Directions: Match each item with its correct description. Write the letter of the correct answer on the space before each item. 9. Brain a. bone that protects the brain 10. Heart b. pair of organs where gas exchange happens . 11. Mouth c. main organ of integumentary system 12. Skull d. main organ of circulatory system 13. Ribs e. main organ of nervous system 14. Skin f. organ responsible for breaking of food into smaller pieces 15. Lungs g. bones that protect heart and lungs.