_Note._--Show by coloring the regions on a blank map of the world. 17. 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 cœlenterates, 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 animals which have differentiated dorsal and ventral surfaces, and right and left sides. Animals which do not have such differentiated surfaces are usually _radially symmetrical_, but sometimes asymmetrical (without symmetry). _Girdle_, the somewhat transparent band frequently found near the anterior end of an earthworm. _Anal opening_, the posterior opening of the food canal. _Setæ_ (singular form, _seta_), small bristles or stiff hairs. In the earthworm these are set in the body wall at definite intervals, and aid in locomotion. _Cuticle_, in the earthworm a delicate, shining cover over the body. _Egg capsules_, small, light-colored, spindle-shaped sacks, about the size and somewhat the shape of a grain of wheat, containing the eggs or young of earthworms. _Directions._ Take a living earthworm to your table and keep it damp by placing it in a wet tray or upon moist paper. Identify the anterior and posterior ends, the dorsal and ventral surfaces, and the right and left sides. Identify also the somites and the girdle, the mouth with its projecting lip, and the anal opening. _Observations._ 1. Watch a living worm for some time. Does it seem to have a definite object in its moving? If so, what is it? Upon what sense or senses does it seem to depend for guidance? Which end usually leads? Why? 2. Over what sort of surface does it move most easily? Why? Watch it closely for some time and discover how it is able to move from place to place. (_Suggestion._ What is the function of the setæ in this process? How can you explain the alternate contraction and expansion of parts?)