Why did Darwin call species like the coelenterate living fossils?

Darwin referred to coelenterates, such as jellyfish and corals, as "living fossils" because they exhibit features and body plans that have remained largely unchanged for millions of years, resembling their ancient ancestors. This term highlights their evolutionary significance, as they provide insights into the early stages of life on Earth. Their simplicity and stability in form illustrate how certain species can persist through time, offering a glimpse into the past of biological diversity.