Definitions

lancelet

lancelet

[lans-lit, lahns-]
lancelet, name for small, fishlike lower chordate (see Chordata), also called amphioxus; it shows many affinities with the vertebrates. There are about 30 lancelet species, most belonging to the genus Brachiostoma (formerly Amphioxus). Lancelets are usually about 1 in. (2.5 cm) long, with transparent bodies tapered at both ends. There is no distinct head and no paired fins. Lancelets are filter feeders and live in shallow marine waters; they can swim through water or wet sand, but are usually found buried in the sand with only the mouth end projecting. Small food particles enter the pharynx through the mouth and are filtered out as the water exits through the gill slits. Respiration probably occurs mostly through the skin. The use of the gill slits for feeding rather than respiration is characteristic of the lower chordates (see tunicate). The lancelet has a dorsal notochord, or stiffening rod, extending from tip to tail, that gives it its characteristic pointed shape. It retains the notochord as the major skeletal support throughout life; in vertebrates the notochord is surrounded and usually replaced by a vertebral column during embryonic development. In the lancelet there is a nerve cord above the notochord, but no brain and no eyes. A ventral blood vessel carries the colorless blood; there is no heart. It is thought that vertebrates evolved from ancestors similar to lancelets. The larva of the lamprey, the most primitive living vertebrate, resembles a lancelet in many respects. Lancelets are classified in the phylum Chordata, subphylum Cephalochordata.

The lancelets (subphylum Cephalochordata, traditionally known as amphioxus) are a group of primitive chordates. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially for food for humans and domesticated animals. They are an important object of study in zoology as they provide indications about the origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, its genome holds clues about evolution, particularly how vertebrates have employed old genes for new functions.

In the issue of Nature on 19 June 2008, the draft genome sequence of the Florida lancelet (Branchiostoma floridae) was reported.

Physical features

Lancelets grow up to about five centimetres long, reaching eight centimetres at the longest. In common with vertebrates, lancelets have a nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres. Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely-packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning 'relating to the head'). Lancelets 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.


  1. brain-like blister
  2. notochord
  3. dorsal nerve cord
  4. post-anal tail
  5. anus
  6. food canal
  7. circulatory 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

The Cephalochordata is traditionally seen as a sister subphylum to the vertebrates, with which it is grouped together into a clade (sometimes called Notochordata) which in turn is the sister group to the simpler still Urochordata. Newer research suggests this may not be the case. The Cephalochordata may be the most basal subphylum of the chordates, while the sister group of the vertebrates may be the urochordates. However, other recent molecular studies (cit. in Benton 2005: 8) place cephalochordates nearer to vertebrates, and "[m]ost authors regard amphioxus as the closest relative of the Vertebrata on the basis of 10–15 [morphological] features that are not seen in tunicates".

The asymmetric nature of juveniles is unique to the cephalochordates and demonstrates (as do certain other features, including the seriated gonads) that lancelets are more derived than would be expected. This is a reminder that the "living fossil" representative of a basal clade has as long an evolutionary history as any other living thing, and thus is more derived than the actual primitive ancestor it otherwise so closely resembles.

The following are the species recognised by ITIS. Other sources, for instance Tudge, show that there might be up to thirty species.

  • Family Asymmetronidae
    • Genus Asymmetron
      • Asymmetron lucayanum
      • Asymmetron maldivense
    • Genus Epigonichthys
  • Family Branchiostomidae
    • Genus Branchiostoma
      • Branchiostoma belcheri
      • Branchiostoma californiense
      • Branchiostoma capense
      • Branchiostoma caribaeum
      • Branchiostoma floridae
      • Branchiostoma lanceolatum
      • Branchiostoma valdiviae
      • Branchiostoma virginiae

In music

Folk musician and marine biologist Sam Hinton performed a song written in 1921 by Philip H. Pope about Amphioxus entitled It's a Long Way from Amphioxus. Sung to the tune of "It's a Long Way to Tipperary," the ditty references now-disputed notions about the place of cephalochordates in the chordate evolutionary timeline:

It's a long way from Amphioxus,
It's a long way to us,
It's a long way from Amphioxus
To the meanest human cuss.
It's good-bye, fins and gill-slits,
Welcome, lungs and hair!
It's a long, long way from Amphioxus,
But we all came from there.

References

Links

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