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Chordates are one of the great group (called a phylum of animals ; a chordate is an animal with a skeleton inside of its body, rather than outside (as the oyster's shell, or the crab's cuticle, for example).
All chordate are characterized by a peculiar piece of skeleton called a notochord, that gives their name. Notochord is a rigid axis, that runs through the body, from head to tail. Other animals use to have a skeleton outside of their body, like a shell, that protect them from danger, like predators, for example. But the chordate's skeleton is rather placed inside the body, making it more stiff, and strong. Chordates use to have muscles connected to their inner skeleton, that allow them to move quickly.
- the vertebrates : their notochord is made of a number of pieces, called vertebrae, that encloses the spinal cord, and form the vertebral column, or spine. Most vertebrates have a bony spine, and a whole skeleton made of bones, but some has cartilage instead. That's us ! In that group, you can find aardvarks, trouts, bats and frogs, but also apes and lizards, for example...
- the tunicates are marine animals : they look shapeless, or at least sponge-like animals, that doesn't much look like us ; in fact, the tunicates' young, called larvae are very different : it has a notochord, like us, and look like some kind of small fish or tadpole. When growing old, however, it fixes itself to a stand (as a rock, for example), and turn itself to this strange animal. The sea squirts and the salps belong to this group.
- the cephalochordates are a group of unfamiliar, fish-like animals, with very few species living today's. They look like some kind of small fish. The first chordates appeared on Earth are believed to be cephalochordate. The lancelet is a cephalochordate
- the hemichordates, or acorn worms, are, as their names suggests, worm-like, marine animals. They only have a short notochord, in the fore part of their body, while the rest is soft, like a worm body. They had been though to be a fourth group of chordate, but most scientists nowadays think that, indeed, hemichordates, with their relatives, the echinoderms, are the chordate closest cousins.
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