Lancelet (dranchiostoma or amphioxus), the lowest known of the vertebrate animals, constituting the order pharyngobranchii of Huxley, the leptocardia of Haeckel, who regards it as a primary division of the branch of vertebrates. This anomalous fish has been found on the coasts of Great Britain and Sweden, in the Mediterranean, on our southern Atlantic coast, and in the Indian ocean. It is from 1 1/2 to 2 in. long, tapering at each end, ribbon-like, translucent, and silvery white; it generally burrows in the sand in deep water, feeding on minute animalcules. Along the back runs a median fin, expanding at the tail into a lancet-shaped caudal; there are no apparent pectorals and ventrals; at the lower surface, extending forward from the anal and branchial apertures, are two lateral folds, which led Pallas to regard it as a gasteropod mollusk. The mouth is a longitudinal fissure, in the front of the head, without jaws, but surrounded by several cartilaginous filaments. The mouth opens into a greatly developed pharynx or throat, the walls of which, strengthened by cartilaginous filaments, are perforated by transverse slits, the whole covered with a thickly ciliated or fringed membrane; this is the respiratory sac, the water entering by the mouth, passing between the branchial slits and over the ciliary fringes filled with blood into the abdominal cavity, and escaping by an opening on the lower surface in front of the vent (porus abdominalis of Muller). From the branchial sac the intestine, having a liver-like organ attached, extends to an oval aperture under the tail.
There is no single contractile cavity or heart, the only exception in vertebrates, the circulation being effected by several contractile dilatations of the great blood vessels, as in the annelids; the blood is colorless. There is no proper skeleton; the vertebral column, moto-chord or chor-da dorsalis, is a semi-gelatinous rod, enclosed in a sheath and supporting the spinal cord, and is composed of 60 to 70 fibrous lamina) loosely attached to each other; there is also a cartilaginous apparatus supporting the mouth, and 70 to 80 hair-like ribs surrounding the branchial cavity. There is no skull, and no expansion of the spinal cord into a brain, though from its anterior extremity are given off nerves to the rudimentary eyes, and perhaps other nerves of special sense. The arrangement of the muscles is fish-like; the skin is thin, but tough and scaleless. The location of the respiratory system in the anterior portion of the intestinal canal is met with also in the cyclo-stome fishes. This most aberrant form is clearly a vertebrate, though it has no brain, and the respiration and circulation of an annelid.
It has been believed by some to favor the idea of Kowalewsky and Kupffer, that the ascid-ians show a kinship to the vertebrates, and it certainly has some remarkable invertebrate features found in no other vertebrate animal. The common species is the B. lanceolatum.
Lancelet (Amphioxus lanceolatus).
1. Upper side. 2. Lower side. 3. Anatomical diagram: A, notochord; B, spinal chord; C, mouth surrounded by cyrrhi; D, greatly dilated pharynx perforated by ciliated clefts; E, intestine terminating in anus F; G, haemal system.