Book - Vertebrate Zoology (1928) 5
Vertebrate Zoology G. R. De Beer (1928)
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Chapter V Gadus, A Chordate with Bone
Externals
The genus Gadus includes the cod, whiting, and haddock. In shape, Gadus differs from the dogfish in being relatively shorter and more compressed from side to side. Gadus belongs to the group of higher bony fish known as the Teleostei, and in these the tail is typically forked and outwardly symmetrical, a condition called homocercal. In Gadus the tail is also outwardly symmetrical, but the tail-fin differs from that of other Teleostei. It represents the hind ends of the dorsal and ventral median fins. It is therefore not a homocercal but a pseudocaudal fin (see p. 325).
There are three dorsal and two ventral median fins. Of the paired fins the pelvic pair is actually anterior to the pectoral pair in position.
The mouth is bounded by tooth-bearing jaws. On the upper side of the snout, slightly behind the mouth, are the nasal pits. Each of these is a cavity communicating to the exterior by two openings, but not in any way connected with the mouth.
The eyes are large. The gill-slits do not open separately to the exterior, but they are covered over by an operculum. The water which emerges from the gill-slits passes between the hind and lower edges of the operculum and the body. There is no open spiracle. The cloaca is shallowed out, so that the anus and the urino-genital apertures are separate ; the former in front of the latter.
Scales
Scales form one of the most obvious features of the fish ; they are arranged in W-shaped rows, overlapping from head to tail. Each row primitively corresponds to the underlying myotome, which is also W-shaped. It is important to notice that the scales are not external, but lie in the mesodermal tissue (from which they are formed) beneath the epidermis. The scales are thin, flat plates of material akin to bone. They are kept throughout life, and enlarge by concentric additions. These scales have nothing to do with the denticles or placoid " scales " of the dogfish.
Fin-rays
The fins are supported by fin-rays, but these, instead of being horny and unjointed like the ceratotrichia of the dogfish, are bony and jointed, and are called lepidotrichia. In the more highly-developed bony fish like Gadus, the lepidotrichia correspond in number to the radials of the axial skeleton, in the dorsal and ventral median fins. There is a lepidotrichium on each side of the tip of each radial, and a joint between them enables the web of the fin to be raised or lowered. At the edge of the fin, between the lepidotrichia, there are some small unjointed horny rays called actinotrichia. These correspond to the ceratotrichia of the dogfish.
Skeleton
The cartilaginous skeleton corresponding to that of the dogfish is present in early stages of development in the bony fish. In the adult, most of this cartilage is replaced by an altogether different skeletal material, viz. bone. Bones which arise in this manner, i.e. replacing pre-existent cartilage , are called cartilage-bones or replacing bones. Some bones, on the other hand, have no cartilaginous precursor at all. These arise independently, as more or less flat plates in relation to the surface of the body, though they may sink deeper. These are dermal or membrane-bones. There is no difference in structure between cartilage-bones and membrane-bones, the distinction applies only to the method of origin. Sometimes a bone which develops as a cartilage-bone in one animal may arise as a membrane-bone in another, and vice versa, though these cases are rare. It is to be noted that as a rule a cartilage- bone represents an ossification in a cartilaginous structure which exists in the dogfish, whereas a membrane-bone is a structure which is wholly unrepresented in the dogfish. There is no doubt that the scales, fin-rays, and bones are kindred structures.
Skull
As in other forms, the skull can be divided into the neurocranium or brain-case, and the splanchnocranium or jaws.
On the floor of the brain-case are the basioccipital, prevomer, and parasphenoid ; in front is the mesethmoid. The roof is formed by the paired nasals, frontals, parietals, and the supraoccipital. The foramen magnum, through which the spinal cord enters the skull, is bounded below by the basi- occipital, above by the supraoccipital, and on each side by the paired exoccipitals.
The auditory capsules are well ossified, and each contains five bones. The lower part of each capsule is made of a prootic in front and an opisthotic behind. Above these are the sphenotic, pterotic, and epiotic bones.
Fig. 35. — Gadus : view of a portion of the skull from the left side, after removal of the lachrymal and suborbital bones, in order to show the palato-pterygo-quadrate arch. {For lettering see p- 70.)
The sides of the brain-case are very incomplete. Anteriorly, there are the paired prefrontals, between the frontal and the parasphenoid on each side. Farther back the paired latero- sphenoids are situated beneath the edge of the frontal and in front of the prootic on each side. A large window is left open in the side of the brain-case, through which many nerves and blood-vessels pass from the skull to the space in which the eye is lodged, called the orbit. Quite at the side there is a string of little bones which bound the orbit below and behind. These bones touch the prefrontal anteriorly and the frontal behind. The most anterior of the string is the lachrymal, and the following ones are the suborbitals and postorbitals. Before leaving the neurocranium, mention must be made of the post-temporal, which touches the epiotic and pterotic behind. It will be noticed again in connexion with the shoulder girdle.
The splanchno- cranium consists of the bony supports of the visceral arches. In the branchial arches there are four elements on each side : pharyngo-, epi-, cerato-, and hypo- branchial. The pharyngo - branchials of the anterior branchial arches are fused together ; the skeleton of the posterior branchial arches is less well developed and ossified. The hypobranchials of the anterior three branchial arches articulate with a median and ventral basibranchial. The fused pharyngobranchials and the ventral elements of the last arch bear teeth.
The skeleton of the first two visceral arches (mandibular and hyoid) is somewhat modified. In the dogfish, the palatoptery goquadrate cartilage forms the margin to the upper jaw, but in the bony fish this is no longer the case. Here it is formed by the paired premaxilla and maxilla. Lying median to these are the paired palatine (which touches the brain-case), the three pterygoids (ecto-, endo-, and metapterygoid), and, farther back, the quadrate. These bones arise in relation to the palato-quadrate arch, and they therefore no longer form the upper boundary to the mouth, but lie at the sides of its roof. The quadrate articulates with a bone of the lower jaw called the articular, and which corresponds to Meckel's cartilage in the dogfish. Here again a new margin to the jaw is formed, by the dentary. The ventro-posterior part of the lower jaw is formed by the angular.
The suspension of the jaws is hyostylic, i.e. the quadrate is connected with the neurocranium by the hyoid arch. The hyoid-arch skeleton consists of an upper hyomandibula which articulates above with the auditory capsule, and is pierced by a foramen for the hyomandibular branch of the facial nerve. Beneath the hyomandibula is the symplectic to which the quadrate is attached. The ventral portion of the arch is made up of the epihyal, ceratohyal, and hypohyal. The epihyal is connected with the symplectic by the interhyal. Below and between the hypohyals is a median basihyal. The ceratohyal bears branchiostegal rays.
The operculum is a posterior extension of the hyoid arch, and it is supported by four bones which are fixed on to the hind edge of the hyomandibula and symplectic. These are the preopercular, opercular, sub-, and interopercular bones.
Pectoral Girdle
The primitive girdle corresponding to that of the dogfish is formed by a dorsal scapula and a ventral coracoid, on each side. Here they are ossified. With these the pectoral fin articulates by means of the radials. These radials are short and fused, and the web of the fin is supported by the lepidotrichia. Another series of bones is plastered on to this primitive girdle from in front. These bones are the cleithrum, post-cleithrum and supra-cleithrum. The supra-cleithrum articulates with the post-temporal so that the pectoral girdle is connected with the skull. There is no clavicle, though this bone is present in more primitive bony fish.
Pelvic Girdle
The pelvic girdle is in the form of a pair of Y-shaped bones lying in the body- wall. The anterior forks of the Y of the bones on each side are joined by a median cartilage. The pelvic fins articulate with the outer side of these bones. The radials in the pelvic fins are even more reduced than those in the pectorals.
Vertebral Column
The centra of the vertebral column are bony discs, concave on both sides, and the vertebrae articulate with one another by means of facets or zygapophyses. Dorsally, each centrum bears a pair of processes which join to form the neural arch. This arch is produced farther into the neural spine. The radial surmounts this and, in the regions of the dorsal fins, is articulated with the lepidotrichia. The radials scarcely project at all into the fins. The spinal nerves emerge between the neural arches.
In the trunk-region, each centrum bears a pair of ventro- lateral processes, to which the (" ventral," see p. 83) ribs are attached. In the region of the tail these processes are directed downwards, and join to form the haemal arches. These are prolonged into the haemal spines, which support the ventral lobe of the tail-fin. The neural arches correspond to basi- dorsals, and the ventro-lateral processes to basiventrals. The notochord is of course obliterated by the centra.
Before leaving the skeleton, it remains to sort out the various bones into cartilage-bones and membrane-bones, and those whose constituents arise in both ways and which may therefore be called mixed bones.
Cartilage-bones.
Membrane-bones.
Mixed bones,
Neuro-cranium.
Basioccipital
Nasal
Sphenotic
Exoccipital
Frontal
Pterotic
Supraoccipital
Parietal
Prefrontal
Prootic
Parasphenoid
Opisthotic
Prevomer
Cartilage-bones. j
Membrane-bones. Mixed bones
Neuro-cranium.
Epiotic (a tendon-bone)
Lachrymal
Mesethmoid
Suborbitals
Laterosphenoid
Postorbitals
Basisphenoid : not always present
Splanchno-cranium .
Quadrate
Endopterygoid Palatine
Metapterygoid
Ectopterygoid
Articular
Premaxilla
Hyomandibula
Maxilla
Symplectic
Dentary
Epihyal
Angular
Ceratohyal
Opercular
Hypohyal
Preopercular
Interhyal
Subopercular
Basihyal
Interopercular
Pharyngobranchial
Branchiostegal rays
Epibranchial
Ceratobranchial
Hypobranchial
Basibranchial
Vertebral Column.
Centra
Ra dials
Appendicular Skeleton.
Scapula
Cleithrurh
Coracoid
(Clavicle)
Pelvic
Post- clei thrum
Radials
Supra-cleithrum
Post-temporal
Dermal Skeleton.
Lepidotrichia
Scales
It will be obvious from this table that the majority if not all the cartilage-bones are ossifications in cartilage which itself is represented in the dogfish. On the other hand, there are no structures of any kind in the dogfish which have any connexion with the membrane-bones. The membrane-bones are of interest from two points of view. A number of them bear teeth : premaxilla, maxilla (not in Gadus, however), prevomer, dentary. Others enter into relations with the lateral- line canal system, and these relations are of importance, for owing to their constancy they enable homologies to be made between bones in fish and in higher Vertebrates. The lateral- line canal of the trunk runs forwards from the tail, and in so doing it pierces the scales. On reaching the head it is protected by a few " lateral-line ossicles," and then passes through the post-temporal to the pterotic and sphenotic. The supra- orbital canal runs forwards over the eye through the frontal and nasal, the infraorbital canal pierces the chain of bones formed by the post-orbitals, infraorbitals, and lachrymal. The hyomandibular canal runs down through the preopercular to the dentary.
Teeth
The teeth are fundamentally similar to the denticles or placoid scales of the dogfish, but instead of being scattered all over the surface of the body, they are restricted to the mouth. They are composed of a core of dentine containing a pulp cavity, and are covered over with a cap of enamel. The bones which bear teeth have been enumerated above.
Nervous System
The brain and spinal cord lie in the long tubular cavity provided by the skull and neural arches of the vertebrae. The spinal cord is essentially similar to that of the dogfish, and calls for no special description. The spinal nerves, each composed of a dorsal and a ventral root, emerge between the neural arches.
- In the brain, the medulla oblongata is not very different from the spinal cord. The cerebellum is well developed and projects downwards and forwards beneath the roof of the midbrain forming the valvula, a structure which is peculiar to bony fish. The roof of the midbrain is produced into optic lobes. The floor of the forebrain projects downwards as the infundibulum, and is attached to the pituitary body and the saccus vasculosus. The latter structure, which is of doubtful significance, is peculiar to fish. It is a region of the brain- floor where the wall is thin, thrown into folds, and very richly supplied with blood-vessels. It has been supposed that its function is to secrete the cerebro-spinal fluid which fills the cavity of the brain and spinal cord, or to estimate the pressure of this fluid. The olfactory lobes are peculiar in that they are situated far forwards, close behind the nasal pits. They are connected with the rest of the brain by long olfactory tracts.
The olfactory nerves are short, which fact is correlated with the length of the olfactory tracts. The optic nerves have no chiasma. The three eye-muscle nerves, oculomotor, trochlear, and abducens, are similar to those of the dogfish, and call for no special comment. The profundus is reduced, the trigeminal has the usual maxillary and mandibular branches.
The facial nerve has ophthalmic, buccal, and hyomandibular branches, innervating respectively the supraorbital, infra- orbital, and hyomandibular lateral-line canals. In addition, the facial nerve has a cutaneous branch which runs upwards and backwards, and divides into three nerves which can be seen immediately underneath the skin. One of these runs along the base of the median dorsal fins ; another runs obliquely down across the side of the body to the median ventral or anal fin ; the last branch runs to the pectoral and pelvic fins.
The auditory nerve calls for no comment. The relations of the glossopharyngeal and vagus nerves to the gill-slits is the same as in the dogfish. The vagus supplies the heart and viscera, and also the lateral line of the trunk. This nerve supplying the lateral line branches, one portion remaining close to the lateral-line canal, and the other runs a little below, at the level of the septum which divides the myotomes into dorsal and ventral portions.
Sense-organs. — The nose is represented by paired nasal sacs on the upper side of the snout, each with two openings, and without connexion with the mouth. The eye is similar in structure to that of the dogfish ; but there is in addition a vascular process extending into the cavity of the eyeball known as the campanula Halleri, which is attached to the retractor lentis muscle.
In the ears, the ductus endolymphaticus no longer maintains its persistent opening to the outside. In the saccule there are two large calcareous concretions or otoliths ; otherwise the structure of the organ is similar to that of the dogfish.
The lateral-line canals have already been mentioned, and their course and innervation described.
Alimentary Canal
The mouth leads into the pharynx, out of which the five pairs of gill-slits open. There is no open spiracle. The gill-arches between the slits are smaller than those of the dogfish, and do not form a broad septum as in that fish. This is correlated with the fact that they are covered over by the operculum. The gills are supported by two rows of branchial rays on each arch.
Behind the pharynx, the oesophagus leads to the stomach which bears a number of blind tubes, the pyloric coeca. The liver has a gall-bladder from which the bile-duct runs to the intestine. The latter receives the pancreatic duct from the pancreas, makes a loop forwards and back again and runs to the rectum, which opens at the anus.
The swim-bladder is to be regarded as a derivative of the alimentary canal, and in many forms it retains its connexion with it by an open duct. This connexion has, however, been lost in Gadus, and the swim-bladder is a closed sac which occupies the dorsal portion of the coelomic cavity, close up against the under side of the vertebral column. Its ventral wall is thick and is covered with the coelomic epithelium ; its dorsal wall is very thin. Inside the bladder is a rete mirabile, a concentration of small blood-vessels forming a gland which secretes oxygen into the bladder. This " red " gland, as it is called, is supplied with blood by the mesenteric artery, like the other viscera. The function of. the bladder is hydrostatic, for by varying the amount of gas which it contains (by passage of gas from the blood to the bladder or vice versa) the fish can accommodate itself to any given depth of water and maintain itself there without muscular exertion. As will be seen later, it probably corresponds to the lung of the air-breathing vertebrates.
Excretory System
A pair of mesonephric kidneys extend longitudinally, dorsal to the swim-bladder and below the vertebral column. At their posterior ends they join, and the single median excretory duct runs ventrally, behind the swim- bladder to a urinary sinus. This opens to the exterior at the urinary aperture.
Genital System
Gadus (and the higher bony fish) differs from the dogfish in that the urinary and genital systems are not intimately connected. The testes are elongated structures suspended in the coelomic cavity on each side of the gut. They do not connect with the kidney, but join one another posteriorly, and send a single duct to open at the genital aperture. In the female, the ovaries correspond in position to the testes. The remarkable thing is that the ovaries are enclosed in sacs which lead by a single duct to the genital aperture. Here, therefore, the eggs are never shed free into the ccelom, to enter the open mouths of oviducts.
Vascular System. — The heart consists of sinus venosus, single auricle, single ventricle, and bulbus arteriosus. It is to be noticed that the muscular conus which was present in the dogfish has disappeared, and has only left its valves as a vestige. The ventral aorta gives off four pairs of afferent branchial arteries, one ascending each of the first four branchial arches. From these arches the blood is collected up into the efferent branchial arteries which run to the lateral dorsal aorta of their side. The lateral dorsal aortae are joined together behind the gill-region to form the single dorsal aorta, and they also join in front of the gills, so that a ring is formed called the circulus cephalicus. Anteriorly the internal carotids run to the head ; behind, the dorsal aorta gives off the subclavian arteries to the pectoral fins, the cceliac and mesenteric arteries to the viscera, and continues backwards between the kidneys to the tail.
The venous system does not differ in essentials from that of the dogfish.
Gadus is a type of one of the most successful group of marine animals. It shows certain important advances over the condition of the dogfish, but when compared with higher forms most of its characters are seen to be specialised and secondary.
Characters of Gadus which show an advance over the conditions in Scyllium :
Bone ;
New marginal skeleton to the jaws ;
New elements added to the pectoral girdle ;
Swim-bladder.
Characters of Gadus which are secondary when compared with other forms :
Continuity of the gonads with their ducts ; Loss of optic chiasma ; Loss of clavicle ; Loss of conus arteriosus ;
Swim-bladder closed, and adapted to hydrostatic function.
Literature
Goodrich, E. S. Vertebrata Craniata : Cyclostomes and Fishes. Black, London, 1909.
Parker, T.J. A Course of Instruction in Zootomy (Vertebrata) . Macmillan , London, 1884.
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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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