![]() ![]() There are about ten thousand living species, more than half of which are passerine, or "perching" birds. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. ( Full article.)īirds are a group of warm-blooded vertebrates constituting the class Aves ( / ˈ eɪ v iː z/), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. The term is derived from Ancient Greek ζῷον, zōion ('animal'), and λόγος, logos ('knowledge', 'study'). Zoology is one of the primary branches of biology. Its studies include the structure, embryology, classification, habits, and distribution of all animals, both living and extinct, and how they interact with their ecosystems. Zoology ( / z oʊ ˈ ɒ l ə dʒ i/) is the scientific study of animals. ![]() In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between taxa. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. Historically, Aristotle divided animals into those with blood and those without. 6,331 groups of genes common to all living animals have been identified these may have arisen from a single common ancestor that lived 650 million years ago. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. The Bilateria include the protostomes, containing animals such as nematodes, arthropods, flatworms, annelids and molluscs, and the deuterostomes, containing the echinoderms and the chordates, the latter including the vertebrates. Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The scientific study of animals is known as zoology. They have complex interactions with each other and their environments, forming intricate food webs. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). As of 2022, 2.16 million living animal species have been described-of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates-but it has been estimated there are around 7.77 million animal species in total. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. ![]()
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