Jovan Hadži theories of animal evolution. He devised a system of classification in which he divided the animal kingdom into six phyla: Protozoa, Parazoa, "Ameria" (animals with no segments), "Oligomeria" (animals with few segments), "Polymeria" (animals with many segments) and Chordata. His choice of characters important for classification was generally discredited by his contemporaries, and the system was never accepted by zoologists. However, due to its simplicity, the system was widely used in science education in former Yugoslavia. His other major theory was that of the origin of metazoa - he developed an existing hypothesis stating that the first multicellular animals resembling
Tullimonstrum characteristics of the well-known modern phyla, it was speculated that it was representative of a stem group to one of the many phyla of worms that are poorly represented today. Similarities with Cambrian fossil organisms were noted. Chen "et al." suggested similarities to "Vetustovermis planus". Others pointed to a general resemblance between "Tullimonstrum" and "Opabinia regalis", although Cave "et al." noted that they were too morphologically dissimilar to be related. In 2016 a morphological study showed that "Tullimonstrum" may have been a basal vertebrate, and thus a member of the phylum Chordata, with one study suggesting "Tullimonstrum" may be closely