The animals are one of the two groups to be considered in this website. In most modern classifications, such as that of the Tree of Life web project (http://tolweb.org/Animals/2374), animals are synonymous with the Metazoa. The phylogeny of the animals, based on the Open Tree of Life (https://tree.opentreeoflife.org) is illustrated below:
All the nodes shown in the above figure, shown as black dots, represent crown nodes of the successive crown groups into which the metazoan crown group has divided.
All the animal groups on the right hand side of the above tree, with the exception of the Cephalochordata, will be considered in the following sections of the website. (The Cephalochordata are excluded because of their very limited fossil record.) These groups belong to several higher categories:
Of the groups on the right hand side of the tree, only three are phyla: Ctenophora, Porifera and Cnidaria. The rest represent higher or, in the case of the Chordata, lower groups:
Of these groups, only the vertebrates have been partially completed to date.
All the animal groups on the right hand side of the above tree, with the exception of the Cephalochordata, will be considered in the following sections of the website. (The Cephalochordata are excluded because of their very limited fossil record.) These groups belong to several higher categories:
- Eumetazoa: all animals except the ctenophores and the sponges (Porifera); in some classifications, ctenophores are included in the eumetazons
- Bilateria: all animals with a body form exhibiting bilateral symmetry
- Deuterostomia: bilaterans in which the anus forms before the mouth during development of the embryo
- Chordata: having at some point in their life cycle several unique features, including a notochord (a stiff cartilaginous rod supporting, the nerve cord)
- Olfactores: all chordates except the Cephalochordata.
Of the groups on the right hand side of the tree, only three are phyla: Ctenophora, Porifera and Cnidaria. The rest represent higher or, in the case of the Chordata, lower groups:
- Protostomia: bilaterans in which the mouth forms before the anus during development of the embryo; includes the arthropods, worms and molluscs
- Ambulacraria: living representatives comprise the echinoderms and the hemichordates
- Cephalochordata: soft-bodied chordates without true vertebrae; living representatives are the lancelets
- Tunicata: chordates with a polysaccharide exoskeleton
- Vertebrata: chordates with vertebrae.
Of these groups, only the vertebrates have been partially completed to date.
Photo credit
Header: Hippopotamus in the river near St.Lucia, by Petter Lindgren [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons.
Header: Hippopotamus in the river near St.Lucia, by Petter Lindgren [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons.