A. Fin Box
B. Nerve Cord
F. Endostyle Groove
G. Secondary Gill Bar
H. Primary Gill Bar
I. Pharyngeal Cavity
J. Metapleural Fold
source: Jon Houseman, Matthew Ford
The notochord is a long rod made of packed muscle running nearly the length of the lancelet or Amphioxus.
It acts like a torsion bar, providing strength and flexibility, enabling the animal to propel itself in a direction
determined by a nerve center located near its oral region. Prior to the notochord's development animals
relied on an external shell to give them form and strength. A hard, external shell limits an animal's size while
also requiring it to shed its outer layer to provide for growth. The notochord would eventually be replaced
by segmented bone in animals that would come to be grouped as vertebrates.