The cephalochordates are the closest living relatives of the vertebrates. The group derives its name from the notochord that extends far forward into the head (farther than the brain, in contrast to vertebrates). The most famous representative of the group is Branchiostoma lanceolatus (also known as Amphioxus, or lancelet). There are about twenty-five living species of cephalochordates. All adults are small, fishlike animals that are rarely longer than 5 centimeters (2 inches). Cephalochordates live in shallow marine or brackish water all over the world. They can actively swim around, but most of the time are sedentary, buried in sand.

Swimming and burying are accomplished through an interaction between the notochord (stabilizing element and anchor point for muscles) and large blocks of muscle segments along the body wall. Unlike the vertebral column of vertebrates, the notochord is an elastic, flexible rod. It prevents the body from shortening when the muscles contract...