what is the difference between protochordata and vertebrata

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Animals that have notochord are categorised into chordata.

Some animals like Balanoglossus, Herdmania, Amphioxus, Saccoglossus etc. have a notochord, which is not present at all stages in their lifecycle, nor does it run the entire length of an animal’s body. Therefore, these animals are kept in a separate sub-phylum named Protochordata under phylum chordata.

In animals where the notochord is present throughout the life cycle and also has given rise to the vertebral column are called vertebrates.The members of sub-phylum Vertebrata under Phylum Chordata are advanced chordates that include animals belonging to classes- Pisces,Amphibia, Reptilia, Aves and Mammalia.