Asymmetronidae
Branchiostomidae
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 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 ("cephalo-" 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.
Taxonomy
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
