Kingdom Animalia Metazoa

Tree of Life

Animalia (Metazoa)

Also see:
Domain Eukaryota

Kingdon Animalia comprises, the metazoans, that are multicellular organisms having eukaryotic cells with a nucleus, a cell membrane, and organelles such as mitochondria. They are heterotrophic (obtain food from external sources) and utilize oxygen for energy catabolism. Some of the oldest known fossils of these organisms are found in rocks ranging from 540-650 million years old corresponding to the Vendian (also called the Ediacaran Period), but the paucity of these fossils make their association with those of the Cambrian almost impossible, and the possibility exists that they are not even animals.

Except for the Ediacaran biota, most animal phyla (some that persist and some that did not) appear in the fossil record rapidy and essentially simultaneously at the base of the Cambrian period, some 570 million years ago, an event known as the Cambrian Explosion. Still in dispute is whether the Cambrian Explosion really was a rapid divergence between different groups or was instead a change in size and the emergence of protective armor such as exoskeletons and shells that facilitated fossilization.

It is widely accepted that animals evolved from flagellate protozoa. If so, the closest living relatives are the choanoflagellates, which are heterotrophic, single-called eukaryotes having the same structure as some sponge cells. Phylogenetic studies place them in a supergroup called the opisthokonts, which also include the fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal sperm, whereas other eukaryotes tend to have anterior flagella.

Also see: Eumetazoa

Parazoans  (note 1) Phylum Porifera (sponges)
"Subkingdom" Eumetazoa (cells truly coorperate - see note 1)   Radial symmetry with 2 layers of cells (diblastic)   Phylum Cnidaria (corals, jellyfish, Hydra)
Phylum Ctenophora (comb jellies)
Phylum Conulariida
Bilaterians (note 2)
Superphylum Deuterostomia  (mouth appears second - see note 3) Phylum Echinodermata (crinoids, starfish, sea urchins, sea cucumbers, etc.)
Hemichordata (acorn worms, graptolites)
Phylum Chordata (Vertebrates and closely related invertebrates all having a notochord)
Phylum Vetulicolia (note 4 - Extinct)
Superphylum Ecdysozoa (molting animals) Phylum Arthropoda (crabs, spiders, insects, etc.)
Phylum Priapulida
Phylum Nematoda (roundworms)
Phylum Onychophora (velvet worms) - see Phylum Lobopodia
Phylum Lobopodia (note 5)
Superphylum Lophotrochozoa
Phylum Annelida (segmented worms)
Phylum Echiura (Spoon worms note 6)
Phylum Brachiopoda (lamp shells)
Phylum Bryozoa ("moss animals")
Phylum Hyolitha
Phylum Mollusca (snails, clams, squid, etc.)
Phylum Phoronida
  1. Some phylogenists believe sponges and eumetazoans evolved separately from single-celled organisms, which would mean that the entire animal kingdom does not form a distinct clade descended from a common ancestor. However, some genetic studies and some morphological characteristics support a common origin and ancestor.
  2. The Bilaterians are distinguished by bilateral morphological symmetry, and are a major group of animals that contains the majority of phyla. Sponges and cnidarians are notable exceptions. Most Bilaterian bodies develop from three different germ layers, called the endoderm, mesoderm, and ectoderm, thus earning the developmental term triploblastic. Nearly all Bilaterians are bilaterally symmetrical, or approximately so. The most notable exception is the echinodermata that are bilaterally symmetrical as larvae, but become radially symmetric as adults.
  3. Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "other mouth") are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes. Deuterostomes are differentaiated by their embryonic development where the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.
  4. Vetulicolia is a recently-erected phylum comprising several early Cambrian fossils from Chengjiang, China that are suggested to have been among the earliest deuterostomes. See - Shu, D.-G., et. al. 2001. Primitive Deuterostomes from the Chengjiang Lagerstätte (Lower Cambrian, China), Nature, 414:419-424.
  5. The Lobopodians are worms with legs that occur in the early Cambrian, and are mainly known from the Burgess Shale and Chengjiang Biota. Most theories hold they are related to arthropods, and like here they are often placed with the predatory Onychophora (or Velvet Worms) that are extant. Some theories place lobopods as a stem group of arthropods, or posit that arthropods arose from lobopods. Clearly, they are a poorly understood group of animals.
  6. Echiura were initially regarded as an annelid group, were then excluded, but newer evidence suggests they are in fact annelids (Hessling and Westheide, 2002), albeit this placement remains unresolved. The Echiura fossilize poorly and the earliest known specimen is from Mazon Creek dating to the Pennsylvanian. see - Hessling, R., and W. Westheide. 2002. Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis. J. Morph. 252:100-113.