Final answer:
The flexible rod between the digestive tube and nerve cord in chordates is called the notochord. It provides structural support during development and guides the formation of the central nervous system. In vertebrates, it is replaced by the spine after embryonic development.
Step-by-step explanation:
The longitudinal, flexible rod located between the digestive tube and nerve cord in chordates is called the notochord. This structure is crucial for the support and structure of chordates during their development.
In chordates, the notochord is strengthened with glycoproteins similar to cartilage and is enveloped by a collagenous sheath. It provides rigidity to counterbalance muscle contractions during movement and serves as a point for muscle attachment. Although the notochord is present in all chordate embryos, its role in adults varies; in vertebrates, it is replaced by the spine after embryonic development, while in other chordates, it remains as the primary axial support.
The notochord's importance during development extends beyond mere support. It is critical for the proper formation of the neural tube, which eventually develops into the central nervous system, consisting of the brain and spinal cord.