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