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How does the notochord of a cephalochordate differ from a vertebral column?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

The notochord of a cephalochordate and a vertebral column differ primarily in their permanence and structural composition.

In cephalochordates, the notochord is a permanent, flexible rod that extends the entire length of the body throughout the animal's life, providing essential structural support for swimming. It consists of a core of cells containing muscle fibers enclosed in a firm sheath.123

In contrast, the vertebrate notochord is typically a transient embryonic structure. In most vertebrates, it is eventually replaced by a bony or cartilaginous vertebral column. During this development, the notochord is either absorbed or incorporated into the vertebral column as the nuclei pulposi of the intervertebral discs. While the cephalochordate notochord remains a singular, non-bony support element, the vertebrate column is a complex, segmented structure that develops through perichordal ossification or the formation of cartilaginous arches around the notochordal sheath.413