These worms are very common and very diverse, having over 10,000 species identified so far. They are found in burrows or tunnels in the depths of the mud or sand of oceans or even just swimming around. Some can extend up to three meters in length. Polychaete worms are capable of burrowing and swimming due to parapodia, which are paired, un-joined lateral outgrowths from the bodies of two different invertebrate groups, mainly marine in habitat. Worms that are permanent burrowers have smooth soft muscle, which helps them get in and out of their burrows. They have two layers of muscle: circular and longitudinal that when contracted aid in the movement of these slinky animals.
Below is a video that illustrates the movement of polychaete worms along with other sea animals. Enjoy!