The notochord is a defining feature of the phylum Chordata.
It is a stiff, flexible rod made of collagen that runs along the body below the hollow dorsal nerve cord. It functions as an anchoring point for muscles, enabling chordates to move their bodies by flexing their muscles against it. The notochord is the precursor of the spine found in all vertebrates, or members of the subphylum Vertebrata. Through evolution, a column of vertebrae eventually replaced the notochord in most (but not all) chordates.
There are two surviving, or extant, groups of chordates that aren't vertebrates: tunicates and lancelets. Tunicates (also called sea squirts) have a notochord only during the larval stage, while lancelets retain their notochord as adults.
Also worth noting is the hagfish, an extant chordate that has a skull but no spine, making it difficult to classify. It has a notochord in all life stages. Whether it's a vertebrate or not is a subject of debate.
161 words for BQ2025