The fully-developed lancelet (Fig. 210) is about two inches long, is colourless or of a light red tint, and has the shape of a narrow lancet-formed leaf. The body is pointed at both ends, but much compressed at the sides. There is no trace of limbs. The outer skin is very thin and delicate, naked, transparent, and composed of two different layers, a simple external stratum of cells, the epidermis, and a thin underlying cutis-layer. Along the middle line of the back runs a narrow fin-fringe which expands behind into an oval tail-fin, and is continued below in a short anus-fin. The fin-fringe is supported by a number of square elastic fin-plates.
In the middle of the body we find a thin string of cartilage, which goes the whole length of the body from front to back, and is pointed at both ends (Fig. 210 i). This straight, cylindrical rod (somewhat compressed for a time) is the axial rod or the chorda dorsalis; in the lancelet this is the only trace of a vertebral column. The chorda develops no further, but retains its original simplicity throughout life. It is enclosed by a firm membrane, the chorda-sheath or perichorda. The real features of this and of its dependent formations are best seen in the transverse section of the Amphioxus (Fig. 211). The perichorda forms a cylindrical tube immediately over the chorda, and the central nervous system, the medullary tube, is enclosed in it. This important psychic organ also remains in its simplest shape throughout life, as a cylindrical tube, terminating with almost equal plainness at either end, and enclosing a narrow canal in its thick wall. However, the fore end is a little rounder, and contains a small, almost imperceptible bulbous swelling of the canal. This must be regarded as the beginning of a rudimentary brain. At the foremost end of it there is a small black pigment-spot, a rudimentary eye; and a narrow canal leads to a superficial sense-organ. In the vicinity of this optic spot we find at the left side a small ciliated depression, the single olfactory organ. There is no organ of hearing. This defective development of the higher sense-organs is probably, in the main, not an original feature, but a result of degeneration.
Underneath the axial rod or chorda runs a very simple alimentary canal, a tube that opens on the ventral side of the animal by a mouth in front and anus behind. The oval mouth is surrounded by a ring of cartilage, on which there are twenty to thirty cartilaginous threads (organs of touch, Fig. 210 a). The alimentary canal divides into sections of about equal length by a constriction in the middle. The fore section, or head-gut, serves for respiration; the hind section, or trunk-gut, for digestion. The limit of the two alimentary regions is also the limit of the two parts of the body, the head and the trunk. The head-gut or branchial gut forms a broad gill-crate, the grilled wall of which is pierced by numbers of gill-clefts (Fig. 210 d). The fine bars of the gill-crate between the clefts are strengthened with firm parallel rods, and these are connected in pairs by cross-rods. The water that enters the mouth of the Amphioxus passes through these clefts into the large surrounding branchial cavity or atrium, and then pours out behind through a hole in it, the respiratory pore (porus branchialis, Fig. 210 c). Below, on the ventral side of the gill-crate, there is in the middle line a ciliated groove with a glandular wall (the hypobranchial groove), which is also found in the Ascidia and the larvæ of the Cyclostoma. It is interesting because the thyroid gland in the larynx of the higher vertebrates (underneath the “Adam’s apple”) has been developed from it.
Behind the respiratory part of the gut we have the digestive section, the trunk or liver (hepatic) gut. The small particles that the Amphioxus takes in with the water—infusoria, diatoms, particles of decomposed plants and animals, etc.—pass from the gill-crate into the digestive part of the canal, and are used up as food. From a somewhat enlarged portion, that corresponds to the stomach (Fig. 210 e), a long, pouch-like blind sac proceeds straight forward (f); it lies underneath on the left side of the gill-crate, and ends blindly about the middle of it. This is the liver of the Amphioxus, the simplest kind of liver that we meet in any vertebrate. In man also the liver develops, as we shall see, in the shape of a pouch-like blind sac, that forms out of the alimentary canal behind the stomach.
Fig. 212—Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body.
Fig. 213—Diagram of the preceding. (From Hatschek.) A epidermis, B medullary tube, C chorda, C1 inner chorda-sheath, D visceral epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome), 3 skeletal plate (sclerotome), 4 cœloseptum (partition between dorsal and ventral cœloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocœl (dorsal body-cavity), II splanchnocœl (ventral body-cavity).)
The formation of the circulatory system in this animal is not less interesting. All the other vertebrates have a compressed, thick, pouch-shaped heart, which develops from the wall of the gut at the throat, and from which the blood-vessels proceed; in the Amphioxus there is no special centralised heart, driving the blood by its pulsations. This movement is effected, as in the annelids, by the thin blood-vessels themselves, which discharge the function of the heart, contracting and pulsating in their whole length, and thus driving the colourless blood through the entire body. On the under-side of the gill-crate, in the middle line, there is the trunk of a large vessel that corresponds to the heart of the other vertebrates and the trunk of the branchial artery that proceeds from it; this drives the blood into the gills (Fig. 210 l). A number of small vascular arches arise on each side from this branchial artery, and form little heart-shaped swellings or bulbilla (m) at their points of departure; they advance along the branchial arches, between the gill-clefts and the fore-gut, and unite, as branchial veins, above the gill-crate in a large trunk blood-vessel that runs under the chorda dorsalis. This is the principal artery or primitive aorta (Fig. 214 D). The branches which it gives off to all parts of the body unite again in a larger venous vessel at the underside of the gut, called the subintestinal vein (Figs. 210 o, 212 E). This single main vessel of the Amphioxus goes like a closed circular water-conduit along the alimentary canal through the whole body, and pulsates in its whole length above and below. When the upper tube contracts the lower one is filled with blood, and vice versa. In the upper tube the blood flows from front to rear, then back from rear to front in the lower vessel. The whole of the long tube that runs along the ventral side of the alimentary canal and contains venous blood may be called the “principal vein,” and may be compared to the ventral vessel in the worms. On the other hand, the long straight vessel that runs along the dorsal line of the gut above, between it and the chorda, and contains arterial blood, is clearly identical with the aorta or principal artery of the other vertebrates; and on the other side it may be compared to the dorsal vessel in the worms.
The cœloma or body-cavity has some very important and distinctive features in the Amphioxus. The embryology of it is most instructive in connection with the stem-history of the body-cavity in man and the other vertebrates. As we have already seen (Chapter X), in these the two cœlom-pouches are divided at an early stage by transverse constrictions into a double row of primitive segments (Fig. 124), and each of these subdivides, by a frontal or lateral constriction, into an upper (dorsal) and lower (ventral) pouch.