What is some of the symbolism of the lulav, etrog, etc.?

The lulav (palm), etrog (citron), hadas (myrtle), and aravot (willow) are joined together, and a blessing made over them during the Sukot holiday (except on the Sabbath). They are symbolic of how we should serve G-d. The etrog is shaped like the heart (considered to be the seat of wisdom), indicating that we should serve G-d with our minds; the lulav is like the spine - our entire body should be used for G-dly purposes; the hadasim are shaped like eyes, for we are enjoined not to go after that which our eyes desire. The arava is like the mouth, for we should fill our mouths with words of Torah, and be vigilant with our speech.