The Evolution of Man, vol 2 | Page 6

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 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, Figure 2.210 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 (Figure 2.210 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, Figure
2.210 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 larvae 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.
(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five
gill-clefts, through the middle of the body.
FIGURE 2.213. 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 coeloseptum (partition
between dorsal and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre
layer, I myocoel (dorsal body-cavity), II splanchnocoel (ventral
body-cavity).)
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 (Figure 2.210 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,