In the picture above, that's the head on the left side. There's one narrow white band near the leading edge, and then at least four thicker white bands after that. Following these first five bands, the white bands become much closer together (see images below).
The "six lines" for which this species are named are the very narrow white stripes running down the length of the body from the head to the tip. Several are visible on the dorsal (upper) surface, and a few are on the ventral (lower) surface.
When stretched out, this ribbon worm is quite long — I estimated about 30 cm. (Some individuals may reach 1 meter or more!) When looking at the center of the ribbon worm, at first I had trouble identifying the upper and lower surface. I could see a difference — one had white speckles and the other was more uniformly brown (see next picture), but I wasn't sure which was which.
I had to go back to the head and trace the upper and lower surfaces, and finally figured out that the upper surface is the speckled one. Now you can identify both in the next two pictures, even as the ribbon worm is twisting:
Many books and images on the Internet show the head and middle portion of Six-lined Ribbon Worms, but I've hardly seen any pictures of the posterior end. Here's a picture of the all white tip!
I'm including one more picture of this beautiful ribbon worm (although it would be easy to show many more!). It's fun to think about the color patterning of this species. Why does it have so many stripes and bands? Does it help blend in with light and shadow? Is it distracting to potential predators? Is it disruptive coloration (breaking up the outline of the animal to confuse predators)? Do you have other ideas?