Escher wondered what the world would be if the basic building block was an octahedron rather than a cube. He discovered that the resulting architecture contained so many angled surfaces that such structures would best be suited for underwater living. He populated his "Atlantis" with flatworms.
The flatworms within the artwork were inspired by drawings in Ralph Buchsbaum’s book “Animals Without Backbones.” Ralph’s book was first published in 1938 and Escher created his lithograph 20 years later. Ralph Buchsbaum sought out meeting Escher and discovered that Escher did indeed have a copy of Ralph’s book. Ralph, of course, obtained a copy of Flatworms and the two remained friends for the remainder of Escher’s life. Coincidentally, both of them suffered colon cancer.
Thanks to the Buchsbaum family for the photo of the original drawing from the book!