This order includes but a single fish, the anomalous Amphioxus lanceolatus, or Lancelet (fig. 259), the organisation of which differs in almost all important points from that of all the other members of the class. The characters of Amphioxus, in fact, are so aberrant, that Haeckel proposes to divide the sub-kingdom Vertebrata into two primary sections - the one (Leptocardia) comprising the Lancelet alone, whilst the other (Tachycardia) includes all other Vertebrates. The order is defined by the following characters, which, as will be seen, are mostly negative: - No skull is present, nor lower jaw (mandible), nor limbs. The notochord is persistent; and there are no vertebral centra nor arches. No distinct brain nor auditory organs are present. In place of a distinct heart, pul-sating dilatations are developed upon several of the great bloodvessels. The blood is pale. The mouth is in the form of a longitudinal fissure, surrounded by filaments or cirri. The walls of the pharynx are perforated by numerous clefts or fissures, the sides of which are ciliated, the whole exercising a respiratory function.
* Cuvier divided the class Pisces into the great orders of the Chondrop-ierygii (or Cartilaginous Fishes), the Acanthopterygii (or Fishes with spinous rays in the paired fins), and Malacopterygii (or Fishes with soft rays in the paired fins). Agassiz divides Fishes, from the character of the scales, into the four orders, Cycloidei, Ctenoidei, Ganoidei, and Placoidei. Muller divides the Fishes into the five orders Leptocardia, (Lancelet), Cyclo-stomata (Lampreys and Hag-fishes), Teleostei, (Bony Fishes), Ganoidei, (Ganoid Fishes), and Selachia (Sharks and Rays).
The Lancelet is a singular little fish, from one to two inches in length, which is found burrowing in sandbanks, in various seas, but especially in the Mediterranean. The body (fig. 259) is semi-transparent, destitute of an exoskeleton, and lanceolate in shape, and is provided with a narrow membranous border, of the nature of a median fin, which runs along the whole of the dorsal and part of the ventral surface, and expands at the tail to form a lancet-shaped caudal-fin. No true paired fins, representing the anterior and posterior limbs, are present. The mouth is a longitudinal fissure, situated at the front of the head, and destitute of jaws. It is surrounded by a cartilaginous ring, composed of many pieces, which give off prolongations, so as to form a number of ciliated cartilaginous filaments or "cirri" on each side of the mouth. (Hence the name of Cirrostomi, proposed by Professor Owen for the order.) The throat is provided on each side with vascular lamellae, which are believed by Owen to perform the function of free branchial filaments. The mouth leads into a dilated chamber (fig. 259, b), which is believed to represent the pharynx, and is termed the "pharyngeal" or "branchial sac." It is an elongated chamber, the walls of which are strengthened by numerous cartilaginous filaments, between which is a series of transverse slits or clefts, the whole covered by a richly-ciliated mucous membrane. This branchial dilatation has given rise to the name Branchiostoma, often applied to the Lancelet. Posteriorly the branchial sac opens into an alimentary canal, to which is appended a long and capacious sac or caecum (h) which is believed to represent the liver. The intestinal tube terminates posteriorly by a distinct anus (a), which is situated at the root of the tail a little to the left of the median line; and the intestinal mucous membrane is ciliated. Respiration is effected by the admission of water taken in by the mouth into the branchial sac, having previously passed over the free branchial filaments before mentioned. The water passes through the slits in the branchial sac, and thus gains access to the abdominal cavity, from which it escapes by means of an aperture with contractile margins situated a little in front of the anus, and called the "abdominal pore" (p). There is no distinct heart, and the circulation is entirely effected by means of rhythmically contractile dilatations which are developed upon several of the great blood-vessels. In other words, the heart retains its primitively tubular condition, and special contractile dilatations are developed upon other vessels (those carrying the blood to the pharynx). The blood itself is colourless. No kidneys have as yet been certainly identified, and there is no lymphatic system. There is no skeleton properly so called. In place of the vertebral column, and constituting the whole endoskeleton, is the semi-gelatinous cellular notochord (n), enclosed in a fibrous sheath, and giving off fibrous arches above and below. The notochord is, further, peculiar in this, that it is prolonged quite to the anterior end of the body, whereas in all other Vertebrates it stops short at the pituitary fossa. There is no cranium, and the spinal cord does not expand anteriorly to form a distinct cerebral mass. The brain, however, may be said to be represented, since the anterior portion of the nervous axis gives off nerves to a pair of rudimentary eyes, and another branch to a ciliated pit, believed to represent an olfactory organ. The generative organs (ovaria and testes) are not furnished with any efferent ducts (oviduct or vas deferens). The generative products, therefore, are shed into the abdominal cavity, and gain the external medium by the "abdominal pore."
Fig. 259. - The Lancelet (Amphioxus lanceolatus), enlarged to twice its natural size. o Mouth; b Pharyngeal sac; g Stomach; h Diverticulum representing the liver; i Intestine; a Anus: n Notochord ; f Rudiments of fin-rays ; p Abdominal pore.