But now I know it comes from Kunstformen der Natur or Art Forms of Nature by Ernst Haeckel.
Ernst Haeckel isn't exactly a mid-century artist, in fact much of his work isn't even from the same century as mid-century art — Art Forms of Nature was first published in 1899.
Besides me finally finding the image of the octopus I've been looking for, I found a book full of gorgeous illustrations of nature.
I mention the book and the images with here because I think they would look outstanding on the wall of any mid-century or mid-century modern home. I love when art plays against the look of a room. The elaborate illustrations and lines of Haeckel's work would play lovingly against the streamlined look and straight lines of mid-century modern furniture.
The best part of it is that the images in Art Forms of Nature are now public domain, so if you can find a high enough resolution image, you're free to print it and hang it on your wall.