90 



PHYSIOLOGY 



simple. These animals live in the water (Fig. 53). At 

 certain seasons of the year primitive, or undifferenti- 



Fig. 53 A.— Sea-urchin. 



ated, cells multiply in their bodies in great numbers. 

 These cells are formed by special reproductive organs 



Fig. 53 B. — Sea-urchin with spines removed. 



which open to the outside by a tube or duct. The organs 

 are of two kinds and are found in different individuals 



