Chordate
Channel: 宇宙のすべての知識 プリンシピア
Date: 27/01/2019

Chordate, by Wikipedia https://en.wikipedia.org/wiki?curid=5131 / CC BY SA 3.0

#Chordates
#Terreneuvian_first_appearances
#Extant_Cambrian_first_appearances
Chordate

A chordate () is an animal belonging to the phylum Chordata; chordates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail, for at least some period of their life cycle. Chordates are deuterostomes, as during the embryo development stage the anus forms before the mouth. They are also bilaterally symmetric coelomates with metameric segmentation and a circulatory system. In the case of vertebrate chordates, the notochord is usually replaced by a vertebral column during development.

Taxonomically, the phylum includes the following subphyla: the Vertebrata, which consists of fish, amphibians, reptiles, birds, and mammals; the Tunicata, which includes salps and sea squirts; and lastly the Cephalochordata, which includes the lancelets. There are also additional extinct taxa such as the Vetulicolia. The Vertebrata are sometimes considered as a subgroup of the clade Craniata, which is consisting of chordates with an already formed skull; the Craniata and Tunicata compose the clade Olfactores.

Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. The world's largest and fastest animals, the blue whale and peregrine falcon respectively, are chordates, as are humans. Fossil chordates are known from at least as early as the Cambrian explosion.

Hemichordata, which includes the acorn worms, has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. The Hemichordata, along with the Echinodermata (which includes starfish, sea urchins, sea cucumbers, and crinoids), form the Ambulacraria, the sister taxon of the Chordates. The Chordata and Ambulacraria form the superphylum Deuterostomia, composed of the deuterostomes.

Attempts to work out the evolutio...