A miniature red train

In a discarded barnacle shell from my low-current aquarium tank, this small polychaete worm had made his home. He came out to see what was happening while I cleaned the tank.

"Where's my food gone?"

Watching him, I noticed something I had not seen so clearly before. See that red dotted line down his central back? See how it's paler at the head end, and also fades towards the back, just after the fifth dark line?

I watched him slither about; that pattern is repeated several times along the body length. But it's not static. The red line runs at speed from tail to head, looking rather like a miniature red train, hurrying so as not to get run into by the one following it. At the head end, it vanishes, but the next one is only a pair of segments behind.

What we're seeing is the main part of the worm's circulatory system, the blood running up to the head in the upper blood vessel. It returns to the tail through a second vessel running down the belly side.

Cross-section of polychaete. Image by Hans Hillewaert, from Wikipedia.

The worm has no heart; the blood vessels contract rhythmically to push the blood along, creating that busy train track effect.

I returned the broken barnacle shell, worm and all, to the tank.