When I took an embryology course last summer, I decided to adapt the hand-tracing aesthetic to the illustration of marine larvae. The result? A Hand Pluteus in 10 easy steps!
PART I: The Body
Step 1. Trace your hand.
PART II: The Skeleton
Step 4. Get a new color. Start with the postoral skeletal rods.
Step 8. Get a new color. The digestive tract runs through the center of the pluteus, and its muscular wall is quite thick. The overall shape changes with peristalsis during feeding, so it's all right to take some liberties here.
Step 9. Now that your pluteus has a gut, you can feed it. Green algal cells would be appropriate.
You don't know what a pluteus is?
Oh. Well. It's the larval form of echinoids (sea urchins and sand dollars) and ophiuroids (brittle stars). They acquire their arms in pairs as they develop, leading to progressively older two-arm, four-arm, six-arm, and eight-arm plutei. You've just drawn a six-arm pluteus, of course. Here's the six-arm pluteus of Dendraster eccentricus, the Western sand dollar, in two focal planes for enhanced 3D effect: