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.
In common with the vertebrates, lancelets have a nerve cord running along their back, pharyngeal gill slits and a tail that runs part of 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 vertebrates notochord (the spine), extends beyond the head. This gives the subphylum its name ("cepahlo-" meaning "relating to the head"). The lancelets also have oral cirri, thin tentacle-like strands that fall in front of the move that act as sensory devices and as filter for the water passing into the body.
Habitat
Lancelets grow up to about five centimetres long. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially.
Taxonmy
Cephalochordata is a sister subphylum to the vertebrates. These two groups together a sister group to the simpler still urochordates. Nielsen suggests that the combined group be called the notochordata though this name is not in common use. The asymmetric nature of juveniles is unique to the cephlochordates, and proves that lancelets do not contain an ancestor of the vertebrates, making the two groups true sisters.
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
References
Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.
Cephalochordata from Berkeley (http://www.ucmp.berkeley.edu/chordata/cephalo.html)
The Cephalochordata are a small subphylum (about 28 species) of of small, 5 to 10 centimetres long, but fascinating animals.
The Cephalochordata are filter feeders, like the Urochordates, with most of the body being occupied by the large pharynx which exists within the atrial cavity.
The Cephalochordata represent a very primitive example of the basic Chordate plan, and in some ways resemble what scientists think the original ancestor to the vertebrates may have looked like.
The lancelets (subphylum Cephalochordata, and traditionally known as the amphioxus) are a group of primitive chordates.
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.
It is the urochordates who are the sister group of the vertebrates, while cephalochordata is the most original and primitive subhylum of the chordates.
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