1. Anatomy

Section 1.  We find in Amphioxus the essential vertebrate features reduced to their simplest expression and, in addition, somewhat distorted.  There are wide differences from that vertebrate plan with which the reader may now be considered familiar.  There are no limbs.  There is an unbroken fin along the median dorsal line and coming round along the ventral middle line for about half the animal’s length.  But two lowly vertebrates, the hag-fish and lamprey, have no limbs and a continuous fin.  There is, as we shall see more clearly, a structure, the respiratory atrium, not apparently represented in the true vertebrate types, at least in their adult stages.  There is no distinct heart, only a debateable brain, quite without the typical division into three primary vesicles, no skull, no structures whatever of cartilage or bone, no genital ducts, no kidneys at all resembling those of the vertebrata, no pancreas, no spleen; apparently no sympathetic chain, no paired sense organs, eyes, ears, or nasal sacs, in all of which points we have striking differences from all true vertebrata; and such a characteristic vertebrate peculiarity as the pineal gland we can only say is represented very doubtfully by the eye spot.

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 ­

   -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.-

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.

The student will observe that here, just as in the case of the ciliated funnel and anus, the Amphioxus is not strictly symmetrical, but twisted, as it were, and so departs from the general rule of at least external bilateral symmetry obtaining among the vertebrates.  It habitually lies on one side in the mud of the sea bottom, and it is probable that this external asymmetry is due to this habit, so that too much classificatory importance must not be attached to it.  The soles and other related fish, for instance, are twisted and asymmetrical, through a similar specific habit, to such an extent that both eyes lie on one side of the animal.

Section 9.  No kidney on the vertebrate pattern is found, but the following structures have, among others, been suggested as renal organs: ­

(a) Certain canals, the brown tubes of Lankester (b.t.L., Figure 2, Sheet 19), a pair of pigmented tubes opening into the atrium at the hind end of the pharynx, lying forward along by the dorsal coelomic canals, and having an internal opening also.

(b) Certain tubuli described by Weiss as situated in a series along the upper corners of the atrial cavity, and communicating, after the fashion, of the “nephridia” of the earthworm, with the coelom and with the exterior (or, rather, with that portion of the animal’s exterior enclosed in by the atrial wall; compare Section 6).

   (c) The general epithelial lining of the atrium.

The reproductive organs (Figure 4, Sheet 20, g.) are masses of cells situated in an isolated part of the coelom in the atrial folds, and, having no ducts, their contents must escape into the atrium by rupture of the body-wall.  Thence they escape either by gill-slits, pharynx and mouth, or, more generally, through the atrial pore.  The animals, like all the vertebrata, are dioecious, i.e., male or female.

Section 10.  The endostyle (end.), in Figures 3 and 4, is a ciliated path or groove on the under side of the pharynx, which is generally supposed to represent the thyroid gland of vertebrates.  The vertebrate thyroid, early in development, is certainly an open and long narrow groove in the ventral side of the pharynx.  The hyper-pharyngeal groove (h.p.) has been in the past compared to the pituitary body, but there is little doubt now that this structure is represented by the ciliated pit.

Section 11.  The student is advised to revise this chapter before proceeding, and to schedule carefully the anatomical features under the headings of (1.) distinctly vertebrate characters, (2.) characters contrasting with the normal vertebrate structure, (3.) facts of doubtful import, with the suggestions given in the text written against them.

2. The Development of Amphioxus

Section 12.  The development of amphioxus, studied completely, is at once one of the most alluring and difficult tasks in the way of the zoologist; but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now.

Section 13.  The phenomena of the extrusion of polar bodies and fertilization are treated of later, and will, therefore, not be considered now.  We will start our description with an egg-cell, which has escaped, of course, since there are no genital ducts, by rupture of the parent, has been fertilized by the male element, and is about to develop into a young amphioxus.  It is simply a single cell, with some power of amoeboid motion, a single nucleus and nucleolus; and in amphioxus its protoplasm is clear and transparent.  Frequently ova are loaded with granules of food store (yolk), which enable the young animal to go far with its development before it is hatched and has to begin fending for itself.  Such an ovum as that of our present type, however being devoid of such yolk (alecithal = without yolk), necessitates a very early start in life, and, for reasons too complicated to state fully here, the development in such a case is considered particularly instructive and primitive by zoologists.

Section 14.  The first thing to be seen in the developing cell is a deepening circular groove (Figure 1, Sheet 21), which divides the ovum into two parts.  Another groove then cuts at right angles to this subdividing the two into four (Figure 2).  Another groove, at right angles to both the former, follows, making the four eight (Figure 3).  And so subdivision goes on.  The whole process is called segmentation or cleavage.

Section 15.  At the end of segmentation we get a hollow sphere of small cells, the cells separating from one another centrally and enclosing a cavity as the process proceeds.  This is the blastosphere, shown diagrammatically in Figure 4, and of which an internal view, rather truer to the facts of the case as regards shape, is given as Figure 5.  The central cavity is the segmentation cavity (s.c.).

Section 16.  Invagination follows (Figure 6).  In this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed.  The space ar. is the archenteron, the primordial intestine, and its mouth is called, the blastopore (bp.).  The outer layer of this double-walled sac is called the epiblast.  For the present we will give the inner lining no special term.  The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches.  This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore.  All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus.

Section 17.  The anus is perforated later near the region occupied at this stage by the blastopore.  Hence the anterior end of the future amphioxus, the head end, is pointing towards the Figure 6, and the letters ep. are marked on the side which will be dorsal.

Section 18.  Figure 7 i. is a dorsal view of the gastrula at a somewhat later stage, and here indications of distinctly vertebrate relationships already appear.  Figure 7 ii. is a cross-section, its position, being shown by cross-lines in 7 i. and 6.  Note first that the epiblast along the mid-dorsal line is sinking in to form what is called the neural plate (n.p.), and simultaneously on either side of it rise the neural folds (n.f.).  Now, at Figure 8, a slightly later stage is represented, and at 9 i. the inturned part is separated from the general external epiblast as the spinal cord.  The remainder of the epiblast constitutes the epidermis.

Section 19.  Reverting to Figure 7 ii., along the dorsal side of the archenteron a thickening of its wall appears, and is gradually pinched off from it to form a cellular rod, lying along under the nervous axis and above the intestine.  This is the notochord (compare Figures 8 and 9).

Section 20.  Finally, we note two series of buds of cells, one on either side of the archenteron in Figure 7 ii.  In 8 these buds have become hollow vesicles, growing out from it, the coelomic pouches.  They are further developed in 9; and in 9 ii., which is a diagrammatic figure, they are indicated by dotted lines.  They finally appear to (? entirely) obliterate the segmentation cavity ­ they certainly do so throughout the body; and their cavities are in time cut off from the mesenteron, by the gradual constriction of their openings.  In this way the coelom (body cavity) arises as a series of hollow “archenteric” outgrowths, and ms. becomes the alimentary canal. mt.c., the metapleural canals, probably arise subsequently to, and independently of, the general coelomic space, by a splitting in the body-wall substance.

Section 21.  Hence, in considering the structure of amphioxus, we have three series of cells from which its tissues are developed: ­

   1.  The epiblast.

2.  Walls of the coelomic pouches, which form (a) an inner lining to the epiblast, (b) an outer coating to the hypoblast, and (c) the mesentery (m.), by which the intestine is supported.  This is the mesoblast.

   3.  The lining of the mesenteron, or hypoblast.

From the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived.  From the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived.  The mass of the body is thus evidently made of mesoblast.  The hypoblast is the lining of the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast.

Section 22.  Figure 9 ii. shows all the essential points of the structure of amphioxus.  Epiblast is indicated by a line of dashes, mesoblast by dots, and hypoblast, dark or black.  The true mouth is formed late by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and fuses with the hypoblast, and is then perforated.  The position of this mouth is at the velum.  The formation of the atrium has been described.  The metapleural folds run forward in front of the velum, as the epipleurs (ep. in Sections 1 and 2), and form an oral hood (b.c.), around which the tentacles appear, and which is evidently not equivalent to the vertebrate mouth cavity, but in front of and outside it.  The anus is formed by a tucking in, the proctodaeum, similar to the stomodaeum.

Section 23.  The formation of the respiratory slits is complicated, and difficult to describe, but, since investigators have still to render its meaning apparent, it need not detain the elementary student.

See Balfour’s Embryology, Volume 2, and Quarterly Journal of Microscopical Science March, 1891.

3. Questions on Amphioxus

1.  Draw diagrams, with the parts named, of the alimentary canal of (a) amphioxus, (b) any craniate; (c) indicate very shortly the principal structural differences between the two.

2.  Describe, with a diagram, the circulation of amphioxus.  Compare it with that of the craniata.

3.  Draw from memory transverse sections, of amphioxus (a) in the oral region, (b) through the pharynx, (c) just anterior, and (d) just posterior to atrial pore.

4.  Describe fully the coelom of amphioxus, and compare it with that of the frog in regard to (a) development, (b) its relation to other organs in the adult.

5.  Compare the atrial cavity and coelom of amphioxus.  To what series of cavities in the frog are the metapleural canals to be compared?

6.  Describe the notochord of amphioxus, and point out its differences from the vertebrate notochord.

7.  Describe, with diagrams, the nervous system of amphioxus, and compare its nervous axis, in detail, with that of a vertebrate.

8.  Compare the genital organs of amphioxus with those of a higher vertebrate.

9.  What structures have been regarded, as renal organs in amphioxus?

10.  What is a gastrula?  With what lower type has the gastrula been compared?  Discuss the comparison.