-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.). "Pharynx" 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.-
Section 6. The coelom, or body cavity, of Amphioxus lies, of course, as in the vertebrata, between the intestinal wall and the body walls, and, just as in the vertebrata, it is largely reduced where gill slits occur. But matters are rather complicated by the presence of an atrial cavity round the pharynx, which is not certainly represented in the vertebrata, and which the student is at first apt to call the body cavity, although it is entirely distinct and different from that space. The mutual relation of the two will become apparent after a study of Figures 10, 11, 12 ([Sheet 21]). Figure 10 gives diagrammatically a section of a very young stage of Amphioxus; P is the pharynx portion of the alimentary canal, coe. is the coelom surrounding it at this stage here as elsewhere; mt.c. are certain lymph spaces, the metapleural canals, between which a small invagination (i.e., a pushing-in), at., of the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal cord. The gill slits, by which P. communicates with the exterior, are not shown. Next [Figure 11] shows the invagination (at.) pushing its way in, and cut off from the exterior by a meeting of the body wall below. Note that at. is a portion of the animal's exterior thus embraced by its body, and that its lining is therefore of the same material as the external integument. In [Figure 12,] at. is developing upward, so that the true body hangs into it. Now imagine the gill slits perforated, as shown by the double-headed arrow in [Figure 12]. Figure 3, on [Sheet 20], is a less diagrammatic representation of a cross-section of the pharyngeal region (vide Figure 1, [Sheet 19]). The student should compare Figure 3, [Sheet 20], and Figure 12, [Sheet 21]. The atrium and metapleural canals are easily recognised in both. In Figure 3 the coelom is much cut up by the gill slits, and we have remaining of it (a) the dorsal coelomic canals (d.c.c.) and (b) the branchial canals (br.c.) in the bars between the slits. The atrial cavity remains open to the exterior at one point, the atrial pore (at.p.).
Section 7. The method of examining cross-sections is an extremely convenient one in the study of such a type as Amphioxus. The student should very carefully go over and copy the six sections on [Sheet 20], comparing Figure 1 as he goes. He should do this before reading what follows. One little matter must be borne in mind. These figures are merely intended to convey the great structural ideas, and they are considerably simplified; they must not be regarded as a substitute for the examination of microscopic sections. [He will notice a number of rounded masses from the body wall. The] -For instance, the body-wall- muscles of Amphioxus are arranged in bundles bent sharply in an arrow shape, the point forward. -A number of these bundles are cut in any one section, and so the even shading of our diagrams, if they professed to be anything more than diagrams, should be broken up into masses.- These -bundles, we may mention-, are called myomeres, and they are indicated in [Figure 1] by lines pointing acutely forward. [Several are consequently cut in any transverse section ([Sheet 20]), and these are the rounded masses he sees.] Similar myomeres, similarly situated, are found in fish, behind the head, and, less obviously, they occur with diminishing importance as the scale of the vertebrata is ascended.
Section 8. If we compare the nervous system of amphioxus with that of any vertebrate, we find at once a number of striking differences. In the first place, the skeletal covering of it, the cranium and the neural arches of vertebrae, are represented only by a greatly simplified connective tissue. In the next, a simple and slight anterior dilatation alone represents the brain. A patch of black pigment anterior to this (e.s.) may or may not be what its name implies an eye-spot. There is a ciliated funnel, c.f. (Figure 1, [Sheet 19]), opening on the left side, which has been assumed to be olfactory in its functions, and in the mouth chamber a ciliated pit (c.p.), which may, or may not, be an organ of taste. The ventral fissure of the spinal cord is absent. The dorsal nerves are without ganglia, and do not come off in pairs, but alternately, one to the left, then one to the right, one to the left, one to the right, and so on. The ventral nerves are very short, more numerous than the dorsal, and never unite with these latter to form mixed nerves.