Fig. 5. Skull of a maleChimaera monstrosa(afterHubrecht).1. nasal capsule.6. auditory capsule.2. cartilaginous appendage to7. interorbital septum.the fronto-nasal region.8. mandible articulating with3. erectile appendage.an outgrowth from the posterior4. foramen by which thepart of the palato-pterygo-quadrate.ophthalmic nerves leave the orbit.9. teeth.5. foramen by which the10. labial cartilage.ophthalmic branch of the Vth nerveII. III. V. VII. IX. X. foraminaenters the orbit.for the passage of cranial nerves.
Fig. 5. Skull of a maleChimaera monstrosa(afterHubrecht).1. nasal capsule.6. auditory capsule.2. cartilaginous appendage to7. interorbital septum.the fronto-nasal region.8. mandible articulating with3. erectile appendage.an outgrowth from the posterior4. foramen by which thepart of the palato-pterygo-quadrate.ophthalmic nerves leave the orbit.9. teeth.5. foramen by which the10. labial cartilage.ophthalmic branch of the Vth nerveII. III. V. VII. IX. X. foraminaenters the orbit.for the passage of cranial nerves.
These singular fish have the skin smooth and in living forms almost or quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are fused to the cranium, and Meckel's cartilage articulates directly with the part corresponding to the quadrate. The skull is distinctly articulated with the spinal column, the notochord is persistent and unconstricted, and the skeletogenous layer shows no trace of metameric segmentation,though in the neural arches this segmentation is readily traceable. The neural arches of the first few vertebrae are fused together and completely surround the notochord, while they do not in other parts of the body. The tail is diphycercal. Of the living genera, inCallorhynchusthere is no trace of calcification in the skeletogenous layer, while inChimaerarings of calcification are found, there being three to five for each vertebra as indicated by the foramina for the exit of the spinal nerves. The pelvic fins are produced into claspers. Besides the living generaChimaera,HarriottaandCallorhynchusa fair number of fossil forms are known, e.g.Ischyodus.
OrderIII.Ganoidei.
The fishes included under the term Ganoidei form a very heterogeneous group, some of which closely approach the Dipnoi, others the Elasmobranchii, others the Teleostei. The great majority of them are extinct, only eight living genera being known; these are all inhabitants of the northern hemisphere, and with the exception ofAcipenser, which is both fluviatile and marine, are entirely confined to fresh water.
The following is a list of the living genera of Ganoids with their respective habitats:—
Acipenser.Rivers and seas of the northern hemisphere.Scaphirhynchus.Mississippi and rivers of Central Asia.Polyodon(Spatularia). Mississippi.Psephurus.Yan-tse-kiang, and Hoangho.Polypterus.Rivers of tropical Africa.Calamoichthys.Some rivers of West Africa.Lepidosteus.Freshwaters of Central and North America and Cuba.Amia.Rivers of Carolina.
Acipenser.Rivers and seas of the northern hemisphere.
Scaphirhynchus.Mississippi and rivers of Central Asia.
Polyodon(Spatularia). Mississippi.
Psephurus.Yan-tse-kiang, and Hoangho.
Polypterus.Rivers of tropical Africa.
Calamoichthys.Some rivers of West Africa.
Lepidosteus.Freshwaters of Central and North America and Cuba.
Amia.Rivers of Carolina.
The exoskeleton is very variable, thus the body may be:—
(a) Naked or with minute stellate ossifications as in the Polyodontidae. (b) Partially covered with large detachedbony plates as inScaphirhynchusandAcipenser. (c) Entirely covered with rhomboidal ganoid scales as inLepidosteus,Polypterus,Palaeoniscusand many extinct forms. (d) Covered with rounded scales shaped like the cycloid scales of Teleosteans as inAmia. (e) Having the trunk and part of the tail covered with rhomboidal scales, and the remainder of the tail with rounded scales as inTrissolepis.
The teeth also are very variable. The endoskeleton shows every stage of transition from an almost entirely cartilaginous state as inAcipenserto a purely bony state as inLepidosteus. Sometimes, as inAcipenser, the notochord persists, and its sheath is unsegmented; sometimes, as inLepidosteus, there are fully formed vertebrae. The tail may be heterocercal, as inAcipenser, or diphycercal as inPolypterus. The cartilaginous cranium is always covered with external membrane bone to a greater or less extent, and the suspensorium is markedly hyostylic. The pectoral girdle is formed of two parts, one endoskeletal and cartilaginous, corresponding with the pectoral girdle of Elasmobranchs, and one exoskeletal and formed of membrane bones, corresponding with the clavicular bones of Teleosteans. The pelvic fins are always abdominal. The fins often, as inPolypterus, have spines (fulcra) attached to their anterior borders.
The order Ganoidei may be divided into three suborders:
(1)Chondrostei.Living generaAcipenser,Scaphirhynchus,PolyodonandPsephurus.(2)Crossopterygii.Living generaPolypterusandCalamoichthys.(3)Holostei.Living generaLepidosteusandAmia.
(1)Chondrostei.Living generaAcipenser,Scaphirhynchus,PolyodonandPsephurus.
(2)Crossopterygii.Living generaPolypterusandCalamoichthys.
(3)Holostei.Living generaLepidosteusandAmia.
Suborder(1).Chondrostei.
The skull is immovably fixed to the vertebral column. By far the greater part of the skeleton is cartilaginous. The notochord is persistent and unconstricted, its sheath is membranous, but cartilaginous neural and haemal arches aredeveloped. Intercalary pieces (interdorsalia) occur between the neural arches, and similar pieces (interventralia) between the haemal arches. The cranium is covered with membrane bone, and teeth are but slightly developed. The tail is heterocercal. Gill rays occur on the hyoid arch, and the gills are protected by a bony operculum attached to the hyomandibular. The skin (1) may be almost or quite naked, (2) may carry bony plates arranged in rows, or may be covered (3) with rhomboidal scales, or (4) partly with rhomboidal, partly with cycloidal scales.
Suborder(2).Crossopterygii.
The exoskeleton has the form of cycloidal or rhomboidal scales. The condition of the vertebral column differs in the different genera. Sometimes, as inPolypterus, there are well-developed ossified vertebrae; sometimes, as in many extinct forms, the notochord persists and is unconstricted. The tail may be diphycercal or heterocercal. The pectoral and sometimes the pelvic fins consist of an endoskeletal axis bearing a fringe of dermal rays.
Suborder(3).Holostei.
The exoskeleton has the form of cycloidal or rhomboidal scales. The notochord is constricted and its sheath is segmented and ossified, forming distinct vertebrae, which are generally biconcave, sometimes opisthocoelous (Lepidosteus). The cartilaginous cranium is largely replaced by bone, and in connection with it we find not only membrane bone, but cartilage bone, as the basi-occipital, exoccipitals, and pro-otic are ossified. The tail is heterocercal. The suspensorium resembles that of Teleosteans, consisting of a proximal ossification, the hyomandibular, which is movably articulated to the skull and a distal ossification, the symplectic. The two are separated by some unossified cartilage. The cartilaginous upper and lower jaws are to a great extent surrounded and replaced by a series of membrane bones.
OrderIV.Teleostei.
The exoskeleton is sometimes absent but generally consists of overlapping cycloid or ctenoid scales. Bony plates are sometimes present, as in the Siluridae, or the body may be encased in a complete armour of calcified plates, as inOstracion. Enamel is however never present, and the plates are entirely mesodermal. The skeleton is bony, but in the skull much cartilage generally remains. The vertebral centra are usually deeply biconcave, and the tail is of the masked heterocercal type distinguished ashomocercal. In the skull the occipital region is always completely ossified, while the sphenoidal region is generally less ossified. The skull has usually a very large number of membrane bones developed in connection with it. The teeth vary much in character in the different members of the order, but are as a rule numerous and pointed, and are ankylosed to the bone. The suspensorium is hyostylic and the jaws have much the same arrangement as in the Holostei. There are five pairs of branchial arches, of which all except the last bear gill rays. A series of dermal opercular bones is developed in connection with these arches. The pectoral girdle consists almost entirely of dermal clavicular bones. The pelvic girdle has disappeared, its place being taken by the enlarged and ossified dermal fin-rays of the pelvic fins.
The group includes the vast majority of living fish (see p. 33).
OrderV.Dipnoi.
The exoskeleton is of two types; dermal bones are largely developed in the head region, while the tail and posterior part of the body may be naked or may be covered with overlapping scales. The cranium remains chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with the cranium, and the suspensorium is autostylic. The gill clefts are feebly developed and open into a cavity covered by an operculum. The notochord is persistent and unconstricted, and the limbs are archipterygia. The pelvic fins are without claspers.
Suborder(1).Sirenoidei[33].
The head has well developed membrane bones. The trunk is covered with overlapping scales and bears no bony plates. Three pairs of teeth are present, two in the upper and one in the lower jaw, the two principal pairs of teeth are borne on the palato-pterygoids and splenials, while the third pair are found in the vomerine region. The tail is diphycercal in living forms. In the extinct Dipteridae it is heterocercal. The pectoral girdle includes both membrane and cartilage bones. The pelvic girdle consists of a single bilaterally symmetrical piece of cartilage.
This suborder is represented by the living generaCeratodus,ProtopterusandLepidosiren, and among extinct forms by the Dipteridae and others.
Suborder(2).Arthrodira.
Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown.
The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera areCoccosteusfrom the European Devonian andDinichthys, a large predatory form from the lower Carboniferous of Ohio.
Scyllium canicula.
I. EXOSKELETON.
The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales.
A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin.
II. ENDOSKELETON.
The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue.
The endoskeleton is divisible into anaxialportion consisting of the vertebral column, skull, and skeleton of the median fins, and anappendicularportion consisting of the skeleton of the paired fins and their girdles.
1.The Axial Skeleton.
A.The Vertebral Column and Ribs.
The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.
These vertebrae are developed round an unsegmented rod, thenotochord, which forms the axial support of the embryo. The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform. The vertebrae are divided into two groups, an anterior group of trunk vertebrae, and a posterior group of caudal or tail vertebrae.
A typical vertebra consists of a middle portion, thecentrum, a dorsal portion, thedorsalorneural arch, which surrounds the spinal cord, and a ventral portion, theventralorhaemal arch, which similarly encloses a space.
The tail vertebrae of the dogfish have this typical arrangement, the trunk vertebrae have the haemal arches modified.
Eachcentrumis a short cylinder of cartilage surrounding an hourglass-shaped cavity occupied by the notochord. Theneural archesare composed of three separate elements, thevertebral neural plates(basidorsalia),intervertebral neural plates(interdorsalia), andneural spines(supradorsalia).
Thevertebral neural platesare in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. Theintervertebral neural platesare polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates,for the exit of the dorsal or sensory roots of the spinal nerves.
Theneural spinesare small patches of cartilage filling up the gaps between the dorsal ends of the neural plates.
Thehaemal arches(basiventralia) differ much in the trunk and tail portions of the vertebral column. In the trunk portion the centra are flattened below, and the two halves of the haemal arch diverge from one another as bluntventri-lateral processesto which short cartilaginous rods, theribs, are attached. Further back at about vertebra 37, the two halves of the haemal arch project downwards and meet forming a complete arch. Further back still, towards the hind end of the tail, the haemal arches bear medianhaemal spines(ventrispinalia).
B.The Skull.
The skull of the dogfish remains cartilaginous throughout the life of the animal, and has consequently a far more simple structure than have the skulls of higher animals, in which complication has been produced by the development of bone.
The skull consists of the following parts:—
(1) a dorsal portion, thecranium, which lodges the brain, and to the sides of which the capsules of the auditory and olfactory sense organs are united. The cranium may be compared to an unsegmented continuation of the vertebral column;
(2) a number of ventral structures, disconnected or only loosely connected with the cranium. These together constitute thevisceral skeletonforming the jaws and supporting the gills.
(1)The Cranium.
TheCraniumis an oblong box, with a flattened floor and a more irregular roof. Its sides are expanded in front owingto the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits.
(a) On the dorsal surface of the cranium the following points should be noticed. First at the anterior end, the large thin-wallednasalorolfactory capsules(fig. 6, 1), each of which is drawn out into a narrow cartilaginous process.
The olfactory capsules have no ventral walls, and are separated from one another by theinternasal septum, which is drawn out into a third slender process. These three processes together constitute therostrum(fig. 6, 2).
Behind the olfactory capsules comes a large, nearly circular, hole, theanterior fontanelle, slightly behind which are the twoophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively formed by the prominentsupra-orbitalandsuborbital ridges. Behind are theauditory capsules(fig. 6, 8), each of which is marked by a pair of prominent ridges, converging towards the middle line to a pair of apertures. These apertures communicate with two canals, theaqueductus vestibuli, which lead into the internal ear. The two ridges lodge respectively theanterior and posterior vertical semicircular canalsof the ear.
(b) The principal structures to be noted in a side view of the cranium are contained in theorbitor eye-cavity. Near the base of the orbit at its anterior end is seen the smallorbitonasal foramen(fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is the largeophthalmic foramen(fig. 6, 5) so prominent in a dorsal view of the skull; through it the ophthalmic branches of the fifth and seventh nerves pass. Slightly further back near the ventral surface is the largeoptic foramen(fig. 6, II.) for the passage of the second nerve. Vertically above the optic foramen, near the dorsal surface, is the very smallforamen for the fourth nerve(fig. 6, IV.). Behind and a little above the opticforamen is another small aperture, theforamen for the third nerve. Behind and slightly below this is the largeforamen for the sixth and main branches of the fifth and seventh nerves(fig. 6, V.). In front of and slightly below this foramen are seen two other small apertures; the more anterior and ventral of these (fig. 6, 4) is for the passage of a vessel connecting the efferent artery of the hyoid gill with the internal carotid artery inside the skull, the more posterior and dorsal is for theinterorbital canal(fig. 6, 3) which unites the two orbital sinuses. Above and very slightly in front of the large foramen for the sixth and main parts of the fifth and seventh nerves, are two small foramina (fig. 6, Va., and VIIa.), through which theophthalmic branches of the fifth and seventh nervesenter the orbit. Behind and slightly below the large foramen just mentioned is a small hole through which the external carotid enters the orbit (fig. 6, 9).
Fig. 6. Lateral view of the skull of a Dogfish(Scylliumcanicula) × 2/3.1. nasal capsule.10. ethmo-palatine ligament.2. rostrum.11. palato-pterygo-quadrate bar.3. interorbital canal.12. Meckel's cartilage.4. foramen for hyoidean artery.13. hyomandibular.5. foramen for the exit of the14. cerato-hyal.ophthalmic branches of15. pharyngo-branchial.Vth and VIIth nerves.16. epi-branchial.6. foramen through which the17. cerato-branchial.external carotid leaves the18. gill filaments, nearly all haveorbit.been cut off short for the7. orbitonasal foramen.sake of clearness.8. auditory capsule.19. extra-branchial9. foramen through which the20. pre-spiracular ligament.external carotid enters theII. III. IV. V. Va. VIIa. foraminaorbit.for passage of cranial nerves.
Fig. 6. Lateral view of the skull of a Dogfish(Scylliumcanicula) × 2/3.1. nasal capsule.10. ethmo-palatine ligament.2. rostrum.11. palato-pterygo-quadrate bar.3. interorbital canal.12. Meckel's cartilage.4. foramen for hyoidean artery.13. hyomandibular.5. foramen for the exit of the14. cerato-hyal.ophthalmic branches of15. pharyngo-branchial.Vth and VIIth nerves.16. epi-branchial.6. foramen through which the17. cerato-branchial.external carotid leaves the18. gill filaments, nearly all haveorbit.been cut off short for the7. orbitonasal foramen.sake of clearness.8. auditory capsule.19. extra-branchial9. foramen through which the20. pre-spiracular ligament.external carotid enters theII. III. IV. V. Va. VIIa. foraminaorbit.for passage of cranial nerves.
Behind the orbit is theauditory capsule. This is marked below by a prominentsurface for the articulation of the hyomandibular, above which is the deeppostorbital groovefor the passage of a blood-vessel, connecting the orbital and anterior cardinal sinuses.
(c) Passing to the posterior end of the cranium: in the centre is seen the largeforamen magnumthrough which the brain and spinal cord communicate. Thenotochordenters the skull just below this foramen, and on each side of the notochord is a projection, theoccipital condyle, by which the first vertebra articulates with the skull.
External to the condyles are the prominentpneumogastric foraminafor the passage of the tenth nerves, and further to the sides, just beyond the posterior vertical semicircular canals, are a pair of deep pits in which lie theforamina for the ninth nerves(fig. 6, IX).
(d) The broad and flat ventral surface of the cranium is continued in front as theinternasal septumand terminated laterally by thesuborbital ridges. At a little behind the middle it is traversed by two shallow grooves along which the internal carotid arteries run. At the divergent ends of these grooves are seen two small apertures through which the external carotids enter the orbit (fig. 6, 9), and at the point where they meet is a single small aperture through which the internal carotid enters the cranium.
(2)The Visceral Skeleton.
TheVisceral skeletonforms a series of seven cartilaginous arches or hoops, surrounding the anterior part of the alimentary canal, and enclosing a wide but rather shallow space.
(a) The first ormandibular archis the largest of the series, and forms the upper and lower jaws. Each half of the upper jaw orpalato-pterygo-quadratebar is formed by a thick cartilaginous rod which meets its fellow in the middle line in front, the two being united by ligament. Each half is connected to the cranium just in front of the orbit by theethmo-palatine ligament(fig. 6, 10), and at its hind end articulates with one of the halves of the lower jaw. Each half of the lower jaw orMeckel's cartilage(fig. 6, 12) is a cartilaginous bar, wide behind but narrow in front, where it is united to its fellow by a median ligament. Imbedded in the tissue external to the upper jaw are a pair oflabial cartilages, and a similar but smaller pair are imbedded in the tissue external to the lower jaw.
The jaws are developed from a structure whose dorsal and ventral portions subsequently become of very different importance. The ventral portion forms both upper and lower jaws, the former being developed as an outgrowth from the latter. The dorsal portion forms only thepre-spiracular ligament(fig. 6, 20), a strong fibrous band containing a nodule of cartilage, and running from the anterior part of the auditory capsule to the point where the jaws are connected with the hyomandibular.
(b) Thehyoid archconsists of a pair of cartilaginous rods which are attached at their dorsal ends to the cranium, and are united ventrally by a broad median plate of cartilage, thebasi-hyal. Each rod is divided into a dorsal portion, thehyomandibularand a ventral portion, thecerato-hyal.Thehyomandibular(fig. 6, 13) is a short stout rod of cartilage projecting outwards, and somewhat backwards and downwards from the cranium, with which it articulates behind the orbit and below the postorbital groove. Its distal end articulates with a rather long slender bar, thecerato-hyal(fig. 6, 14), which is in its turn attached to the side of thebasi-hyal. Thebasi-hyalis a broad plate, rounded in front and drawn out behind into two processes to which the two halves of the first branchial arch are attached. The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, thegill rays. The hyoid arch forms the mainsuspensoriumor means by which the jaws are attached to the cranium. This attachment is chiefly brought about by a series of short ligaments which connect the posterior ends of both upper and lower jaws with the hyomandibular, but there is also a ligament connecting the lower jaw with the cerato-hyal. The attachment of the jaws to the cranium is also partially effected by the pre-spiracular and ethmo-palatine ligaments.
(c) Each of the fivebranchial archesis a hoop, incomplete above and formed of four or more pieces of cartilage. The most dorsal elements, thepharyngo-branchials, are flattened, pointed plates whose free inner ends run obliquely backwards, and terminate below the vertebral column. They are connected at their outer ends with the short broadepi-branchials(fig. 6, 16) which lie at the sides of the pharynx. From the epi-branchials arise the longcerato-branchials(fig. 6, 17) which run forwards and inwards along the ventral wall of the pharynx. The first four cerato-branchials are connected with small rods, thehypo-branchials, which run backwards to meet one another in the middle line. The last two pairs of hypo-branchials and the fifth cerato-branchials are connected with a broad median plate, thebasibranchial. Along the outer sides of the second, third and fourth cerato-branchials are found elongated curved rods, theextra-branchials(fig. 6, 19). The epi-branchials and cerato-branchials bear gill rays along their posterior borders.
C.The Skeleton of the Median Fins.
Thedorsal finshave a skeleton consisting of a series of short cartilaginous rods, thebasalsor basalia, which slope obliquely backwards. Their bases are imbedded in the muscles of the back, while their free ends bear a number of small polygonal cartilaginous plates, theradialsor radiale. Associated with this cartilaginous skeleton are a number of long slender horny fibres, the fin-rays, which have been already referred to in connection with the exoskeleton. The skeleton of the other median fins mainly consists of these fibres, the cartilaginous portion being reduced or absent.
2.The Appendicular Skeleton.
This includes the skeleton of the two pairs of limbs and of their respective girdles.
The Pectoral girdleforms a crescent-shaped hoop of cartilage, incomplete above and lying just behind the visceral skeleton. The mid-ventral part of the hoop is the thinnest portion, and is drawn out in front into a short rounded process which is cupped dorsally and supports part of the floor of the pericardium (fig. 7, 1). On each side of this flattened mid-ventral portion the arch becomes very thick and bears on its outer border a surface with which the three basal cartilages of the fin articulate. The dorsal ends or scapular portions of the girdle form a pair of gradually tapering horns.
The Pectoral finarticulates with the pectoral girdle by means of three basalia or basal cartilages, thepropterygium,meso-pterygiumandmeta-pterygium. The most anterior and the smallest of these is thepropterygium(fig. 7, 5),while the most posterior one, themeta-pterygium(fig. 7, 3), is much the largest. Along the outer borders of the three basalia are arranged a series of close set cartilaginous pieces, theradiale. The propterygium supports only a single radial, which is however much larger than any of the others. The meso-pterygium also supports only a single radial which divides distally.
Fig. 7. Semidorsal view of the pectoral girdle and finsof a Dogfish(Scyllium canicula) × 2/3.The gaps between the radiale are blackened.1. hollow in the mid-ventral part5. propterygium.of the pectoral girdle which6. propterygial radial.supports the pericardium.7. meso-pterygial radial.2. dorsal (scapular portion) of8. meta-pterygial radial.pectoral girdle.9. outline of the distal part of3. meta-pterygium.the fin which is supported4. meso-pterygium.by horny fin-rays.
Fig. 7. Semidorsal view of the pectoral girdle and finsof a Dogfish(Scyllium canicula) × 2/3.The gaps between the radiale are blackened.1. hollow in the mid-ventral part5. propterygium.of the pectoral girdle which6. propterygial radial.supports the pericardium.7. meso-pterygial radial.2. dorsal (scapular portion) of8. meta-pterygial radial.pectoral girdle.9. outline of the distal part of3. meta-pterygium.the fin which is supported4. meso-pterygium.by horny fin-rays.
The meta-pterygium bears about twelve long narrow radials, the first nine of which are traversed by a transverse joint at about two-thirds of the way from their origin. Succeeding the radials are a series of small polygonal pieces of cartilage arranged in one or more rows and attached to the ends of the radials, and finally the fin is completed by the dermal fin-rays.
Fig. 8. Dorsal view of the pelvic girdle and fins of amale Dogfish(Scyllium canicula).1. pelvic girdle.3. clasper.2. basi-pterygium.4. radiale.
Fig. 8. Dorsal view of the pelvic girdle and fins of amale Dogfish(Scyllium canicula).1. pelvic girdle.3. clasper.2. basi-pterygium.4. radiale.
The Pelvic Girdleis much smaller than the pectoral. It is formed of a stout nearly straight bar of cartilage placed transversely across the ventral region of the body. The bar has no dorsal or lateral extensions, and is terminated by short blunt processes. It bears on its posterior surface a pair of facets with which the pelvic fins articulate.
The Pelvic Finis smaller and more simply constructed than is the pectoral. It consists of a long, somewhat curvedrod, thebasi-pterygium(fig. 8, 2), running directly backwards on the inner side of the fin, and articulating in front with the pelvic girdle. From its outer side arise a series of about fourteen parallel cartilaginous radials which bear smaller polygonal pieces. The anterior one or two of these radials may articulate independently with the pelvic girdle. In the adult male dogfish the distal end of the basi-pterygium bears a stout rod nearly as long as itself, and grooved on the dorsal surface. This is the skeleton of theclasper(fig. 8, 3).
I. EXOSKELETON.
The exoskeleton includes
(1)Scales.These are of the type known ascycloidand consist of flat rounded plates composed of concentrically arranged laminae of calcified matter, with the posterior margin entire. The anterior end of each scale is imbedded in the skin and is overlapped by the preceding scales.
(2) Theteeth. These are small, pointed, calcified structures arranged in large groups on the premaxillae, mandible, vomer, and superior and inferior pharyngeal bones.
(3) Thefin-rays. These are delicate, nearly straight bony rods which support the fins.
II. ENDOSKELETON.
The endoskeleton of the Codfish, though partially cartilaginous, is mainly ossified.
It is divisible into anaxial portion, including the skull, vertebral column, ribs, and skeleton of the median fins, and anappendicular portion, including the skeleton of the paired fins and their girdles.
1.The Axial Skeleton.
A.The Vertebral Column.
This consists of a series of some fifty-two vertebrae, all completely ossified.
It is divisible into two regions only, viz. thetrunkregion, the vertebrae of which bear movable ribs, and thecaudalortailregion, the vertebrae of which do not bear movable ribs.
Trunk vertebrae.
These are seventeen in number; the ninth may be described as typical of them all. It consists of a short deeply biconcavecentrumwhose two cavities communicate by a narrow central canal. From the dorsal surface of the anterior half of the centrum arise two strong plates, the dorsal orneural processes, which are directed obliquely backwards and meet forming the dorsal orneural arch. This is produced into a long backwardly-directed dorsal orneural spine.
From the lower part of the anterior edge of each neural arch arise a pair of blunt triangular projections which overhang the posterior half of the preceding centrum, and bear a pair of flattened surfaces which correspond to the anterior orprezygapophysesof most vertebrae, they differ however from ordinary prezygapophyses in the fact that they look downwards and outwards. From the posterior end of the centrum arise a pair of short blunt processes each of which bears an upwardly- and inwardly-directed articulating surface corresponding to apostzygapophysis.
The two halves of the ventral arch form a pair of largeventri-lateral processeswhich arise from the anterior half of the centrum and pass outwards and slightly backwards and downwards.
Behind these there arises on each vertebra a second outgrowth which is small and flattened, and like the ventri-lateral process serves to protect the air-bladder. The surface of the centrum is marked by more or less wedge-shaped depressions, one in the mid-dorsal line, and two on the ventral surface immediately mesiad to the bases of the ventri-lateral process. There are also a number of smaller depressions.
The space between one centrum and the next is in the fresh skeleton filled up by the gelatinous remains of thenotochord.
The first few vertebrae differ from the others in having very short centra and no ventri-lateral processes.
The first vertebra comes into very close relation to the posterior part of the skull, articulating with the exoccipitals. In the next few vertebrae the centra gradually lengthen, and at the fourth or fifth vertebra the ventri-lateral processes appear and gradually increase in size as followed back. They likewise gradually come to arise at a lower level on the centrum, and also become more and more downwardly directed, till at the last trunk vertebra they nearly meet.
Theneural spinesof the anterior trunk vertebrae are much longer than those of the posterior ones, that of the first vertebra being the largest and longest of all, and articulating with the skull. The spinal nerves pass out through wide notches or spaces between the successive neural arches.
Caudal vertebrae.
The caudal vertebrae are about thirty-five in number, each consists of a centrum with a slender backwardly-directed dorsal or neural arch, similar to those of the posterior trunk vertebrae. The two halves of the ventral or haemal arch however do not form outwardly-directed ventri-lateral processes, but arise on the ventral surface of the centrum, and passing downwards meet and enclose a space; they thus form a complete canal, and are prolonged into a backwardly-directed ventral orhaemal spine. The anterior haemal arches are much larger than the corresponding neural arches, but when followed back they gradually decrease in size, till at about the twenty-fourth caudal vertebra they are nearly as small as the neural arches. The last caudal vertebra is succeeded by a much flattenedhypuralbone orurostyle, which together with the posterior neural and haemal spines supports the tail-fin.
B.The Ribs.
Theribsare slender, more or less cylindrical bones attached to the poster-dorsal faces of the ventri-lateral processes of all the trunk vertebrae except the first and second. The earlier ones are thicker and more curved; the later ones thinner and more nearly straight. The ribs are homologous with the distal parts of the haemal arches of the caudal vertebrae.
Associated with the ribs are a second series of rib-like bones, theintermuscular bones. These are slender, curved bones which arise from the ribs or from the ventri-lateral processes at a distance of about an inch from the centra, and curve upwards, outwards and backwards. In the anterior region where the ventri-lateral processes are short they arise from the ribs, further back they arise from the ventri-lateral processes.
C.The Unpaired or Median Fins.
These are six in number, three beingdorsal, onecaudaland twoanal.
Thedorsalandanalfins each consist of two sets of structures, thefin-raysand theinterspinous bones. Each fin-ray forms a delicate, nearly straight, bony rod which becomes thickened and bifurcated at its proximal or vertebral end, while distally it is transversely jointed and flexible, frequently also becoming more or less flattened.
The first dorsal fin has thirteen rays, the second, sixteen to nineteen, the third, seventeen to nineteen. The first anal fin has about twenty-two, the second anal fourteen. In each fin the posterior rays rapidly decrease in size when followed back.
Theinterspinous bonesof the dorsal and anal fins alternate with the neural and haemal spines respectively, and form short, forwardly-projecting bones, each attached proximally to the base of the corresponding fin-ray.
Thecaudal finconsists of a series of about forty-three rays which radiate from the posterior end of the vertebral column, being connected with the urostyle or hypural bone, and with the posterior neural and haemal spines without the intervention of interspinous bones. Like the other fin-rays those forming the caudal fin are transversely jointed, and are widened and frayed out distally.
The tail-fin in the Cod ishomocercal, i.e. it appears to be symmetrically developed round the posterior end of the vertebral column, though in reality a much greater proportion is attached below the end of the vertebral column than above it. It is a masked heterocercal tail.
The Skull.
Owing to the fact that very little cartilage remains in the skull of the adult Codfish, its relation to the completely cartilaginous skull of the Dogfish is not easily seen. Before describing it therefore, the skull of the Salmon will be described, as it forms an intermediate type.
THE SKULL OF THE SALMON[36].
The Salmon's skull consists of (1) thechondrocranium, which remains partly cartilaginous and is partly converted into cartilage bone, especially in the occipital region, (2) a large series of plate-like membrane bones.
The Chondrocranium.
This is an elongated structure, wide behind owing to the fusion of the large auditory capsules with the cranium, and elongated and tapering considerably in front; in the middle it is much contracted by the large orbital cavities.
Dorsal surface of the Cranium.
In the centre of the posterior end of the dorsal surface is thesupra-occipital(fig. 9, A, 1) with a prominent posteriorridge. It is separated by two tracts of unossified cartilage from the large series of bones connected with theauditory organ. The first of these is theepi-otic(fig. 9, 2), which is separated by only a narrow tract of cartilage from the supra-occipital, and is continuous laterally with the largepterotic(fig. 9, A, 3) which overlaps in front a smaller bone, thesphenotic(fig. 9, 4). Both epi-otic and pterotic are drawn out into rather prominent backwardly-projecting processes.
Fig. 9. A. dorsal and B. ventral view of the cranium of a Salmon(Salmo salar) from which most of the membrane bones have beenremoved (afterParker). Cartilage is dotted.1. supra-occipital.12. opisthotic.2. epi-otic.13. alisphenoid.3. pterotic.14. orbitosphenoid.4. sphenotic.16. foramen for passage of an5. frontal.artery.6. median ethmoid.17. pro-otic.7. parietal.18. articular surface for8. lateral ethmoid.hyomandibular.9. parasphenoid.II. VII. IX. X. foramina for the10. vomer.passage of cranial nerves.11. exoccipital.
Fig. 9. A. dorsal and B. ventral view of the cranium of a Salmon(Salmo salar) from which most of the membrane bones have beenremoved (afterParker). Cartilage is dotted.1. supra-occipital.12. opisthotic.2. epi-otic.13. alisphenoid.3. pterotic.14. orbitosphenoid.4. sphenotic.16. foramen for passage of an5. frontal.artery.6. median ethmoid.17. pro-otic.7. parietal.18. articular surface for8. lateral ethmoid.hyomandibular.9. parasphenoid.II. VII. IX. X. foramina for the10. vomer.passage of cranial nerves.11. exoccipital.
The greater part of the remainder of the dorsal surfaceis formed of unossified cartilage which is pierced by three large vacuities orfontanelles. The anterior fontanelle is unpaired, and lies far forward near the anterior end of the long cartilaginous snout, the two larger posterior ones lie just in front of the supra-occipital and lead into the cranial cavity. In front of the orbit the skull widens again, and is marked by two considerablelateral ethmoid(fig. 9, 8) ossifications. In front of these are a pair of deep pits, thenasal fossae, at the base of which are a pair of foramina through which the olfactory nerves pass out; they communicate with a space, themiddle narial cavity, seen in a longitudinal section of the skull.
The long cartilaginous snout is more or less bifid in front, especially in the male (fig. 9).
Posterior end of the Cranium.
Theforamen magnumforms a large round hole leading into the cranial cavity, and is bounded laterally by the twoexoccipitalsand below by them, and to a very slight extent by thebasi-occipital, the three bones together forming a concaveoccipital condyleby which the vertebral column articulates with the skull.
The exoccipitals are connected laterally with a fourth pair of auditory bones, theopisthotics, and just meet the epi-otics dorsolaterally, while dorsally they are separated by a wide tract of unossified cartilage from the supra-occipital.
The opisthotics are connected laterally with the pterotics.
Side of the Cranium.
At the posterior end is seen thebasi-occipitalin contact above with theexoccipital, which is pierced by a prominent foramen for the exit of the tenth nerve. In front of this lies a small foramen, sometimes double, for the ninth nerve.