Chordates

•	A chordate is an animal that for at least some of its life has a hollow nerve cord, a notochord, pharyngeal pouches and a tail that extends beyond the anus. •	 Chordates belong to the phylum Chordata. •	They are either vertebrates or closely related invertebrates. **Four Key Characteristics** •	Hollow nerve cord: runs along the back part of the body. Nerves branch from this cord to connect to internal organs, muscles and sense organs. •	Notochord: a long supporting rod that runs through the body below the nerve cord. •	Pharyngeal pouches: paired structures in the throat. In some chordates slits develop that connect the pouches to the outside that then may develop gills used for gas exchange. •	Tail: contains bones and muscles and is used for swimming. **Vertebrate Chordates** •	96% of chordates are in the subphylum Vertebrata and are called vertebrates. •	Vertebrates are animals that have backbones. •	A vertebrate’s backbone is part of an endoskeleton or internal skeleton.
 * What is a Chordate?**

**Nonvertebrate Chordates** •	Nonvertebrates are animals that lack a backbone. •	There are two subphyla that do not have backbones: tunicates and lancelets. •	Both tunicates and lancelets are soft- bodied marine organisms. **Tunicates** •	Filter-feeding tunicates belong to the subphylum Urochordata. •	The larva has all of the chordate characteristics. •	The adult tunicates on the other hand, do not have a notochord or a tail. •	Most tunicates are sea squirts. **Lancelets** •	Lancelets belong to the subphylum called Cephalochordata. •	They live on the sandy ocean bottom. •	Unlike the adult tunicate, the adult lancelet has a head region that contains a mouth. •	The mouth opens into a long pharynx. It has up to 100 pairs of gill silts. •	They use the pharynx for gas exchange. Lancelets can also exchange gas through their body surface. This is because they are extremely thin. •	They have a closed circulatory system. •	Also they do not have a true heart. Instead, the walls of the major blood vessels contract to push blood through the body. **Respiration and Reproduction** •	Chordates take in oxygen and get rid of carbon dioxide through their lungs or gills. •	Chordates reproduce sexually and asexually through budding. •	Some are separate males and females but others like tunicates are hermaphrodites: meaning both male and female at the same time.

**Origin** •	Chordates are deuterostomes. This means a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the anus. At this point the later specialized function of any given embryonic cell has not yet been determined. Deuterostomes are one of the two groups of animals that have true body cavities (coeloms), and are believed to share a common ancestor. •	Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by analyzing biochemical differences, mainly in RNA. One study says that deuterostomes started before 900 million years ago and the earliest chordates around 896 million years ago.