Chapter CXXX: Section 2: The vertebral column is devoid of vertebrae; it is

throughout life a rod of gelatinous tissue, the notochord (Figure 1, n.c.), surrounded by a cellular sheath. Such a rod is precursor to the vertebral column in the true vertebrates, but, except in such lowly forms as the lamprey, is usually replaced, partially (e.g., dog-fish) or wholly (as in the rabbit) by at first cartilaginous vertebrae whose bodies are derived from its sheath. Further, while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain, and is there at its termination enclosed by the middle portion of the skull, in Amphioxus it reaches far in front of the anterior extremity of the nervous system, to the end of the animal's body.* On this account the following classification is sometimes made of those animals which have a notochord:--

-Chordata_ (= Vertebrata, as used by Lankester).

1. Having the notochord reaching in front of the brain.
Cephalochorda = Amphioxus.

2. Having the notochord reaching anteriorly to the mid-brain, a
brain of three primary vesicles and a skull.
Craniata = all "true vertebrata": fishes, amphibia, reptiles,
birds, and mammals (Vertebrata of Balfour).

3. Having the notochord confined to the tail.
Urochorda = the ascidians, or sea-squirts, certain forms of life
only recently recognised as relatives of the vertebrata.

* The anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be.

Section 3. Figure 1, Sheet 19, shows the general anatomy of Amphioxus. We recognise four important points of resemblance to the earlier phases of the higher and the permanent structure of the lower members of the vertebrata, and it is these that justify the inclusion of amphioxus in this volume. In the first place there is the--

-Notochord_.
In the next, just above it (at s.c.) we find--

-A Dorsal Tubular Nervous Axis_,
the spinal cord. Thirdly, the pharynx (ph.) is perforated by--

-Respiratory Slits_,
though these, instead of being straight slashes, are modified from
a U-shape [slant very much forward and are much more numerous than
in any true vertebrate.]. -And-, Fourthly, there is, as we shall
see, a--

-Vertebrate Type of Circulation_.
[And finally the body-wall muscles are divided into--]

[-Myomers_.]

Section 4. The alimentary canal of Amphioxus commences with an "oral cavity," not represented in our vertebrata, surrounded by a number of cirri, or tentacles, supported by a horny substance which seems to be chitin, a common skeletal material among invertebrates. A velum (v.) forms a curtain, perforated by the mouth and by two smaller hyoidean apertures, between the oral cavity and the pharynx (ph.). "Pharyux" is here used in a wider sense than in the true vertebrata; it reaches back close to the liver, and is therefore equivalent to pharynx + oesophagus + a portion or all of the stomach. The [so-called] hyoidean apertures are not equivalent to the similarly-named parts of the vertebrata. Behind the pharynx the intestine (int.) runs straight out to the anus (an.), which opens not in the middle line, as one might expect, but in the left side! The liver lies usually on the creature's right, and instead of being a compact gland, is simply bag-like.

Section 5. The circulation is peculiarly reduced (Figure 2). The cardiac aorta (c.ao.) lies along the ventral side of the pharynx, and sends branches up along the complete bars between the gill slits. There is no -distinct- heart, but the whole of the cardiac aorta is contractile, and at the bases of the aortic arches that run up the bars there are contractile dilatations that assist in the propulsion of the blood. Dorsal to the pharynx, as in fishes, there is a pair of dorsal aorta (d.ao.) that unite above the liver (compare the frog, for instance), and thence run backward as a median dorsal aorta (d.ao.'). A portal vein (p.v.) bring blood back from the intestine (and apparently from the whole posterior portion of the animal) to the liver. Thence hepatic veins (hep.) take it to the cardiac aorta.

{Lines from First Edition only.}
-When we remember that in the embryonic vertebrate the heart is at
first a straight tube, this circulation appears even more strikingly
vertebrate in its character than before.-