Cephalochordata

? Lancelets
Lancelet (Branchiostoma lanceolatum)
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Cephalochordata Owen, 1846
Families
Asymmetronidae Branchiostomidae

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 in southern regions. 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 life, 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 cephalochordatas, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning 'relating to the head').

There are only two genera and about 30 species of cephalochordates recognized, the genera being Branchiostoma (originally Amphioxus, 23 species) and Asymmetron (6 species). The common name lancelet, or amphioxus, is generally used for all cephalochordates, but sometimes is applied to just Branchiostoma.

Lancelets are blade-shaped (tapered at both ends) and translucent, and 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 1 inch (2.5 cm) in length, some speices may grow up to about five centimetres long, eight centimetres at the longest. In Asia, lancelets are harvested commercially for food that is eaten by both humans and domesticated animals. They are an important object of study in zoology as they provide indications about the origins of the vertebrates.

Physical features

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 lack blood cells, a major heart, a kidney, and a brain, although there is a cluster of nerves.

1. brain-like blister
2. notochord
3. dorsal nerve cord
4. post-anal tail
5. anus
6. food canal
7. blood system
8. atriopore
9. overpharynx lacuna
10. gill slit
11. pharynx
12. vestibule
13. oral cirri
14. mouth opening
15. gonads (ovary/testicle)
16. light sensor
17. nerves
18. metapleural fold
19. hepatic caecum (liver-like sack)

Taxonomy

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.

• Family Asymmetronidæ
  • Genus Asymmetron
    • Asymmetron lucayanum
    • Asymmetron maldivense
  • Genus Epigonichthys
• Family Branchiostomidæ
  • Genus Branchiostoma
    • Branchiostoma belcheri
    • Branchiostoma californiense
    • Branchiostoma capense
    • Branchiostoma caribæum
    • Branchiostoma floridæ
    • Branchiostoma lanceolatum
    • Branchiostoma valdiviæ
    • Branchiostoma virginiæ

References

ISBN links support NWE through referral fees

• Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.*Cephalochordata from Berkeley

Links

• A special issue of Amphioxus Research (I)
• A special issue of Amphioxus Research (II)

Credits

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