Thanks to Carol, we finally got to see a pilidium! This is the larval stage of a nemertean, or ribbon worm. It was in a plankton sample collected about 1 kilometer off Bodega Head.
Here's a view of the basic anatomy:
Modified from A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae (Second Edition) by Smith and Johnson (1996)
Amazingly, the juvenile ribbon worm develops inside this larva that looks like a little hat. Pilidium comes from the word pileos for "felt cap."
The pilidium is a very graceful swimmer and it also has a fascinating feeding behavior. It uses both long cilia and flexing movements of its lobes (the "ear flaps") to direct water currents and food particles towards its mouth. Here's a picture of the cilia along the edges of the lobes.
Once the juvenile ribbon worm is ready, it breaks out of the episphere, consumes the larval body, and crawls away. In this case, we got lucky, because the development of the swimming stage was nearly complete.
One day after we looked at it, we noticed that the "cap" looked smaller. And the next day, the juvenile ribbon worm had emerged!
Below are pictures of the tiny ribbon worm. The first image shows the entire ribbon worm not long after metamorphosis. The second image includes a ruler for scale (with millimeter marks).
Can you believe how small it is?
Eric was fortunate to capture a few moments in the life of this pilidium and juvenile ribbon worm on film:
We're not sure which species of ribbon worm this is. In fact, most pilidium larvae haven't been matched to their ribbon worm adults! But note that this juvenile ribbon worm has a caudal cirrus, the narrow extension at the tail end. Not all ribbon worms have caudal cirri, but the two ribbon worms I've shown on the blog do — Micrura verrilli and Micrura wilsoni.