This time, we start with vocabulary.   Cnidarian.   It’s a Greek word built on the simpler word meaning knídē  or “nettle.”  And a “nettle,” as a noun, is a stinging plant.   For animals, being cnidarian means you share something in common with those plants. You sting. You’re a jellyfish, a coral, a Portuguese man o’ war, an anemone.   (Don’t you love saying the word anemone , the way it rolls off the tongue? Get ready to pronounce it many times in the paragraphs ahead.)   C