lancelet

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.

amphioxus Another name for the lancelet: see Cephalochordata.

Branchiostomidae (lancelets; order Amphioxiformes) A family of small, fish-like, marine, coastal animals. Lancelets are the only living representatives of the subphylum Cephalochordata. Although definitely not true fish, lancelets (e.g. Branchiostoma lanceolata) are considered fairly close to the vertebrate ancestral lineage. They have an almost transparent body, no eyes or brains, a permanent notochord extending into the head and a dorsal hollow nerve cord, gill slits, and segmented muscle blocks. They spend most of their lives buried in the sediments beneath the world's oceans.