With over a million species of animals described, a systematic classification is essential, and it also helps in assigning a position to newly described species. Although all animals are multicellular, they differ in the fundamental features used for classification: arrangement of cells, body symmetry, nature of coelom and patterns of digestive, circulatory and reproductive systems. The first basis is the level of organisation. In sponges, cells are loose aggregates with some division of labour, which is the cellular level. In coelenterates, cells performing the same function form tissues, the tissue level. Platyhelminthes and higher phyla show the organ level, where tissues form organs specialised for a function. Annelids, arthropods, molluscs, echinoderms and chordates show the organ system level, where organs associate into systems with a specific physiological function. Organ systems vary in complexity. A digestive system with a single opening serving as both mouth and anus, as in Platyhelminthes, is incomplete, whereas a complete one has separate mouth and anus. A circulatory system is open when blood leaves the heart and directly bathes cells, and closed when blood flows through vessels. The second basis is symmetry. Sponges are mostly asymmetrical. In radial symmetry, any plane through the central axis gives identical halves, as in coelenterates, ctenophores and echinoderms. In bilateral symmetry, only one plane gives identical left and right halves, as in annelids and arthropods.