Book - Vertebrate Zoology (1928) 6

From Embryology

Chapter VI Ceratodus, A Chordate with a Lung

Vertebrate Zoology G. R. De Beer (1928)

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Externals

Ceratodus is the Australian lung-fish, a group of great importance, whose only other living representatives are Protopterus in Africa and Lepidosiren in South America. It is not an uncommon thing for ancient and primitive groups of animals to have gone extinct everywhere except for definite, small, and isolated regions of the earth. These animals are therefore an example of discontinuous geographical distribution. In shape, Ceratodus is typically a fish. The median fins are remarkable in that the dorsal, caudal, and ventral fins are all continuous with one another. Further, the tail-fin is symmetrical and pointed, and resembles the primitive diphy cereal type, such as is found in Cyclostomes. There is, however, a certain amount of doubt as to whether the tail-fin of Ceratodus is primitive. When a tail such as this is derived secondarily by simplification from another type (as is the case in the eel, for example) it is called gephyrocercal.


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Fig. 39. — Ceratodus : view from the left side. (Partly after Goodrich.) e, eye ; pc and pi, pectoral and pelvic " archipterygial " fins ; tf, tail- fin (diphy cereal ?).



The paired fins are elongated and leaf- like. They have a central axis bearing radials both in front and behind, a condition known as biserial, and they conform to the type known as " archipterygial." Both the paired fins and the dorsal fin are covered with scales.


The gill-slits, five in number, are protected by an operculum. The spiracle is closed. The anterior nostrils are on the ventral surface of the snout, and behind them are the posterior nostrils which open into the mouth. There is a cloaca into which the alimentary, excretory, and genital systems open, as also do a pair of abdominal pores.


Scales

The scales are thin and covered with spines which must not be mistaken for denticles. They overlap one another from before backwards as in Gadus, and they also extend over the dermal bones of the skull and the paired fins, and the dorsal fin.


Fin-rays

The dermal fin-rays are jointed and made of fibrous substance. They differ from the lepidotrichia of Gadus in being more numerous than the radials, and in being covered over by scales. No actinotrichia are present, and it is uncertain whether these rays, which are called camptotrichia, represent the ceratotrichia or the lepidotrichia of Gadus.


Skull. — The skeleton is largely cartilaginous, and little of this primitive skeleton is replaced by bone. The neurocranium forms a complete case enclosing the brain, olfactory and auditory capsules, and several vertebrae are plastered on to its hind end. As some of these bear ribs, the latter appear to articulate with the skull, and are called cranial ribs. The only cartilage-bone in the neurocranium represents one of the neural arches which have been incorporated as just described. The membrane-bones which cover the dorsal surface of the neurocranium are sunk beneath the surface of the skin and are themselves overlain by scales. These bones are very modified and secondary, there is a preponderance of median unpaired bones, and as they cannot well be compared with those of other forms, there is little advantage in studying them in detail. On the underside of the neurocranium are to be found a parasphenoid, prevomer, and paired ptery go-palatines.


The splanchnocranium is important because of the manner in which the upper jaw is fastened on to the skull. The quadrate is directly attached to the neurocranium by cartilaginous processes : a basal process and an otic process. There is also an ascending process. This method of suspension of the jaws is called autostylic ; the hyomandibula plays no part in it. The relations of the basal, otic and ascending processes to the neighbouring nerves, veins, and arteries are important, and most of them will be found to be identical in all the remaining groups of vertebrates. The ascending process lies between and separates the profundus from the maxillary branch of the trigeminal {i.e. it is situated between Y 1 and V 2 ) ; the palatine nerve runs down behind and forwards beneath the basal process ; while the facial nerve (hyomandibular branch) and jugular vein pass on the inner and under side of the otic process.


The premaxilla and maxilla have disappeared, and consequently there are no teeth round the edge of the jaws. In the lower jaw the dentary is very much reduced. Teeth are carried on the prevomer, pterygo-palatine, and splenial (membrane-bone). An angular is present in the lower jaw.


In the hyoid arch there are a very small hyomandibula, and well- developed ceratohyal, hypohyal, and basihyal. The skeleton of the branchial arches also is not very well developed, and the arches do not carry any branchial rays.


The operculum is supported by opercular and subopercular bones.


Vertebral Column

The vertebral column, which is continuous in front with the hind end of the skull, is made up of paired basidorsal and basiventral cartilaginous elements, which do not interrupt the notochord. The basidorsals rise up into bony neural arches and neural spines which are attached to the jointed radials supporting the median fin. The basiventrals in the hinder region form haemal arches carrying haemal spines and radials supporting the ventral median fin. Farther forwards the basiventrals are produced into ribs. These do not extend into the horizontal septum between the dorsal and ventral portions of the myotomes, like the true or " dorsal " ribs of Scyllium. Instead, they bend down and lie just outside the outer lining of the coelom. From their position they are known as " ventral " or pleural ribs.


Limbs and Girdles

The primitive pectoral girdle is cartilaginous and composed of paired dorsal scapular regions, and ventral coracoid regions, which latter are joined to one another in the midventral line. Overlying this are the membrane-bones, clavicle, clei thrum, and the post- temporal which connects the girdle with the hinder part of the skull.


The pelvic girdle is formed of a median Y-shaped cartilage with the prongs directed backwards and articulating with the pelvic fins.


The fins are covered with scales. Their endoskeleton is cartilaginous and composed of a long central axis of about twenty pieces, tapering away to the tip. On each side of this axis are radials (pre- and postaxial). Beneath the scales are the camptotrichia.


Teeth

The plates of teeth, which are firmly attached to the prevomer, pterygo-palatines, and splenials, are the result of fusion of separate teeth.


Fig. 40. — Ceratodus : skeleton of the pectoral fin, showing the " archipterygial " structure, with an axis (a), bearing preaxial (pr) and postaxial (po) radials.


Alimentary Canal

In its main lines the alimentary canal does not differ much from that of Scyllium, with a spiral valve in the intestine. Its most interesting and important feature is that in the floor of the oesophagus there is an opening (the glottis) leading to a tube or trachea which passes up round the right side of the gut to the lung. This is a large sac with highly vascular walls surrounding a cavity which is subdivided into little chambers or " cells." In the lung the blood can be oxygenated when the water in which the animal lives becomes polluted, and the animal rises to the surface to take in air through the nostrils. It is this capacity of breathing by means of lungs and gills which is responsible for the name of the group Dipnoi, to which Ceratodus belongs. The lung is homologous with the swim-bladder of Gadus.


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Fig. 41. — Ceratodus : diagram of the relations of the lung, heart, and vascular system seen from the ventral side (combined from diagrams after Baldwin Spencer, simplified). aa, anterior abdominal vein ; ab, afferent branchial artery ; b, brachial vein ; ea, efferent branchial artery ; ej, external jugular vein ; g, glottis ; ijt internal jugular vein ; /, lung ; la, left auricle ; //, left lateral abdominal vein ; Ipa, left pulmonary artery ; Ipc, left posterior cardinal vein ; o, oesophagus ; p, pharynx ; pv, pulmonary vein ; rl, right lateral abdominal vein ; rpa, right pulmonary artery ; rpc, right posterior cardinal vein (remnant) ; rvcs, right vena cava superior ; t, truncus arteriosus ; vci, vena cava inferior.


Vascular System

Blood is supplied to the lung by branches of the last (6th) efferent branchial artery, which can now be called pulmonary arteries. The right pulmonary artery runs direct to the lung, but the left passes down under the gut and up again on the right side parallel with the windpipe or trachea. This shows that the primitive position of the lung was ventral, and that it moved up the right side, whither it is followed by the left pulmonary artery. Blood leaves the lung by the pulmonary veins, which unite to form one vein. This vein also passes down on the right side of the gut and goes right through the sinus venosus to open into the left side of the auricle. The auricle itself is partially divided into two by a septum, so that the blood (oxygenated) from the lung comes in on the left, and that from the rest of the body (deoxygenated) enters on the right from the sinus venosus. The ventricle is single, but the conus arteriosus is nearly divided into two by enlarged valves.


The ventral aorta leads forwards from the conus and gives off four pairs of afferent branchial arteries. The two collecting vessels in each gill-arch join to form the efferent branchial arteries which combine to form the dorsal aorta. The arrangement of the valves in the conus and truncus arteriosus is such that the blood from the sinus venosus tends to go into the posterior branchial arches (and so to the pulmonary arteries), while that from the pulmonary vein gets into the anterior arches. There is therefore an attempt to separate the circulation of the freshly-oxygenated blood from that of the impure blood which should go and be oxygenated.


There are two important points to notice in the venous system. The posterior cardinal on the right side loses its connexion with the ductus Cuvieri. Instead, it has developed a new connexion with the sinus venosus, forming the inferior vena cava. The ductus Cuvieri can also be called the superior vena cava (right and left). The lateral abdominal veins unite in the midventral line and so give rise to an anterior abdominal vein, which runs into the right ductus Cuvieri close to its connexion with the sinus venosus.


The hindmost portions of the posterior cardinal veins bring blood from the posterior region of the body to the kidneys, and so form renal portal veins. Blood leaves the kidneys by the left posterior cardinal and the inferior vena cava.


Urino-genital System

The excretory system is similar to that of Scyllium. The mesonephric kidneys are elongated, and connect by means of Wolffian ducts with the cloaca. The testis is connected with the kidney by vasa efferentia, and the Wolffian duct functions as a vas deferens. In the female, the eggs from the ovary are shed freely into the ccelom, and enter the openings of the Mullerian ducts which lead them to the cloaca.


Nervous System

The most remarkable feature of the nervous system is the formation of cerebral hemispheres in the telencephalon (end-brain). They are hollow outgrowths from the diencephalon projecting forwards side by side. A transverse section in the region of the brain of Ceratodus therefore would show a pair of cavities, not a single cavity as in lower forms. The cavities of the cerebral hemispheres are the so-called first and second ventricles of the brain ; they communicate with the cavity of the rest of the forebrain (third ventricle) through the foramina of Monro. This is the first appearance in the vertebrate series of the organs which mean so much in the supremacy of man over other animals. In Ceratodus, the roof of the cerebral hemispheres is still membranous. The cerebellum is small.


As regards the sense-organs, the eyes and ears present no striking features. It must be remembered that the nasal sacs each have two openings. The lateral line is somewhat degenerate, and in some regions may consist of a groove instead of a canal.


Characters of Ceratodus which are lacking in other living fish, but present in Amphibia :

Respiratory lung ;

Pulmonary arteries and veins ;

Divided auricle, and conus arteriosus ;

Vena cava inferior ;

Anterior abdominal vein ;

Anterior and posterior nostrils : the latter within the mouth ; Autostylic suspension of jaws ; Ascending process ; Cerebral hemispheres.

Characters of Ceratodus which are primitive when compared with other fish :

Cloaca ;

Contractile conus ; Uninterrupted notochord ; Diphycercal tail ? Spiral valve in intestine.


Characters of Ceratodus which are secondary and specialised :

Loss of Maxilla and Premaxilla ;

Median membrane-bones over the skull ;

Lack of ossification in the cartilaginous neurocranium ;

Specialised tooth-plates ;

Fusion of vertebrae on to the back of the skull ;

Ventral ribs ;

Rotation of the lung to the dorsal position.


It will be obvious from these tables that Ceratodus and the Dipnoi generally are very remarkable animals. On the one hand, they have a surprising number of characters which no other fish possess, and which are typical of Amphibia ; yet, on the other hand, their relationship with the Amphibia cannot be very close, because of the large number of specialised characters which they show.


Literature

Goodrich, E. S. Vertebrata Craniata: Cyclostomes and Fishes. Black, London, 1909.

Huxley, T. H. Contributions to Morphology : Ichthyopsida. On Ceratodus forsteri. Proceedings of the Zoological Society of London, 1876.

Spencer, W. B. Ceratodus : The Blood-vessels. Macleay Memorial Volume, 1892.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
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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