Lancelet
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Lancelet, also called amphioxus, primitive fishlike animal that lacks a backbone but has many characteristics of vertebrates (animals with backbones). Many scientists believe that vertebrates may have evolved from an early ancestor similar to lancelets. Lancelets occur worldwide in shallow, marine environments, where they burrow into loose gravel or sand, leaving only their mouths protruding. Some Asian people consider lancelets a great delicacy, and the animal is commercially harvested in China.
Lancelets grow to a length of about 5 cm (2 in). They have an eel-like body shape with a well-defined mouth and tail. Most are semitransparent. Lancelets share a number of features in common with vertebrates, including a dorsal nerve cord that is supported by a stiff rod known as the notochord—a forerunner of the backbone; a system of pharyngeal slits between the mouth and stomach, used to strain food particles from water; and a series of segmented, V-shaped muscles along the torso.
Unlike vertebrates, lancelets do not develop a true brain, cranial nerves, or sensory structures, such as eyes or ears. Lancelets do not have a heart but they have a circulatory system in which a large blood vessel pulsates to move oxygenated blood to various organs in the body. Veins return blood to the gills, where it is replenished with oxygen. The sexes of lancelets are distinct—the sex organs occur in pairs along the body and produce gametes that are released into the water after the animal reaches maturity.
Lancelets have no hard body parts, such as bone, and as a result these animals only fossilize under special conditions. Fossils of animals believed to be lancelets found in south China and British Columbia date back to the early Cambrian Period, an interval of geologic history that began about 570 million years ago. These fossils may represent animals close to those from which vertebrates evolved.
Scientific classification: Lancelets make up the families Branchiostomatidae and Epigonichthyidae of the order Amphixi.
Lancelets grow to a length of about 5 cm (2 in). They have an eel-like body shape with a well-defined mouth and tail. Most are semitransparent. Lancelets share a number of features in common with vertebrates, including a dorsal nerve cord that is supported by a stiff rod known as the notochord—a forerunner of the backbone; a system of pharyngeal slits between the mouth and stomach, used to strain food particles from water; and a series of segmented, V-shaped muscles along the torso.
Unlike vertebrates, lancelets do not develop a true brain, cranial nerves, or sensory structures, such as eyes or ears. Lancelets do not have a heart but they have a circulatory system in which a large blood vessel pulsates to move oxygenated blood to various organs in the body. Veins return blood to the gills, where it is replenished with oxygen. The sexes of lancelets are distinct—the sex organs occur in pairs along the body and produce gametes that are released into the water after the animal reaches maturity.
Lancelets have no hard body parts, such as bone, and as a result these animals only fossilize under special conditions. Fossils of animals believed to be lancelets found in south China and British Columbia date back to the early Cambrian Period, an interval of geologic history that began about 570 million years ago. These fossils may represent animals close to those from which vertebrates evolved.
Scientific classification: Lancelets make up the families Branchiostomatidae and Epigonichthyidae of the order Amphixi.
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