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
Neuroscience of sleep to have evolved, the functions of sleep should have provided a substantial advantage over the risk it entails. In fact, studying sleep in different organisms shows how they have balanced this risk by evolving partial sleep mechanisms or by having protective habitats. Thus, studying the evolution of sleep might give a clue not only to the developmental aspects and mechanisms, but also to an adaptive justification for sleep. One challenge studying sleep evolution is that adequate sleep information is known only for two phyla of animals- chordata and arthropoda. With the available data, comparative studies have been used to determine