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The lancelets (subphylum Cephalochordata, and traditionally known as the amphioxus) are a group of primitive chordates. 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, lancelets have a nerve cord running along their back, pharyngeal gill slits and a tail that runs past the anus. Also like humans, muscles are banded. Unlike the vertebrates however the dorsal nerve chord 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 membrane. 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 fall in front of the mouth that 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
- abdominal porus
- overpharynx lacuna
- gill's slit
- pharynx
- mouth lacuna
- mimosa
- mouth gap
- gonads (ovary/testicle)
- light sensor
- nerves
- abdominal ply
- liver like sack
Habitat
Lancelets grow up to about five centimetres long, being eight centimetres at the longest. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially.
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 urochordates. But newer research shows this is not the case. It is the urochordates who are the sister group of the vertebrates, while cephalochordata is the most original and primitive subhylum of the chordates. The asymmetric nature of juveniles is unique to the cephlochordates, and proves that lancelets do not contain an 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 Asymmetronidæ
- Genus Asymmetron
- Asymmetron lucayanum
- Asymmetron maldivense
- Genus Epigonichthys
- Genus Asymmetron
- Family Branchiostomidæ
- Genus Branchiostoma
- Branchiostoma belcheri
- Branchiostoma californiense
- Branchiostoma capense
- Branchiostoma caribæum
- Branchiostoma floridæ
- Branchiostoma lanceolatum
- Branchiostoma valdiviæ
- Branchiostoma virginiæ
- Genus Branchiostoma
See also
References
Links
- Error in the Genealogy of Human
- The Amphioxus Song
- A special issue of Amphioxus Research (I)
- A special issue of Amphioxus Research (II)
Cefalocordat | Lanzettfischchen | Cephalochordata | Brankiostomulo | Cephalochordata | 두색동물 | Branchiostoma | Schedellozen | ナメクジウオ | Lansettfisker | Bezczaszkowce | Cephalochordata | Ланцетник | Kopijovce | Suikulaiset | 文昌鱼
"Lancelet"