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, causing it to bend sideways instead. This creates an undulating (wavy) body movement much like that of fishes. However, poor fin development makes cephalochordates relatively inept swimmers, and as a consequence they spend most of their time (except when they disperse and reproduce) buried in sand with only their front end exposed.
When they are buried, their head sticks out to filter out food particles from the water. In this process, water is driven through the mouth opening into the mouth cavity and back out into the environment through pharyngeal gill slits. In the process, food particles suspended in the water are caught in a sheet of mucus that covers the inside lining of the pharyngeal slits. Cephalochordates may have up to 200 pharyngeal gill slits, making their filter feeding very efficient. The slits are separated from one another by so-called gill bars, which are supported by cartilage rods. During the evolution of vertebrates (about 500 million years ago) from a cephalochordate-like ancestor, these cartilage rods eventually gave rise to the jawbones of vertebrates.
Cephalochordates have a closed circulatory system (the blood is enclosed in blood vessels) but lack a central pump (heart). Instead, the blood is propelled by pulsation (rhythmic contraction and relaxation) of several blood vessels. The blood contains no pigments or cells and is thought to function largely in nutrient distribution rather than in gas exchange and transport. The central nervous system of the cephalochordates is very simple. A dorsal nerve cord extends through the length of the body, giving rise to segmentally arranged nerves. No brain is detectable. The skin is rich in sensory nerve endings that probably help produce a sense of touch and are important for burrowing. A number of cephalochordates have some photosensors near the front and back ends of their body, but in general (unlike vertebrates) they lack any eyes or organs to sense gravity.
Cephalochordates reproduce by releasing their eggs and sperm into the water, where they are fertilized externally. The fertilized eggs develop into free-swimming larvae that drift in the water for up to 200 days, feeding on plankton and other suspended matter, before settling down as adults.
see also Phylogenetic Relationships of Major Groups.
Kathrin F. Stanger-Hall
Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.
Kluge, Arnold G. Chordate Structure and Function, 2nd ed. New York: Macmillan Publishing, 1977.