Chordates
Chordates are a phylum of animals that includes vertebrates, which are animals with a backbone. They are characterized by several key features, including a notochord, a dorsal nerve cord, a post-anal tail, and a ventral heart. However, one of the options listed is incorrect.

Incorrect Option: They must have notochord at every point of their life (Option A)
The incorrect statement is that chordates must have a notochord at every point of their life.

- The notochord is a flexible, rod-like structure that runs along the length of the body in the early stages of development of chordates. It provides support and serves as a precursor to the backbone or vertebral column in vertebrates. While chordates do possess a notochord at some point in their life cycle, it is not a permanent structure in all chordates.
- In some chordates, such as the cephalochordates (e.g., lancelets) and the earliest vertebrates (e.g., jawless fish), the notochord persists throughout their entire life. These animals are referred to as "chordates with persistent notochord."
- However, in most vertebrates, the notochord is present only during the embryonic stages of development. It is gradually replaced by the vertebral column as the animal develops. The vertebral column provides better support and protection for the spinal cord. Therefore, adult vertebrates do not have a notochord.
- Examples of chordates without a notochord in their adult stage include fish, amphibians, reptiles, birds, and mammals. These animals have a notochord only during their embryonic development.

Conclusion
In conclusion, the correct answer is option A: They must have a notochord at every point of their life. While chordates do possess a notochord at some point in their life cycle, it is not a permanent structure in all chordates. Most chordates, particularly adult vertebrates, do not have a notochord as it is replaced by the vertebral column during development.