He turned and said, "Those dark areas are the pyloric caeca!"
We had observed the way these small Sunflower Stars looked dark in the center (see below), but without looking closer, we had assumed it was pigment. Now with a better view under the microscope, we could see that it wasn't pigment. The body wall of the sea star was transparent, so we were seeing through to the inside of the sea star and actually viewing the digestive system!
Here's a diagram with an explanation (below). A sea star has two stomachs — the cardiac stomach is the one you might be more familiar with; it's everted (extended outside of the animal) when the sea star is feeding. The pyloric stomach is located above the cardiac stomach, and it has branches that reach into the arms. These branches are called pyloric caeca (there are two in each arm).
Modified from Biology of the Invertebrates (Pechenik 2010)
Many sea stars store energy in their pyloric caeca during the spring and summer. In the fall and winter they transfer that energy to their gonads for reproduction.
When sea stars are older, their body wall is no longer transparent, so the pyloric caeca can't be seen. It was a treat to be able to see both the polygon-shaped pyloric stomach and the pyloric caeca in this juvenile Sunflower Star!
(Note that the newest arms, the shortest ones, don't yet have pyloric caeca, but they will develop them later.)