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Definition: lancelet from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Any of a variety of marine animals about 2.5 cm/1 in long. They have no skull, brain, eyes, heart, vertebral column, centralized brain, or paired limbs, but there is a notochord (a supportive rod) which runs from end to end of the body, a tail, and a number of gill slits. Found in all seas, lancelets burrow in the sand but when disturbed swim freely. (Genus Amphioxus, phylum Chordata, subphylum Cephalocordata.)

Taxonomically they are significant since the notochord may be regarded as the precursor of the backbone (spinal column).

Summary Article: lancelet
From The Columbia Encyclopedia

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.

The Columbia Encyclopedia, © Columbia University Press 2018

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