that of a fluid. Such 



