Which one of the following statements is totally wrong about the occurrence of the notochord?

Tadpole is the free-swimming larva of tunicates, having a tadpole-like tail containing the notochord and nerve cord. Among vertebrates, while the notochord is retained by the adults of the lower vertebrates of class Agnatha (hagfish and lampreys), in higher vertebrates (cartilaginous fish, bony fish, reptiles, amphibians, birds, mammals) it is replaced by the vertebral column. In humans, it is present in the initial stage which is further replaced by a backbone in the later stages. Amphioxus has notochord throughout their life. The dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells. So, the correct answer is option C.