Lancelet

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Lancelets

Lancelet (Branchiostoma lanceolatum)

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Cephalochordata Owen, 1846

Families

Asymmetronidae
Branchiostomidae

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 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.

1 Physical features
2 Taxonomy
3 References
4 Links

[edit] Physical features

Lancelets grow up to about five centimetres long, being eight centimetres at the longest. In common with the vertebrates, lancelets have a nerve cord running along the back, 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 lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning 'relating to the head'). The 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.

brain-like blister
notochord
dorsal nerve cord
post-anal tail
anus
food canal
blood system
atriopore
overpharynx lacuna
gill slit
pharynx
vestibule
oral cirri
mouth opening
gonads (ovary/testicle)
light sensor
nerves
metapleural fold
hepatic caecum (liver-like sack)

[edit] 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 ITIS. Other sources (see for instance Tudge) show that there are 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