I mean, I don't have the encyclopedia anymore, so I guess I can't really prove it to you. But it was sort of my introduction to the idea that not everything in an encyclopedia is necessarily true, so it's pretty memorable for me. I recognize the irony of using another encyclopedia here, but Wikipedia mentions in the History section of its Pain in Animals article, "Researchers remained unsure into the 1980s as to whether animals experience pain." (https://en.wikipedia.org/wiki/Pain_in_animals#History ) I obviously don't know the story about how the line ended up in World Book, but I know they employ fact checkers for every line in the book, so presumably in the 50's the line reflected some kind of scientific consensus, as bizarre as that sounds now.