AM'PHIOXIUS (Gk. afirpi, on both sides + u.rys, sharp). A small, bilateral, translucent, marine animal. about two or three inches long, thought by some to be an offshoot of the primitive vertebrate stock, and by others to be a degenerate, primitive vertebrate. The amphioxus or "lancelet." has no well-defined brain, but a persistent and unsegmented noto chord. The muscles are arranged in sixty-two V-shaped myomeres dovetailed into one another. The single mouth and anus are in the median line. There are no limbs, eyes, sympathetic nerves, or genital ducts. The gill-slits, which are numerous and supported by bars, open from the mouth into the atrial chamber. which has one opening to the exterior, the atrial pore. The best-known species is Amphioxus lanecolatus, which dwells buried in sand near the seashore line. fts food, which consists mainly of diatoms, is sucked into its month. The adults swim about in the evening only, but the young are very ac tive. The segmentation of the egg is complete, and results in the formation of a blastosphere, which invaginates to form a gastrula. The
medullary groove is formed by a sinking of the ectoderm along the mid-dorsal line. The cavity of the gastrula becomes the gut of the adult. In the active early life of the embryo the ecto derm is ciliated. The simplicity of its develop ment has made the amphioxus a favorite object of study for the descriptive and experimental embryologist. if the two cells which are the result of the first segmentation are separated, each cell will develop into a complete individual one-half the size of the normal embryo. Incom plete separation results in the formation of double or Siamese-like twins. Compare BALANO GLOSSI:S; and consult A. Willey, Anyhiosus and the Ancestry of the Vertebrates (New York, IS94). See the articles on the evolution of the alimentary. circulatory, muscular, nervous, and respiratory systems, under ALIMENTARY SYS TEM, etc.