76 ELEMENTARY ZOOLOGY. 



81. The Functions of the Complex Animals. The 

 higher animal starts in life as a single cell, similar to those 

 spoken of in Section 80. Their earliest activities, there- 



FIG. 8. Types of tissue cells, i, white (ameboid) blood cells; 2, red blood cells; 

 3, ovum; 4, sperm cells; 5, ciliate epithelium; 6, sensory epithelium (from retina of the 

 eye); 7, flagellate (collared) epithelium; 8, a nettling cell from Hydra; 9, a mucus 

 glandular cell; 10, connective-tissue cells; n, fat cells; 12, a bone cell; 13, an unstriate 

 (plain) muscle cell; 14, a ganglion (nerve cell) with its branches. 



Questions on the Figure. Compare the cells in this figure with 

 those of the preceding and notice the similarities of differentiation. 

 By reference, discover the functions of these various types of cells 

 and see in what ways their structure fits them for their work. What 

 points about them are constant? What most variable? 



fore, are similar to those of the one-celled animals. Ow- 

 ing, however, to cell-division and to the fact that the 

 cells remain together when they divide, these bodies in 



