Most Polychaeta have pairs of parapodia, paddle-like appendages,
running down the sides of their worm-like bodies. The parapodia
are unjointed fleshy appendages, with upper and lower lobes
terminating in a set of stiff bristles. These bristles may be
used for walking, swimming or digging, depending on the
lifestyle of the polychaete in question. A few polychaetes,
known as fire-worms, have bristles which come equipped with a
stinging poison. The prostomium, the front portion of the head
which extends out above the mouth, bears four eyes, palps and
tentacles, an impressive array of sensory organs. The eyes of
some polychaetes are quite complex, and a single individual may
have eyes using two different mechanisms for vision. The
polychaete Fabricia has eyes at both ends of its body, a useful
thing to have, since it usually swims backwards. In contrast to
free-swimming species, sessile polychaetes will often lack
complicated sense organs on their heads. Rather, they may come
equipped with large spirals of feathe