Amphioxus, or LANCELET, one of the lowest backboned animals, abundantly distributed on the sandy coasts of warm and temperate seas. It was regarded as a Gasteropod by Pallas, who first observed it; but in 1831 Yarrell remarked the presence of a cellular rod representing a rudimentary backbone. Since then, the structure and development of amphioxus have been thoroughly studied, and its importance as a persisting primitive vertebrate has been universally recognised. It can hardly be called a fish, for there is a greater difference in structure between it and any fish, than there is between a mammal and a bird. The compressed body, which rarely measures more than two inches in length, is pointed at both ends, whence the name amphioxus (Gr. amphi, 'both,' and oxus, 'sharp'). The mouth is fringed with a circlet of fine processes borne on a cartilaginous ring, and the back exhibits a very delicate fin, which is continued uninterrupted into the tail, and thence for a short distance along the ventral surface. The body is scaleless, and tolerably transparent, and the transverse muscle-segments are beautifully seen. There is no bony skeleton, but the backbone is represented by a simple cellular rod (notochord) running from tip to tip, and bearing dorsal cartilaginous rods which suggest vertebral spines. There is no hint of skull or limbs, and apart from the notochord, the skeleton is represented solely by a development of cartilaginous rods about the mouth, and on the walls of the pharynx. The spinal cord, which lies as usual above the rudimentary 'backbone,' has a slight anterior swelling, faintly suggesting a brain, and in this a pigmented spot, probably representing the eye, is imbedded. The sensitiveness of the animal to light and sound is due to the abundant presence of sense-cells throughout the skin. The mouth-cavity is separated by a movable flap from the wide anterior half of the alimentary canal, which forms a respiratory pharynx comparable to that of Ascidians. The wall is ciliated and pierced by a hundred or more slits, through which the water taken in by the mouth passes to the exterior, bathing in its course a set of blood-vessels spread out between the slits. In the embryo, these clefts open directly to the exterior, but are afterwards covered by two lateral folds of skin which unite to form an outer chamber (atrial chamber), with a single posterior aperture (the abdominal pore). The heart is a simple tube—in fact, only the largest of many contractile regions on the principal vessels. The blood is colourless.
The sexes are separate, and the reproductive organs form a row of cell-clumps on the wall of the body-cavity. These open separately into the outer chamber above mentioned, and thence the elements find their way out by the abdominal pore. The development very closely resembles that of the Ascidians (q.v.), which are also survivors of the ancestral vertebrates. Several varieties of A. 'anceolatus have been described, and another genus (Epigonichthys) was found on the Australian coast by the Gazelle expedition. The old name of amphioxus was Branchiostomum, and the titles Leptocardia and Acrania, sometimes used, refer to the simple tubular heart and absent skull. See works by Halschek (trans. 1893) and Willey (1894).