Lancelet (Branchiostoma lanceolatum)
Cephalochordata (or lancelets, traditionally known as amphioxus, plural amphioxi) is a subphylum of marine invertebrates of the phylum Chordata. They are usually found in shallow parts of temperate or tropical seas. As with all chordates—a group that includes tunicates (subphylum Urochordata), hagfish (class Agnatha), and all vertebrates (class Vertebrata)—cephalochordates have a notochord, a hollow dorsal nerve cord, and pharyngeal slits (or pharyngeal pouches). Cephalochordates retain the notochord throughout their lives, unlike tunicates and vertebrates that have the notochord only during early (embryonic, larval) stages. Unlike vertebrates, cephalochordates and tunicates lack a backbone or vertebral column.
The notochord of cephalochordata, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning "relating to the head"). Lancelets are blade-shaped (tapered at both ends), yielding the name amphioxus, which comes from the Greek for "both (ends) pointed."
With only about 30 species, it would be easy to overlook this subphylum and its importance. Yet, in Asia, lancelets are harvested commercially for food that is eaten by both humans and domesticated animals, and they are an important object of study in zoology as they provide indications about the origins of the vertebrates. Cephalochordates also play a key role in certain food chains, with sometimes thousands per square acre of sand, and they contribute to the diversity of forms and behaviors that makes nature so fascinating to people.
There are only two genera of cephalochordates recognized—Branchiostoma (originally Amphioxus, about 23 species) and Asymmetron (about six species). The common name lancelet, or amphioxus, is generally used for all cephalochordates.
Cephalochordates normally stay buried in sand, with only the head protruding to filter feed and breathe through its gill slits (Towle 1989). They swim with a spiring motion. Often only one inch (2.5 centimeters) in length, some species may grow up to about five centimeters long, eight centimeters at the longest.
In common with the vertebrates, cephalochordates have a nerve cord running along the back, and pharyngeal gill slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres. Unlike the vertebrates, however, the dorsal nerve cord is not protected by bone, but a rather simpler notochord made up of a cylinder of cells that are closely-packed to form a toughened rod.
The cephalochordates also have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. The water exits the body via the atriopore.
Cephalochordates have been known as a "living fossil," tracing to over 500 million years ago (Garcia-Fernandez 2006a). Although cephalochordates lack hard parts, making fossilization difficult, the Middle Cambrian Burgess Shale has yielded some fossils that appear to be cephalochordates, and Early Cambrian deposits in China yielded Yunnanozoon" that is reportedly a cephalochordate (Chen et al. 1995).
In 1778, the amphioxus was described incorrectly as a mollusk (Garcia-Fernandez 2006a). In the early twentieth century, the song "Its a long way from amphioxus" was written, authored by marine biologist Philip H. Pope and students at Biological Laboratories of the Cold Spring Harbor Laboratories (Garcia-Fernandez 2006a). Popularized in the 1960s, this song contains the chorus:
Cephalochordata is traditionally seen as a sister subphylum to the vertebrates, and are grouped together as a sister group (sometimes called Notochordata) to the simpler still Urochordata. But newer research suggests this may not be the case. Urochordates may be the sister group of the vertebrates, while Cephalochordata may be the most basal subphylum of the chordates. The asymmetric nature of juveniles is unique to the cephalochordates and indicates (as do certain other features, including the seriated gonads) that lancelets do not include the direct ancestor of the vertebrates.
The following are the species recognised by Integrated Taxonomic Information System. However, other sources (such as Trudge 2000) recognize up to thirty species.
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