The notochord is best described as

• A. A segmented skeletal structure found in adult animals
• B. A flexible, longitudinal rod providing supportive axis in chordates
• C. A rigid skull bone protecting the brain
• D. A dorsal nerve cord

Answer: (B)

The notochord is a flexible, longitudinal rod that runs along the back of chordates and provides a sturdy axis against which the body can bend as muscles contract. This setup acts like a hydrostatic skeleton, giving support and enabling efficient swimming in many chordates. In many vertebrates, the notochord is later replaced by the vertebral column, though remnants may persist in some tissues. It is not a segmented skeleton like the vertebral column, not a rigid skull bone, and not the dorsal nerve cord itself.