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Endangered Hawaiian Monk Seal
Enrique R Aguirre Aves/Oxford Scientific/Getty Images

Chordata is the phylum that includes all the animals that have a notochord, or nerve cord, at some stage of their development. Animals in this phylum are known as chordates. The most well-known chordates are the vertebrates - animals that have a spine.

Marine chordates include cetaceans, pinnipeds, sirenians, fish and tunicates (ascidians).

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