Lancelet (redirected from Lancelets)

lancelet, name for small, fishlike lower chordate (see Chordata), also called amphioxus; it shows many affinities with the vertebrates. There are about 30 lancelet species, most belonging to the genus Brachiostoma (formerly Amphioxus). Lancelets are usually about 1 in. (2.5 cm) long, with transparent bodies tapered at both ends. There is no distinct head and no paired fins. Lancelets are filter feeders and live in shallow marine waters; they can swim through water or wet sand, but are usually found buried in the sand with only the mouth end projecting. Small food particles enter the pharynx through the mouth and are filtered out as the water exits through the gill slits. Respiration probably occurs mostly through the skin. The use of the gill slits for feeding rather than respiration is characteristic of the lower chordates (see tunicate). The lancelet has a dorsal notochord, or stiffening rod, extending from tip to tail, that gives it its characteristic pointed shape. It retains the notochord as the major skeletal support throughout life; in vertebrates the notochord is surrounded and usually replaced by a vertebral column during embryonic development. In the lancelet there is a nerve cord above the notochord, but no brain and no eyes. A ventral blood vessel carries the colorless blood; there is no heart. It is thought that vertebrates evolved from ancestors similar to lancelets. The larva of the lamprey, the most primitive living vertebrate, resembles a lancelet in many respects. Lancelets are classified in the phylum Chordata, subphylum Cephalochordata.

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amphioxus

 or lancelet

Any of certain small marine chordates (invertebrate subphylum Cephalochordata) found widely on tropical and subtropical coasts and less commonly in temperate waters. Seldom more than 3 in. (8 cm) long, they resemble small, slender fishes without eyes or a definite head. They are grouped in several species in two genera (Branchiostoma or Amphioxus, and Epigonichthyes or Asymmetron). They spend much of their time buried in gravel or mud on the ocean bottom, though they are able to swim, and feed by filtering food particles from water passing through their gill slits. They have no brain or distinct heart.

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lancelet [′lans·lət]

(zoology)

The common name for members of the subphylum Cephalochordata.

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Lancelet 

Branchiostoma lanceolatum, or Amphioxus lan-ceolatus), a chordate animal of subphylum Acrania (Cephalo-chordata).

The body of the lancelet is transparent, up to 8 cm long, and pointed at both ends. The notochord extends into the anterior end of the body. The caudal fin is lancet-shaped. The lancelet inhabits the sandy bottom of temperate and warm seas (in the USSR, the Black Sea), usually at depths of 10–30 m. It burrows into the sand, leaving the anterior part of its body exposed. It feeds on plankton, which is drawn with water into the pharynx by the movement of the cilia of the epithelium around the mouth.

Lancelets are dioecious. The eggs develop in the open water and the larvae are free-swimming. The lancelet was first described by P. S. Pallas (1774), who considered it a mollusk and classified it as Li max lanceolatus. A. O. Kovalevskii, who studied the development of the lancelet, showed that it belongs to the Chordata (subphylum Acrania), intermediate between the lower chordates (tunicates) and the vertebrates.

REFERENCES

Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Zhizn’ zhivotnykh, vol. 4, part 1. Moscow, 1971.