Metazoa are multicellular, eukaryotic organisms in the biological kingdom Animalia

Diversity

The blue whale is the largest animal that has ever lived.

Size

Further information: Largest organisms and Smallest organisms

The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to at least 190 tonnes and measuring up to 33.6 metres (110 ft) long.^[54][55][56] The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes^[54] and measuring up to 10.67 metres (35.0 ft) long.^[54] The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes.^[57] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm,^[58] and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.^[59]

Numbers and habitats

The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,^[60] along with their principal habitats (terrestrial, fresh water,^[61] and marine),^[62] and free-living or parasitic ways of life.^[63] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.^[64] Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.^[65][66][a]