The Evolution of Man, vol 2 | Page 5

surrounded by threads of beard; b anus, c gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g small intestine, h branchial cavity, i chorda (axial rod), underneath it the aorta; k aortic arches, l trunk of the branchial artery, m swellings on its branches, n vena cava, o visceral vein.
FIGURE 2.211. Transverse section of the head of the Amphioxus. (From Boveri.) Above the branchial gut (kd) is the chorda, above this the neural tube (in which we can distinguish the inner grey and the outer white matter); above again is the dorsal fin (fh). To the right and left above (in the episoma) are the thick muscular plates (m); below (in the hyposoma) the gonads (g). ao aorta (here double), c corium, ec endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel, ld partition between the coeloma (sc) and atrium (p), mt transverse ventral muscle, n renal canals, of upper and uf lower canals in the mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal body-cavity), si principal (or subintestinal) vein, sk perichorda (skeletal layer).)
Johannes Muller classed the lancelet with the fishes, although he pointed out that the differences between this simple vertebrate and the lowest fishes are much greater than between the fishes and the amphibia. But this was far from expressing the real significance of the animal. We may confidently lay down the following principle: The Amphioxus differs more from the fishes than the fishes do from man and the other vertebrates. As a matter of fact, it is so different from all the other vertebrates in its whole organisation that the laws of logical classification compel us to distinguish two divisions of this stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the Craniota (man and the other vertebrates). The first and lower division comprises the vertebrates that have no vertebrae or skull (cranium). Of these the only living representatives are the Amphioxus and Paramphioxus, though there must have been a number of different species at an early period of the earth's history.
Opposed to the Acrania is the second division of the vertebrates, which comprises all the other members of the stem, from the fishes up to man. All these vertebrates have a head quite distinct from the trunk, with a skull (cranium) and brain; all have a centralised heart, fully-formed kidneys, etc. Hence they are called the Craniota. These Craniotes are, however, without a skull in their earlier period. As we already know from embryology, even man, like every other mammal, passes in the earlier course of his development through the important stage which we call the chordula; at this lower stage the animal has neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even after the formation of the primitive vertebrae has begun, the segmented foetus of the amniotes still has for a long time the simple form of a lyre-shaped disk or a sandal, without limbs or extremities. When we compare this embryonic condition, the sandal-shaped foetus, with the developed lancelet, we may say that the amphioxus is, in a certain sense, a permanent sandal-embryo, or a permanent embryonic form of the Acrania; it never rises above a low grade of development which we have long since passed.
The fully-developed lancelet (Figure 2.210) 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 (Figure 2.210 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 (Figure 2.211). 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