Although Chordata is normally attributed to William Bateson 1885, Gill 1873 indicated that this name was introduced by Ernst Haeckel and was in prevalent use for many years before 1880. Chordates are animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. Taxonomically, the phylum includes the subphyla Vertebrata, including mammals, fish, amphibians, reptiles, birds; Tunicata, including salps and sea squirts; and Cephalochordata, comprising the lancelets. Members of the phylum Chordata are bilaterally symmetric, deuterostome coelomates, and the vertebrate chordates display segmentation. More than a century ago, Hatschek (Hatschek, 1888) introduced the names Zygoneura (now Protostomia), Ambulacraria and Chordonia (now Chordata) for three major groups of the Bilateria, based on the structure and position of the central nervous system (CNS). The Zygoneura were characterized by a paired longitudinal ventral nerve cord [a cluster of nervous cells including their cell bodies, as opposed to a nerve, which is a bundle of axons (Richter et al., 2010); the ventral nerve cord may be specialized into rows of ganglia connected by connectives] and the Chordonia by an unpaired dorsal neural tube. The division of the Bilateria into Protostomia and Deuterostomia =Cordonia + Ambulacraria) (Grobben, 1908) is still universally accepted and is now supported by numerous phylogenomic studies (Hejnol et al., 2009; Wheeler et al., 2009; Edgecombe et al., 2011). However, the interpretation of the dorsal/ventral orientation of the two groups has been challenged, and it now appears that the two longitudinal nerve cords re homologous.
The phylum Hemichordata including the acorn worms has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the phylum Echinodermata, including starfish, sea urchins, and sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion.
Of the more than 65,000 living species of chordates, about half are bony fish of the class Osteichthyes.
Author And Initials
Haeckel [E. H. P. A.]
2018 01 01 - 00:00