Animalia, also known as Metazoa, is a kingdom of living organisms that includes multicellular eukaryotic creatures. Animals are heterotrophic, meaning they obtain their food by consuming other living organisms. They are also characterized by their ability to move spontaneously and rapidly in response to stimuli. The scientific study of animals is known as zoology, and the study of animal behavior is known as ethology.
As of 2022, there are approximately 2.16 million known animal species on Earth, with over 85,000 being vertebrates, around 65,000 being arthropods, and over 1.05 million being insects. Animals come in a wide range of sizes, from the microscopic to the gargantuan. The smallest animal is the rotifer, which measures only 0.00033 inches in length, while the largest animal is the blue whale, which can grow up to 110 feet long.
Animals have complex interactions with each other and their environments, forming intricate ecosystems. They play important roles in the food chain, with some animals acting as predators and others as prey. Some animals are also important for pollination, seed dispersal, and nutrient cycling.
The classification of animals has evolved over time. Historically, Aristotle divided animals into those with blood and those without, while Carl Linnaeus created the first hierarchical classification for animals in 1758 with his Systema Naturae. Jean-Baptiste Lamarck expanded this classification into 14 phyla by 1809, and in 1874, Ernst Haeckel divided the animal kingdom into the multicellular animals (Animalia) and the unicellular organisms, which are no longer considered animals.
The vast majority of animal species belong to two large superphyla: the protostomes and the deuterostomes. Protostomes include arthropods, mollusks, flatworms, annelids, nematodes, and other phyla, while deuterostomes include echinoderms, hemichordates, chordates, and the vertebrates, which are the most successful subphylum.
The earliest complex animals were present in the Ediacaran biota of the late Proterozoic, and nearly all modern animal