Chondrycthyes have notochord replaced with cartilaginous vertebral column.

Why is the notochord included in Vertebrata?
The notochord is a defining feature of the phylum Chordata, which includes both invertebrate and vertebrate animals. In vertebrates, the notochord is present during embryonic development and plays a crucial role in the early formation of the vertebral column. While the notochord typically disappears in most vertebrates as the embryonic vertebral column develops, it persists throughout life in some groups, such as the Chondrichthyes (cartilaginous fish).

Importance of notochord in Chondrichthyes
- The notochord provides structural support and flexibility to the body of Chondrichthyes, which lack true bony vertebrae.
- It serves as a foundation for the development of the vertebral column and helps in the movement of these fish.

Inclusion of notochord in Vertebrata
- The presence of the notochord, even if persistent throughout life in some groups like Chondrichthyes, is a key characteristic that aligns these organisms with other vertebrates.
- Despite the variations in the fate of the notochord among different vertebrate groups, its initial presence and role in embryonic development are crucial for the classification of these organisms within the phylum Vertebrata.
In conclusion, while the notochord may persist in some vertebrates like Chondrichthyes, its inclusion in the phylum Vertebrata is based on its fundamental role in the early development of the vertebral column and its presence in the common ancestor of all vertebrates.