The largest prehistoric organisms include both vertebrate and invertebrate species. Sources differ for vertebrate and invertebrate animals. "Daphnia" are prey for a variety of both vertebrate and invertebrate predators. Nevertheless, invertebrates possess mechanisms that appear to be precursors of these aspects of vertebrate immunity. The evolutionary origin of the vertebrate dentition remains contentious. Fibronectin has numerous functions that ensure the normal functioning of vertebrate organisms. Lampreys are the only extant vertebrate to have four eyes. In many terrestrial ecosystems, they constitute one of the largest parts of the vertebrate biomass. The various fish groups account for more than half of vertebrate species. The most important exception is a primitive vertebrate, the hagfish. The most abundant cells in vertebrate blood are red blood cells. The tsetse are obligate parasites that live by feeding on the blood of vertebrate animals. A bone is a rigid organ that constitutes part of the vertebrate skeleton in animals. It is not covered by connective tissue, which is characteristic of vertebrate heart anatomy. The dentition of lungfish is different from that of any other vertebrate group.