Protochordates are considered as the connecting link between vertebrates and invertebrates. These include three groups: Hemichordata, Urochordata and Cephalochordata.
- In Hemichordata, notochord is like structure is found in the roof of the buccal cavity which is called stomochord.
- In Urochordata, the notochord is usually present at larval stage and is restricted to tail. It disappears in the adult during retrogressive metamorphosis.
- In Cephalochordata, the notochord is persistent throughout the life and extends from tail up to the head region.
- Cyclostomata is a class under the subphylum Vertebrata in which notochord is replaced by the vertebral column.
So, the correct answer is D.