Starfish Anatomy!
Starfish (Class Asteroidea) are five-legged, star-shaped echinoderms found all over the bottom of the ocean floor. They are marine invertebrates and typically have a central disc with five arms, however, some species (such as Pycnopodia helianthoides) have more.
Starfish have two sides - an aboral and oral surface. The aboral, or upper, surface is covered by distinct textures that varies by species and has a madreporite, where water enters and leaves their systems. Surface spines are also more distinct on this surface. Connected to the madreporite is a stone canal that connects to a ring canal, which helps regular water in the ampulla in each tube foot!
On the opposite side is a starfish's oral surface. Here you can see the tube feet down the midline of its arms, spines towards the side, and its mouth in the middle.
Now, I have an image of a dissected starfish; where this all gets interesting. We cut open one of the arms so the gonads (reproductive organs), ambulacral ridge, and pyloric stomach are semi-visible. Below the semi-dissection is a better image of the ring canal after removing the pyloric stomach (we were not able to keep the stone canal intact, but this is where the madreporite would connect to!).
While this dissection only shows the gonads visibly, starfish anatomy is very interesting in the way that you can peel back/remove the gonads to expose more organs underneath. They're almost layered like a sandwich, which is more visible in a drawn cross-section like this:
Lastly, here's a 40x microscope photo of a cross-section slide of a starfish arm that shows the tube feet (the paired downward-facing structures underneath the pointer).
