The theme of recent posts: tentacles! Here is a dissected squid, with certain squid guts highlighted in color. This poster-sized image is used to teach kids about squid guts in the aquarium class rooms. I based this on a previous diagram that we at Exhibits deemed too ugly to reprint - it was as if someone had hurriedly traced an illustration out of a book with a marker and then enlarged it. So I redrew the whole thing freehand in pencil, then filled with pen and ink, scanned, vectorized, cleaned up, colored.
I added "freehand" because I am always asked if I draw "freehand". It's a bit insulting - though maybe I should see it as a compliment? There are very few instances in which I will trace: if I have to make duplicate (in which case, the first one is freehand), and if I want photographic accuracy for really precise scientific illustration. Even then, I only trace outlines from photos that I've taken. I certainly don't trace other people's images.