Class Polychaeta, the bristleworms, is the largest and most primitive group of annelids, and the majority are marine. They are often strikingly beautiful and very colorful and, unlike the other two groups of annelids (Classes Clitellata and Pogonophora), they show enormous variation in form and lifestyle. Apart from the head and terminal segments, all the segments are identical, each with a pair of flattened, fleshy lobelike paddles called parapodia, which are used for swimming, burrowing and creating a feeding current. The bristles, or chaetae, on the parapodia are immensely variable between species. In the sea mice, for example, they form a protective mat over the back of the worm and give the animal a furry appearance. The bristles of fireworms, on the other hand, are long and poisonous for defense, and are shed readily if a worm is attacked.