Difference between revisions of "Cephalochordata" - New World Encyclopedia
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| − | + | '''Cephalochordata''' (or '''lancelets''', traditionally known as '''amphioxus''') is a subphylum of marine [[invertebrate]]s 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 [[tunicate]]s (subphylum Urochordata), [[hagfish]] (class Agnatha), and all [[vertebrate]]s (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 that and vertebrates that have the notochord during their larval, not adult, stage. Unlike vertebrates, cephalochordates and tunicates lack a backbone or vertebral column. | |
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| + | The notochord of cephalochordatas, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (''cephalo-'' meaning 'relating to the head'). | ||
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| + | The member of Cephalochordata most prominent is the blade-shaped translucent animal called ''Branchiostoma'' (or amphioxus), which has the common name of ''lancelet'' (Towle 1989). A lancelet normally stay buried in sand, with only its head protruding to filter feed and breathe through its gill slits (Towle 1989). 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 [[vertebrate]]s. | ||
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| + | There are about 28 known species of cephalochordates (Towle 1989). Lancelets grow up to about five centimetres long, being eight centimetres at the longest. | ||
==Physical features== | ==Physical features== | ||
| − | + | In common with the [[vertebrate]]s, lancelets have a nerve cord running along the back, and pharyngeal gill slits and a tail that runs past the anus. Also like vertebrates, the [[muscle]]s 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. | |
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| + | 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. | ||
[[Image:lancetnikinside.png|400px]]<br clear="both"> | [[Image:lancetnikinside.png|400px]]<br clear="both"> | ||
Revision as of 02:38, 13 December 2006
| Lancelets | ||||||
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 Lancelet (Branchiostoma lanceolatum) | ||||||
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Asymmetronidae |
Cephalochordata (or lancelets, traditionally known as amphioxus) 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 that and vertebrates that have the notochord during their larval, not adult, stage. 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').
The member of Cephalochordata most prominent is the blade-shaped translucent animal called Branchiostoma (or amphioxus), which has the common name of lancelet (Towle 1989). A lancelet normally stay buried in sand, with only its head protruding to filter feed and breathe through its gill slits (Towle 1989). 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.
There are about 28 known species of cephalochordates (Towle 1989). Lancelets grow up to about five centimetres long, being eight centimetres at the longest.
Physical features
In common with the vertebrates, lancelets 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 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)
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 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
ReferencesISBN links support NWE through referral fees
- Template:RefTudgeVariety
- Cephalochordata from Berkeley
Links
- Error in the Genealogy of Human
- The Amphioxus Song
- A special issue of Amphioxus Research (I)
- A special issue of Amphioxus Research (II)
- Amphioxus and the T-box gene
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