How do protochordates differ from vertebrates?

Protochordates and vertebrates are both members of the phylum Chordata, sharing fundamental characteristics such as a hollow dorsal nerve cord, gill slits, and a notochord at some stage in their life cycle.1

The primary distinction between the two groups is that protochordates—which include tunicates and cephalochordates—are invertebrates that lack a backbone. In contrast, vertebrates (subphylum Vertebrata) possess a vertebral column.1

While the notochord serves as a stiff supporting rod in both groups, its fate differs significantly:
* In protochordates: The notochord often remains unchanged throughout the animal's life, or it may be present only during the larval stage and disappear in adulthood, as seen in some sea squirts.23
* In vertebrates: The notochord is typically replaced by or enclosed within a bony or cartilaginous vertebral column during development.23

Recent research suggests that protochordates evolved from the same ancestral stock that eventually gave rise to the vertebrates.1