Metazoans

Metazoans, or animals, are a clade of multicellular, heterotrophic organisms within the kingdom Animalia. They are distinguished from unicellular organisms by differentiated tissues and, in most groups, organs, a nervous system, and muscular tissue that enables movement. Most metazoans ingest food and digest it internally, and development proceeds from a zygote through cleavage and gastrulation to form tissues and organs. Most species are diploid and reproduce sexually, though asexual modes occur in some lineages.

Historically, sponges (Porifera) are considered the most basal metazoans and lack true tissues, while the remainder form the more derived clade Eumetazoa with true tissues. Metazoans display a range of body plans, from radial symmetry in some groups to bilateral symmetry in most, with organization that ranges from simple tissue layers to complex organ systems. The presence of a body cavity, segmentation, and elaborated nervous and circulatory systems varies among lineages, and sponges lack true nervous and muscular systems.

Origins and fossil record: The earliest metazoan fossils appear in the late Precambrian, about 600–700 million years ago, with molecular data suggesting an even earlier origin for multicellularity. The major lineages include the sponges, cnidarians, and ctenophores, as well as the bilaterians such as arthropods, mollusks, annelids, echinoderms, and chordates.

Ecology and significance: Metazoans occupy nearly every habitat and play central roles in ecosystems as predators, herbivores, detritivores, and symbionts. They are economically and scientifically important, contributing to food webs, nutrient cycling, and the study of biology, development, and medicine.