amphioxus, any of certain members of the invertebrate subphylum Cephalochordata of the phylum Chordata. Amphioxi are small marine animals found widely in the coastal waters of the warmer parts of the world and less commonly in temperate waters. Both morphological and molecular evidence show them to be close relatives of the vertebrates. A brief treatment of amphioxi follows. For full treatment, see cephalochordate.
Amphioxi are seldom more than 8 cm (3 inches) long and resemble small, slender fishes without eyes or definite heads. They are grouped in two genera—Branchiostoma (also called Amphioxus) and Epigonichthyes (also called Asymmetron)—with about two dozen species. The chordate features—the notochord (or stiffening rod), gill slits, and dorsal nerve cord—appear in the larvae and persist into adulthood.
Amphioxi spend much of their time buried in gravel or mud on the ocean bottom, although they are able to swim. When feeding, they let the anterior part of the body project from the surface of the gravel so that they can filter food particles from water passing through their gill slits. At night they often swim near the bottom. They burrow into sand using rapid movements of the body, which is tapered at both ends and is covered by a sheath (the cuticle).
The animals swim by contracting the muscle blocks, or myotomes, that run from end to end on each side of the body. The blocks on each side are staggered, producing a side-to-side movement of the body when swimming. Amphioxi are not buoyant, and they sink quickly when they stop swimming. A dorsal fin runs along the entire back, becomes a caudal fin around the tip of the tail, and then continues as a ventral fin; there are no paired fins.
The notochord runs through the body from tip to tip, providing a central support. A slight bulge distinguishes the anterior end of the nerve cord. Although there is no brain or cranium, growing evidence suggests that the vertebrate brain evolved from a portion of the nerve cord in a lancetlike ancestor. Blood flows forward along the ventral side and backward along the dorsal side, but there is no distinct heart.
The oral cavity of amphioxi is furnished with a hood whose edges are lined with cirri; these are fringelike structures that form a coarse filter to screen out particles too large to be consumed. Water is directed through the small mouth into the pharynx by the action of cilia on the gill slits. Food particles in the passing water are caught by the mucous lining of the gill basket and passed into the gut, where they are exposed to the action of enzymes. Unlike other chordates, amphioxi are capable of a digestive process called phagocytosis, in which food particles are enveloped by individual cells.
Above the pharynx is the excretory system made up of the nephridia, which opens into an excretory canal leading to the atrium. The endostyle corresponds to the thyroid in vertebrates, since it seems to produce iodinated, tyrosine molecules, which may function as regulatory substances, much like hormones, in amphioxi.
Male and female amphioxi are identical in outward appearance and differ internally only in the nature of the gonads, or reproductive glands, which form in rows on the wall of the atrial cavity. Breeding takes place several times a year in tropical regions but only once in temperate areas. Sacs containing eggs or sperm burst and discharge their contents into the water through an opening on the underside of the body. Eggs are fertilized in the water, and after about two days microscopic ciliated larvae develop from the fertilized eggs. The larvae are carried with ocean currents for several weeks before metamorphosing into juvenile amphioxi and taking up life in sandy sediments.
Along parts of the coast of China, amphioxi are so numerous that they constitute the basis of a fishing industry. What appear to be fossilized amphioxi have been found recently in sediments about 525 million years old.