While the main image depicts a very early naturalist simply observing a lion, two additional smaller images depict animals that raised problems of classification. The starfish, sometimes difficult to appreciate as an animal, seemed to be an invertebrate but wore its skeleton on the outside. The zebra, clearly a vertebrate and related to the horse, had caused problems in prior classifications when it was not clear which characteristics should be employed. It was the subject of Academicians William King Gregory, "The Horse in the Tiger's Skin."(1926) and of Stephen Jay Gould, "What, if anything, is a zebra?" (1981).