I can provide a few clues. It's very small, only a couple of millimeters long. It's marine (this picture was taken while it was under water).
Here's another individual (below). [The yellow/orange area is the stomach.]
Need more clues? It's the larval, swimming stage of a familiar invertebrate found along rocky shores. It doesn't really look anything like this once it undergoes metamorphosis into a juvenile form. But all of the stages (larva, juvenile, adult) have arms.
This larval stage will swim and feed in coastal waters for ~45 to more than 60 days. The individuals in these pictures are 39 days old. Once they return to shore and metamorphose into juveniles, they'll have five arms and a water vascular system that supports tubefeet for locomotion.
Do you have a guess now?
The answer is in the very next picture!
Yes! These are larval Bat Stars (Patiria miniata) — the stage in the pictures above is formally called a bipinnaria larva. Aren't they wonderful? Many people have encountered these adult sea stars in local tidepools, but are unfamiliar with the early life stages because they are microscopic. Welcome to the amazing world of larval sea stars!