Vertebrates and tunicates share a notochord and a dorsal, hollow nerve cord.
A part of the subphylum Tunicata, tunicates are marine invertebrate creatures. It belongs to the phylum Chordata, which is made up of all creatures with dorsal nerve cords and notochords. The term "urochordates" is still occasionally used to refer to these animals, even though the subphylum was once known by that name.
The notochord is a long, rod-like structure that forms dorsally to the neural tube and ventrally to the gut. The notochord is mostly made up of a core of glycoproteins that are enclosed in a collagen fiber sheath that is wound into two opposing helices. In vacuolated, turgid cells, the glycoproteins are retained.
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