How does segmentation in cephalochordates compare with segmentation in vertebrates?

Cephalochordates and vertebrates exhibit a high degree of similarity in their body segmentation, suggesting that this metameric pattern was present in their common ancestor. In both groups, the body is largely composed of serially repeated units called metameres, which include blocks of muscles known as myotomes. This segmentation also extends to nerves, excretory structures, and certain body cavities, serving to enhance coordination during locomotion.12

Despite these similarities, there are notable differences in how this segmentation manifests and develops:

• Structural Scope: In vertebrates, segmentation is fundamentally tied to the development of the somite, which gives rise to the sclerotome (forming the vertebral column and ribs), the dermatome, and the myotome. While cephalochordates share the segmental muscle organization, they lack the bony or cartilaginous vertebrae that characterize the vertebrate axial skeleton.3415
• Persistence: In many vertebrates, such as four-footed animals, the clear evidence of segmentation becomes largely erased in the adult form, whereas it remains highly conspicuous in the adults of fishes and elongated terrestrial vertebrates.3

While both groups are segmented, this trait likely evolved independently from the segmentation seen in annelid worms and arthropods.167