STRUCTURE AND FUNCTION 6i 



1 7. How may corals form a reef ? Why do they, as a 

 rule, form a reef instead of adding directly to the mainland ? 



18. Give Darwin's theory regarding the way a coral 

 atoll may have been formed. 



19. Where are fossil corals found in abundance ? What 

 does their presence prove ? 



20. What is polymorphism ? Give an illustration. What 

 may be a disadvantage of polymorphism ? What may be 

 an advantage ? 



21. In what ways is this group of economic importance ? 



4. A STUDY OF WORMS 



To show cells associated even more closely than in 

 coelenterates, forming tissues and systems of organs. 



A Study of Earthworms 



The Living Earthworm 

 Materials. 



Living earthworms, some of which are left undisturbed 

 from day to day, in damp earth with leaves of various 

 plants scattered upon it. 



Definitions. 



Anterior end, the head end, usually the leading end. 



Posterior end, the end opposite the anterior end. 



Ventral surface, the lower surface, usually the one which 

 contains the mouth. 



Dorsal surface, the one opposite the ventral surface. 



Somites, the rings or segments of which some animal 

 bodies are composed. 



Bilateral symmetry, the symmetry usually shown by ani- 

 mals which have differentiated dorsal and ventral surfaces, 



