Book - Vertebrate Zoology (1928) 37
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXXVII The Origin of Chordates, and Their Radiation as Aquatic Animals
From studies on all the groups of chordates and comparisons between them, it is possible to arrive at an idea as to what the original chordates must have been like. They were small, bilaterally symmetrical, and no part of them was sufficiently hard or resistant to be capable of preservation by fossilisation. They were marine animals, as are their lowest representatives at the present day. Some of these may now be considered, for, although they are specialised often to the point of degeneracy, and are of no use in the interpretation of the higher chordates, they show some characters which assist in estimations of the relations which the chordates bear to other animals. Balanoglossus is a " worm-like " form, with the following chordate features : gill-slits, ciliated grooves assisting in the process of feeding, a dorsal nerve-cord which for a short portion of its length is tubular, and a skeletal structure in the anterior region of the body which is held to represent a rudimentary notochord. From the latter possession it is classed as a Hemichordate. The body is divided into three regions, " proboscis," " collar," and " trunk," and is adapted to its mode of life, which is burrowing in the sand. Its chordate affinities are obvious from the characters just mentioned, but other features ally it to a number of invertebrates, and especially the Echinodermata. The free-swimming larval form of Balanoglossus, the Tornaria, is very similar indeed to the larval forms of the Echinoderms, in general form, in the arrangements of the bands of cilia which it carries, and in the method of origin of the mesoderm. The latter arises as three pairs of pouches from the archenteron (strictly, two pairs and an anterior median pouch which represents a fused pair). Such coelomic sacs are enterocoelic, like the anterior gut- diverticula of Amphioxus. The three sets of coelomic pouches persist in the adult Balanoglossus, and it is interesting to notice that the first two sets, forming the cavities of the " proboscis " and of the " collar," have openings to the exterior. These openings are coelomostomes, comparable to the water-pores of the Echinoderms, and the connexions which are occasionally found in Craniates between the premandibular somites and the hypophysis (" proboscis-pores ").
Allied to Balanoglossus are the Pterobranchia, which show a slight trace of a notochord, but no dorsal tubular nerve- cord. One of them, Cephalodiscus, has the three sets of coelomic pouches, each with a coelomostome, and a pair of gill-slits. It is not free-swimming but sessile, reproducing actively by budding. The other, Rhabdopleura is not only sessile but colonial, for the buds formed remain in connexion with the parent stock. Rhabdopleura has the three sets of coelomic pouches, and coelomostomes, but no gill- slits.
The next form to consider is Phoronis, which is worm- like with the anterior end modified into a row of tentacles. The anterior region of the body corresponding to the proboscis is reduced to a flap overhanging the mouth, so that the body contains only two sets of coelomic pouches. The larval form of Phoronis which is called the Actinotrocha, has ciliated bands reminiscent of those of the Tornaria. Phoronis has nephridia, and as these structures are also present in Amphioxus, it is possible that they were present in the original common ancestor from which all these forms were descended.
Phoronis is related to the Ectoproctous Polyzoa, and to the Brachiopoda. All these forms, so far as is known, tend to have coelomic pouches developed as enterocoels, and usually showing a tripartite arrangement. Many of them have open coelomostomes. The larval forms usually have a ciliated band passing behind the mouth, and cleavage of the egg is indeterminate. These features distinguish the chordates and their allies from the other great group of invertebrates comprising the Annelida, Arthropoda, and Mollusca.
There is reason to believe that the concentration of nerve-cells to form a central nervous system out of the more primitive diffuse nerve-net took place in the region of greatest stimulation. This is the ventral side in Annelida, Arthropoda, and Mollusca, all of which typically crawl on the ventral surface. The fact that the central nervous system of chordates is dorsal seems to show that the ancestral chordates were not ventral crawlers, but pursued a free-living pelagic existence, receiving the greatest stimulation on the dorsal side from the surface of the sea.
Returning now from these more or less distant allies to true chordates, the next group to consider is one which, like the Hemichordates, has left the main line of chordate evolution and become specialised in different directions : the Urochordata. These preserve the notochord in the tail in the larval stage only, and the dorsal tubular nerve-cord of the larva degenerates in the adult. They possess gill-slits, and a typical well-developed endostyle, used in connexion with the ciliary method of feeding. Their development is also typical of chordates. One group of these animals, the Larvacea, retain the larval structure throughout life, with the tail and notochord. The others pass through a free-swimming larval stage, and then undergo a retrograde metamorphosis into sessile animals, losing the tail, notochord, and larval eyes and organs of balance. These are the Ascidiacea or sea-squirts. Some of these are solitary, but most are colonial, reproducing extensively by asexual reproduction or budding, as is commonly the case with sessile forms. Others, forming the group of Thaliacea or salps, have returned to a free-swimming existence, but retaining many traces of former sessile habits ; in particular the habit of budding, which is very prevalent. Some of them have a true alternation of sexually produced (from fertilised eggs) and asexually produced (from buds) generations, and one form is further interesting in that the sexually produced generation is nourished during its development by the mother by means of a placenta (Salpa).
None of these forms, however, exhibit the typical chordate segmentation of the body, which enables them to swim in definite directions instead of being carried aimlessly about at the mercy of currents . The immediate ancestors of Amphioxus and of the higher chordates were elongated, compressed from side to side and deep from dorsal to ventral edge. As a consequence, they were able to bend the body from side to side, and perform undulatory movements. The body was made up of several segments. Each segment was separated from the ones in front and behind by septa or partitions, and stretching from septum to septum were the myotomic muscle- fibres. When the myotome on one side of a segment contracts, the septa bounding that segment come closer together, and the body becomes concave on that side. The advantage of having several segments made it possible for the body to bend in several places. By bending alternately right and left in successive regions of the body, and making the bends pass down the length of the body by throwing the myotomes into contraction in succession, the undulatory movements are produced which enable the organism to swim. These movements were made still more efficacious by lengthening the body, which was accomplished by the development of an extension behind the anus forming the tail. After bending in any place, the body became straight (before bending in the opposite direction), and this was effected not only by the relaxation of the myotome on that side, and the contraction of the myotome on the opposite side, but by the possession of a stiff yet elastic rod running along the whole length of the animal : the notochord. This is the typical primitive method of chordate locomotion which persists not only in the fish, but also in the lowest land-vertebrates.
The fact that the animal moved in a definite direction had the consequence that the front end was further specialised by a concentration of sense-organs, which ultimately was to bring about the formation of a head.
It has been held, with some degree of probability, that the habit of swimming in a definite direction was evolved in response to the constant direction of flow of water in rivers or large estuaries, and that the evolution of the early true chordates took place in such surroundings.
The ciliary method of feeding which these animals possessed limited the size of the particles of food which they could ingest, and the size to which they could grow.
The earliest chordates of which fossil remains are known are the Ostracoderms (from the upper Silurian and Devonian), which recent work has shown to be related to the Cyclostomes, especially as regards the brain, auditory organs, and bloodvessels. This raises some interesting problems, because the Ostracoderms possessed denticles, bone, and paired fins, all of which structures are lacking in Cyclostomes. The mode of life of Petromyzon, Bdellostoma, and Myxine is undoubtedly degenerate with their sucking mouth, but they would be more degenerate than otherwise expected if they had lost the structures possessed by the Ostracoderms. The curious fossil Palasospondylus (from the Devonian) may be related to these forms, in all of which evolution had proceeded far enough for the formation of a definite head.
The first true fish appear to have been cartilaginous (together, the cartilaginous fish are called Chondrichthyes) and related to the Selachii. Among them may be mentioned Acanthodes (upper Silurian), Cladoselache (Devonian) interesting for the structure of its paired fins, and Pleuracanthus (Permian). All these forms had true biting jaws, two pairs of paired fins, and heterocercal tails. True Selachii related to Heterodontus (the Port Jackson Shark) appeared in the Carboniferous. At the present day, the Selachii are represented by the true sharks (and dogfish), and by the rays (Raia, Torpedo), which have become adapted to living on the sea bottom and have become flattened in consequence. Their pectoral fins have expanded and fused with the sides of the body. The gill-slits are on the under surface, the spiracle is above. One member of the rays, Pristis the saw-fish, has returned to an active mode of life. The angel-fish Rhina is intermediate in form between the sharks and the rays.
Another group of cartilaginous fishes diverged in the Devonian and gave rise to the Holocephali, represented at the present day by Chimaera.
The bony fish or Osteichthyes appear in the Devonian, and the lungs which they possess were probably in connexion with the poor oxygen- content of the fresh water in which they lived. The Dipnoi were represented by Dipterus (Devonian), and the non-Dipnoan bony fish, or Teleostomi were represented by Osteolepis (also Devonian). These two forms were closely related, and they had the following characters in common : blunt lobate fins, a pair of external and a pair of internal nostrils, the general arrangement of the bones of the roof of the skull, heterocercal tails, and, most important of all, cosmoid scales. There is no doubt that they had a common ancestor, perhaps in the Silurian, and from a close relative of this ancestor the Tetrapods arose. One of the Osteolepidoti, vSauripterus, had fins from which the structure of the penta- dactyl limb of the Tetrapod might be derived. In the Ccelacanths, which are Teleostomes related to the Osteolepidoti, there is definite evidence of the presence of a lung, for it was calcified and fossilised. The Dipnoi evolved into the forms living at the present day, and became more and more adapted to life in rivers which are liable to dry up. The wide and discontinuous distribution of Ceratodus (Australia), Lepidosiren (South America) and Protopterus (Africa) to-day is evidence of the antiquity of the group. The evolution of these forms went on parallel to that of the early Tetrapods, and independently from them.
On the Teleostome side, another group arose in the Devonian from some relatives of the Osteolepidoti : the Palaeoniscoidea. These fish are characterised by the possession of scales of the type called palseoniscoid . Cheirolepis resembled Osteolepis in the structure of its skull, but its eyes were larger and the heterocercal tail was more accentuated. This provision for more active swimming was probably connected with the improvement of the eyes as sense-organs. Polyp terus, alive to-day, may be regarded as a descendant of the Palaeoniscoids. It has palaeoniscoid scales, and preserves the open spiracle. It inhabits certain rivers in Africa. On the other hand, the Palaeoniscoids also gave rise to the sturgeons. Chondrosteus (Jurassic) is already like the sturgeon Acipenser. These animals preserve the open spiracle, but the palaeoniscoid structure of the scales is lost. Sturgeons are both fluviatile and marine.
Another line of evolution from the Palaeoniscoids leads to the higher bony fish or Holostei. These fish lose the open spiracle and their tails assume the homocercal pattern. At the same time the radials of the paired fins become reduced, and the web of the fin is supported mostly by the dermal fin-rays or lepidotrichia. In the median dorsal and ventral fins the lepidotrichia correspond in pairs to the radials, so that the fins can be lowered and raised. Of the Holostei, two groups are primitive. One of these contains Amia, an inhabitant of the rivers of North America. Its lung is still highly vascular and supplied by pulmonary arteries, and in the region of the tail its vertebral column consists of separate hypo- and pleurocentra. The other group contains Lepidosteus, likewise an inhabitant of North American rivers. Its scales are of the peculiar pattern known as lepidosteoid, with a covering of ganoin. The lung is vascular, but supplied by arteries from the dorsal aorta. It is worth noticing that the primitive Osteichthyes are almost exclusively inhabitants of fresh water.
In the remaining highest bony fish or Teleostei (a term not to be confused with Teleostomi), the scales lose the layer of ganoin and become thin and transparent. The lung becomes modified into a swim-bladder, and loses the vascular spongy walls characteristic of a lung. It functions as a hydrostatic organ of adaptation to different depths, and this illustrates the fact that the Teleostei are a group which has reinvaded the sea from fresh water. Rivers do not possess sufficient depth to necessitate a swim-bladder. A number of Teleosts, however, are inhabitants of fresh water, to which they have presumably returned from the sea. The Teleosts have radiated into a great many different lines, and are the most successful of the fish. They have become specialised to various modes of life, but they must be regarded as a sterile side branch on the tree of vertebrate evolution, for their specialisations have prevented them from evolving into anything further. Although some of them, such as Periophthalmus, are capable of coming out on dry land and hobbling about, they cannot compete with the true land- vertebrates, which are less specialised but more progressive descendants of their ancestors, the pre-Osteolepids. Of the adaptations which Teleostei have undergone, one of the most interesting is the modification in connexion with the habit of living on the sea-bottom, and which has resulted in the " flat fish." When hatched, these fish, of which Solea (the sole) is an example, are normal and symmetrical in form, but they undergo a metamorphosis as a result of which they lie on one side on the bottom. The head becomes twisted so that the eye of the " underside " (right or left, according to the species) moves on to the " upper side." It is interesting to compare this flattened condition of the body with that of the rays. The modifications in the two groups are totally different, but both are adaptations to one and the same mode of life, and this accounts for what similarity there is between them.
The so-called flying fishes, of which Exocoetus is an example, have enlarged pectoral fins, and are capable of prolonged leaps through the air rather than of true flight. Lastly, attention may be called to certain deep-sea fish (Edriolychnus) which are not only of a peculiar shape, but are remarkable in that the males are dwarfed and degenerate, and live attached to the females on which they are parasitic.
Literature
Delage, Y., et Herouard, E. Zoologie Concrete, 8, les Procordes. Schleicher Freres, Paris, 1898.
Regan, C. T. Dwarfed Males parasitic on the Females in Oceanic Angler- Fishes. Proceedings of the Royal Society, B, vol. 97, 1925.
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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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