During a low tide this afternoon, I photographed a small cluster of Sandcastle Worms (Phragmatopoma californica) in the rocky intertidal zone. In the first photo you can see two size classes -- this is a head-on view of the worm tube openings. Sandcastle worms have free-swimming, planktonic larvae that settle gregariously. I wonder if the two different sizes represent worms that settled in different years (e.g., larger in 2010, smaller in 2011), or if they just settled at different times within one year?
In this second photo notice the disc-like purplish black operculum (lower right) sealing the opening to the tube. This is similar to the "trapdoor" of a snail -- a hard covering that protects the animal from desiccation at low tide.
And here is a photograph of a Sandcastle Worm larva taken under a microscope (the larva is ~1 mm long). The anterior end (or the head end) is on the left. Two bands of cilia (they look like short hairs) encircle the larva and are used for locomotion and feeding. The long bristles (or setae) can be extended out to the sides and are used for defense.