Book - Vertebrate Zoology (1928) 2
Vertebrate Zoology G. R. De Beer (1928)
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Chapter II Amphioxus, a Primitive Chordate=
A thorough knowledge of the type which forms the subject of this chapter is fundamental for the study of vertebrates.
Externals
Amphioxus lanceolatus is a small animal (about 2 inches long) found in shallow seas with a sandy bottom in which it burrows. It is elongated and pointed at each end, from which fact it derives its name. The body is compressed from side to side and is capable of rapid movement by swimming, though it usually stays embedded in sand, feeding with only the mouth protruding. Both ends of the animal are expanded into thin vertical fins, which are joined by a shallow fin running all along the middle line of the back. The fin also extends a short distance forwards from the hind end on the ventral side. This median fin is of simple structure ; it is supported by a row of stifTeners known as " fin-ray boxes." These are made of connective tissue, and they are more numerous than the segments of the body. Along the dorsal fin the fin-ray boxes are arranged in a single row, but along the short ventral fin there is a double row of fin-ray boxes.
Fig. 3. — Amphioxus seen from the left side. a, anus ; ap, atriopore ; be, buccal cirrhi ; g, gonad ; gs, gill-slits ; n, notochord ; t, tail.
The epidermis is only one-cell thick, as in many invertebrates. Underlying the epidermis is the mesodermal dermis, which, in Amphioxus, takes the form of gelatinous connective tissue. Epidermis and dermis together form what is ordinarily known as the skin. Beneath the skin the myotomes of the body are arranged in a continuous row from front to rear.
Seen from the side and visible through the skin they are markedly V-shaped, with the apex pointing forwards. Those of one side alternate with those of the other.
The ventral side of the front end of the animal is expanded to form the oral hood. The sides of the oral hood (which are not quite symmetrical) bear a fringe of buccal cirrhi, each one supported by a small jointed skeleton. The cirrhi bear sense-organs. On the under surface of the oral hood the epithelium is modified in places into a ciliated organ (the organ of Miiller, or wheel-organ) whose function it is to create a current of water flowing towards the mouth. Slightly to the right of the middle line, a small depression opens into the cavity of the oral hood, known as Hatschek's pit. (The development of this interesting structure is described on p. 167.)
The mouth is situated at the hind end of the oral hood, and is a circular aperture pierced through a vertical transverse plate, the velum. The size of the opening is regulated by a circular sphincter muscle. In addition, there are twelve velar tentacles arising from the rim of the mouth, provided with sense-organs. Their function is to act as a strainer across the mouth-opening.
The anus opens not at but near the hind end of the body on the ventral surface, slightly to the left of the middle line owing to median position of the ventral fin. Just in front of the ventral fin is another aperture, the atriopore, the significance of which will be understood with a knowledge of the structure of the atrium.
Other external features to be noticed are the olfactory (or Kolliker's) pit on the left side of the body very near the front end, and the metapleural folds of the atrium. The gonads can also be seen from the outside, as a row of sacs between the mouth and the atriopore.
Alimentary System
The gut leads straight from the mouth to the anus without any loops or kinks. The anterior half of it is the pharynx, the posterior is the intestine. A blind outpushing is given off on the right side from the front of the intestine, forming the so-called liver- diverticulum.
The gut is suspended by a dorsal mesentery, and its lining is ciliated. It is surrounded by a thin coat of smooth muscle.
The side walls of the pharynx are perforated by a large number of gill-slits, openings which slant forwards from below up. For this reason several gill-slits will be cut in a single transverse section.
Ciliary Mode of Feeding. — Along the whole length of the dorsal wall of the pharynx runs a ciliated groove known as the hyperpharyngeal groove. Anteriorly this groove connects with two tracts of ciliated cells, the peripharyngeal bands which pass round one on each side behind the mouth and down to the floor of the pharynx. There they join the endostyle, which extends all the way back through the pharyngeal region. The endostyle consists of four tracts of glandular cells, separated by tracts of ciliated cells. The glandular cells secrete a sticky mucus which, by the action of the cilia, is driven forwards and sideways up the gill-bars, and round the peripharyngeal bands, where food particles become entangled in it. The food particles have been swept into the mouth with the current of water made by the cilia of the wheel-organ and gill-bars. Mucus and food then get carried into the hyperpharyngeal groove and back to the intestine. By this means the food is carried safely back through the pharynx as by a moving stairway, and is not lost with the water which streams out through the gill-slits.
The ciliary method of feeding is primitive. From the nature of its mechanism it can only supply particles of food of small size, and therefore it can only occur in smallish animals. In higher forms in which other methods such as biting or sucking have been adopted for procuring food, the endostyle is no longer required to secrete a mucus " fly- paper " ; it becomes modified in a most striking way and gives rise to the thyroid gland (see p399).
Fig. 7. — Amphioxus : transverse section through the endostyle showing the cilia, the four tracts of glandular cells (gc), the subendostylar coelom (se), and the ventral aorta (va).
Fig. 8. — Amphioxus : transverse section through two primary (pg) and one secondary (sg) gill-bars. ae, atrial epithelium ; b, blood-vessel ; c, coelomic cavity in the primary gill- bars ; sk, skeletal rods of the gill-bars.
Fig. 9. — Amphioxus : view of the dorsal portion of the pharynx and gill- slits (gs), showing the nephridia («). on, opening of the nephridium into the atrium ; pg, primary gill-bar ; s, synapticulum ; sg, secondary gill-bar.
Atrium. — The gill- slits do not open directly to the outside world but into a cavity known as the atrium, which in its turn opens to the exterior near its posterior extremity by the atriopore. On the right side of the body (but not on the left) the atrium extends back behind the atriopore as a blind sac nearly as far as the anus. The atrium has been formed by folds of the body- wall above the gill-slits growing down on each side (the metapleural folds) and meeting underneath what is the true ventral surface of the animal. The space of the atrium therefore represents a portion of the outside world, and is lined entirely by ectoderm. The low ridges running along the ventro-lateral edge of each of the atrial folds are the metapleural folds, and between them, meeting in the middle line, are the epipleurs, in the form of horizontal shelves. These close off the atrial cavity (see p. 170). The atrium is closed in front so that all the water which enters it does so through the gill-slits and passes out of the atriopore. Where the pharynx passes into the intestine, a pair of conical outpushings of the atrium project into the dorsal ccelomic cavities, one on each side, forming the so-called " brown funnels " (see p. 15). The function of the atrium is to protect the pharyngeal region, which is very vulnerable owing to the gill-slits.
Respiratory System
At early stages the gill-slits corresponded to the segmentation of the body, but more and more of them are formed (up to 180) and the correspondence is lost. The gill-slits are separated from each other by gill-bars, the inner surface of which is covered by endodermal, the outer by ectodermal tissue (forming the inner wall of the atrium). There are two kinds of gill-bars : primary, and secondary or tongue-bars. All the bars have a skeletal rod (composed of a chitin-like substance) passing down them and stiffening them. The rods of the secondary bars end simply at their ventral ends, while those of the primary bars bifurcate. Another difference is that the primary bars contain a portion of ccelomic cavity while the secondary bars do not. The bars are strongly ciliated, and by the activity of these cilia water is forced through the slits into the atrium. As the water passes between the gill-bars, the blood circulating in the blood-vessels of the latter becomes oxygenated. There are three vessels in each primary bar and two in each secondary bar ; the vessels in the secondary bars are connected with those in the primary bars by vessels running in the synapticula, or connecting struts.
Vascular System
Running forward under the floor of the pharynx beneath the endostyle is the ventral aorta. There is no specialised heart, but this aorta is contractile, and propels the blood into the afferent branchial arteries which run to the primary gill-bars. At the base of the bars these arteries swell into little contractile bulbils and divide into the three vessels which run up the bars. The secondary bars obtain blood in their two vessels indirectly from the primary bars through the vessels in the synapticula. Branches are sent to the excretory organs (nephridia, see p. 15) which are thereby enabled to extract the excretory products from the blood.
From the gill-bars the blood is collected into the efferent branchial vessels which run to the lateral dorsal aortae, one on each side of the mesentery, just above the hyperpharyngeal groove. Behind the pharynx they join to form the single dorsal aorta, which carries blood back to the posterior regions of the body. In the septa separating each pair of adjacent myotomes, segmental vessels leave the aorta and distribute blood locally. The blood is collected up again into the sub- intestinal vessel which runs forwards beneath the intestine from the hind end of the body. It breaks up into capillaries in the region of the liver- diverticulum, and so forms a hepatic portal system. From the liver the vessel runs forwards beneath the endostyle of the pharynx as the ventral aorta. There are also paired cardinal veins running in the body-wall at the level of the gonads, and extending forwards in the region of the pharynx and backwards to the tail. These veins connect with the subintestinal vessel by transverse veins, the ductus Cuvieri, on each side, which bridge across the ccelom. The blood is colourless.
Coelom
The relations of the ccelom are of great importance. The myotomes of the body are separated by septa (between the segments), but they do not fit the septa closely. Small spaces are left which are remnants of the myoccels.
Behind the pharynx the relations of the ccelom are quite simple and typical. The gut is suspended by a dorsal mesentery in a spacious splanchnoccel. In the pharyngeal region, however, the relations are slightly complicated by the presence of the gill-slits. Since these slits are openings from the gut to the outside (morphologically, ignoring the atrium) they form connexions between the gut-wall and the body- wall, and thereby necessarily obliterate the ccelom in places. The ccelom, therefore, is restricted to the regions between the slits, i.e. to the gill-bars. The coelom is perfectly normal above and below the level of the gill-slits. Accordingly, there are a pair of dorsal coelomic cavities, separated from one another in the middle line by the dorsal mesentery ; and a ventral coelomic cavity known from its position as the sub- endostylar coelom. The latter is in open communication with the dorsal ceeloms on each side by means of the coelomic canals in the primary gill-bars. There are no coelomic canals in the secondary or tongue bars, for they are later developments which divide the original gill-slits into two. The relations of the coelom are not difficult to understand when it is remembered that relicts are left between the slits in the primary gill-bars.
Into each dorsal coelomic cavity a conical outpushing of the atrium projects, from the region just behind the gill-slits. The apex of the cone points forwards, and so lies dorsal to the hindmost gill-slits. These structures are the so-called " brown funnels," of doubtful significance (see p. 12).
Excretory System. — The excretory organs of Amphioxus are remarkable in that they are nephridia. In all other chordates the excretory organs are coelomoducts or mesodermal kidneys (see p. 31). The nephridia lie over the gill- slits, project into the dorsal coelomic cavities and extend a short way down the coelomic canals in the primary bars in the form of small bent tubes. Each nephridium bears bunches of flame-cells or solenocytes, like hollow pins with a whip or flagellum hanging down inside from the head, and serving to flush out the contents. There is no internal opening to the nephridia, which derive the products which they excrete from the blood-vessels and ccelomic fluid by diffusion. The nephridia open into the atrium (that is, morphologically to the outside) by small pores situated near the top of the secondary gill-bars. They are segmental in origin.
Fig. 10. Amphioxus : transverse sections through the body in the regions of A, Kolliker's pit ; B, Hatschek's pit ; C, the anterior region of the pharynx ; D, the posterior region of the pharynx ; E, between pharynx and atriopore ; F, the atriopore ; G, between atriopore and anus ; H, the anus. a, anus ; op, atriopore ; at, atrium ; be, buccal cirrhi ; bf, brown funnel ; c, ccelom ; da, dorsal aorta ; dc, dorsal coelomic canal (in the region of the pharynx) ; df, dorsal fin ; dn, dorsal nerve-root ; e, eye-spot ; en, endostyle ; ep, epipleur ; fr, fin-ray box ; g, gonad ; gs, gill-slit ; hg, hyperpharyngeal ciliated groove ; Hp, Hatschek's pit ; i, intestine ; Kp, Kolliker's pit ; /, liver ; Ida, lateral dorsal aorta ; m, myotome ; mc, myocoel ; mp, metapleural fold ; n, notochord ; nc, nerve-cord ; oh, oral hood ; p, pharynx ; pa, extension of the atrium behind the atriopore ; sc, subendostylar coelomic canal ; vf, ventral fin ; vn, ventral nerve-root.
There is another nephridium at the front of the animal, lying dorsal to the oral hood near the middle line. It opens into the pharynx just behind the mouth, and is known as Hatschek's nephridium. No chordates other than Amphioxus are known to possess nephridia.
Genital System
The sexes are separate, but very similar in appearance. The gonads are pouches of germ-cells arranged in a row on each side of the body from about the ioth to the 36th segments, in the region of the gill-slits. When these pouches are full they bulge into the atrium ; but they must not be considered as lying in the atrium, for they are separated from it by the whole thickness of the body- wall. The segmental arrangement of the pouches is more or less preserved. When ripe, the germ-cells burst out of the pouches and pierce the body- wall, thus finding themselves in the atrium. From here they make their way to the outside through the atriopore. The cavity of the pouches is, of course, ccelomic.
Skeleton
Reference has already been made to the skeletal supports of the buccal cirrhi and to those of the gill-bars. The most important skeletal structure of Amphioxus is, of course, the notochord. This elastic rod extends from end to end of the animal, dorsal to the gut and ventral to the nerve- cord. Its extreme extension, almost to the tips of the anterior and posterior fins, is noteworthy.
Nervous System
The central nervous system consists of a straight tube running all the way down the back of the animal, dorsal to the notochord and ventral to the fin-ray boxes. Kolliker's pit on the left side of the snout represents the spot where the cavity of the tube opened to the exterior at earlier stages (the neuropore).
The cavity of the nerve-tube is enlarged at its front end forming the cerebral vesicle. At the same time the external diameter of the tube remains the same ; its walls are here therefore thinner. This specialisation is in Amphioxus the only indication of a brain. At the front end of the nerve-tube is a pigment-spot, to be regarded as a visual organ. Other such pigment-spots, or primitive " eyes," are to be found further back near the central canal.
On each side of the body the nerve-cord gives off nerves, which are of two kinds, dorsal and ventral. In each segment on each side of the body there is one dorsal nerve-root and one bunch of ventral nerve-roots. The ventral roots are distributed solely to the muscle-fibres in the myotome of that segment, and are " motor " nerves. The dorsal roots are concerned with transmitting impulses received from the sense-organs all over the skin (especially numerous on the buccal cirrhi), and with innervating the smooth musculature of the gut and atrium. The axons which go to make up the sensory or afferent fibres of the dorsal nerve-roots are derived directly from the sensory cells in the skin. The sensory cells therefore convey their impulses direct to the central nervous system on the plan characteristic of many invertebrates. In all forms above Amphioxus, the impulses are collected from the sensory cells by axons derived from other nerve-cells, whose nuclei lie in swellings or ganglia on the dorsal nerve- roots. Amphioxus is therefore primitive in not possessing these ganglia or nerve- cells.
The most anterior two pairs of roots are dorsal, and have no ventral roots corresponding to them. They innervate the sense-organs of the snout, oral hood, and buccal cirrhi.
While reviewing the foregoing description of Amphioxus an important analysis can be made. In the light of knowledge of other forms, the characteristics of an animal can be divided into two classes : primitive and specialised. There are those characters which are developed and perfected in the process of evolution to the next stage, and which are therefore simpler at the stage in question (in this case Amphioxus). A primitive character of this kind is shown by the vascular system of Amphioxus. There are also negative characters, for the later evolutionary stage may possess structures which the present stage lacks. The absence of a specialised head in Amphioxus is an example of a primitive negative character of this kind. Then there are characters of which it cannot be said that they are simpler than those of the next evolutionary stage, nor that they lead on to them, but which can be considered as historically primitive in the sense that they occur at early stages but not at later ones. The ciliary method of feeding is an historically primitive character of this kind : it preceded the jaw-method of feeding in time, but was not simpler than the latter method, nor did it lead up to it.
All primitive characters imply the possibility of progress in evolution. On the other hand, there are certain characters which have not only not contributed to the progress in evolution to the next stage, but have debarred their possessors from ever evolving to that stage. Such specialised or secondary characters are typified by the atrium of Amphioxus.
The analysis may conveniently be set out in tabular form :
Amphioxus
Primitive Characters.
Ciliary mode of feeding, with endostyle ; Epidermis one-cell thick ; Afferent nerve-fibres derived from sensory cells ; Complete row of segmented myotomes from front to rear ; Very slight specialisation of brain ; No specialised head ; No paired limbs or paired sense-organs ; No specialised heart ; Gonads segmental, without special ducts ; Nephridia ; segmentally arranged ; Simple and unbranched liver diverticulum.
Specialised Characters. Atrium ;
Extra large number of gill-slits, having lost correspondence with the segmentation of the body ; Tongue-bars ;
Asymmetry of oral hood and early development ; Extreme anterior extension of the notochord.
The large number of its primitive characters show that Amphioxus is a primitive animal, i.e. related to the original ancestors from which all chordates evolved. The secondary characters which Amphioxus possesses, however, show that it is not on the direct line of chordate descent.
Literature
Bourne, G. C. An Introduction to the Study of the Comparative Anatomy of Animals. Vol. 2. Bell, London, 191 5.
Delage, Y., et Herouard, E. Zoologie Concrete. Vol. 7. Les Procord£s. Schleicher Freres, Paris, 1898.
Goodrich, E. S. On the Structure of the Excretory Organs of Amphioxus. Quarterly Journal of Microscopical Science. Vol. 45, 1902 ; and Vol. 54, 1910.
Orton, J. H. The Ciliary Mechanisms on the Gill and the Mode of Feeding in Amphioxus. Journal of the Marine Biological Association. Vol. 10, 1913.
Willey, A. Amphioxus and the Ancestry of the Vertebrates. Columbia University Press, 1894.
- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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