This is my favorite annelid. It is called a sea mouse, and from the top it is very difficult to distinguish as a worm as it is covered in seate (or chaete, depending on who you ask). It is not until you take a look at its underside, that you can see the segmentation common to worms, and the parapodia special to polychaetes.
The sea mouse can reach 6 to 8 inches (15-20 cm), and lives on the sea floor in from subtidal areas to moderately deep waters. (This particular sea mouse was caught in a benthic trawl at ~150 ft (50 m). They are active hunters and feed on other polychaete worms.
There is some interest in the optical properties of their seate. The seate projecting from the parapodia are often a copper color, but can turn a bluish-green when see from the right angle (and when not covered in mud). The crystalline formation of the seate is far more efficient in handling light than current man-made optical fibers .