Felix Dujardin made his largest contribution to cell theory when he proposed the classification of a new type of single-celled organism which he called rhizopoda. Felix Dujadin's rhizopoda are now known as protozoans.
Felix Dujardin is also well known for his work in refuting Christian Gottfried Ehrenburg's theory that microscopic organisms had organs with functions similar to the organs of vertebrates. When studying foraminifera, Felix Dujardin also discovered a formless life structure that he named sarcode but which we know today as protoplasm. He is also well known for his wide-ranging research into invertebrate groups, such as echinoderms, cnidarians and helminths.