Solution

Amphioxus also known as lancelet is a fish-like marine chordate. Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head and therefore they are placed in subphylum Cephalochordata. Hence, option C is correct.