Polychaeta: More on Morphology

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 feather-like tentacles, often brilliantly colored and quite beautiful. With these, the worm fans the water for food particles and small critters.

The earliest polychaetes had no jaws, but some later polychaetes developed hard jaws, which are sometimes mineralized with iron oxide. Such polychaete jaws are fairly common in the fossil record, and are known as scolecodonts. These are borne on a muscular proboscis which allows the jaws to be either retracted or extended. Some polychaete jaws are rather complex, having as many as nine individual parts.