An Introduction to the Study of Fishes/Chapter 4
CHAPTER IV.
MODIFICATIONS OF THE SKELETON.
The lowermost sub-class of fishes, which comprises one form only, the Lancelet (Branchiostoma [s. Amphioxus] lanceolatum, possesses the skeleton of the most primitive type.
Fig. 28.—Branchiostoma lanceolatum.
a, Mouth; b, Vent; c, abdominal porus.
Fig. 29.—Anterior end of body of Branchiostoma (magn.)
d, Chorda dorsalis; e, Spinal chord; f, Cartilaginous rods; g, Eye;
h, Branchial rods; i, Labial cartilage; k, Oral cirrhi.
The vertebral column is represented by a simple chorda dorsalis or notochord only, which extends from one extremity of the fish to the other, and, so far from being expanded into a cranial cavity, it is pointed at its anterior end as well as at its posterior. It is enveloped in a simple membrane like the spinal chord and the abdominal organs, and there is no trace of vertebral segments or ribs; however, a series of short cartilaginous rods above the spine evidently represent apophyses. A maxillary or hyoid apparatus, or elements representing limbs, are entirely absent.
- [J. Müller, Ueber den Bau und die Lebenserscheinungen des Branchiostoma lubricum, in Abhandl. Ak. Wiss. Berlin, 1844.]
The skeleton of the Cyclostomata (or Marsipobranchii) (Lampreys and Sea-hags) shows a considerable advance of development. It consists of a notochord, the anterior pointed end of which is wedged into the base of a cranial capsule, partly membranous partly cartilaginous. This skull, therefore, is not movable upon the spinal column. No vertebral segmentation can be observed in the notochord, but neural arches are represented by a series of cartilages on each side of the spinal chord. In Petromyzon (Fig. 30) the basis cranii emits two prolongations on each side: an inferior, extending for some distance along the lower side of the spinal column, and a lateral, which is ramified into a skeleton supporting the branchial apparatus. A stylohyal process and a subocular arch with a palato-pterygoid portion may be distinguished. The roof of the cranial capsule is membranous in Myxine and in the larvæ of Petromyzon, but more or less cartilaginous in the adult Petromyzon and in Bdellostoma. A cartilaginous capsule on each side of the hinder part of the skull contains the auditory organ, whilst the olfactory capsule occupies the anterior upper part of the roof. A broad cartilaginous lamina, starting from the cranium and overlying part of the snout, has been determined as representing the ethmo-vomerine elements, whilst the oral organs are supported by large, very peculiar cartilages (labials), greatly differing in general configuration and arrangement in the various Cyclostomes. There are three in the Sea-lamprey, of which the middle one is joined to the palate by an intermediate smaller one;
Fig. 30.—Upper (A) and side (B) views, and vertical section (C) of the skull of
Petromyzon marinus.
a, Notochord; b, Basis cranii; c, Inferior, and d, Lateral process of basis; e, Auditory capsule; f, Subocular arch; g, Stylohyal process; h, Olfactory capsule; i, Ethmo-vomerine plate; k, Palato-pterygoid portion of subocular arch; l-n, Accessory labial or rostral cartilages; with o, appendage; p, lingual cartilage; q, neural arches; r, Branchial skeleton; s, Blind termination of the nasal duct between the notochord and œsophagus.
the foremost is ring-like, tooth-bearing, emitting on each side a styliform process. The lingual cartilage is large in all Cyclostomes.
There is no trace of ribs or limbs.
- [J. Müller, Vergleichende Anatomie der Myxinoiden. Erster Theil. Osteologie und Myologie, in Abhandl. Ak. Wiss. Berlin, 1835.]
The Chondropterygians exhibit a most extraordinary diversity in the development of their vertebral column; almost every degree of ossification, from a notochord without a trace of annular structure to a series of completely ossified vertebræ being found in this order. Sharks, in which the notochord is persistent, are the Holocephali (if they be reckoned to this order, and the genera Notidanus and Echinorhinus). Among the first, Chimæra monstrosa begins to show traces of segmentation; but they are limited to the outer sheath of the notochord, in which slender subossified rings appear. In Notidanus membranous septa, with a central vacuity, cross the substance of the gelatinous notochord. In the other Sharks the segmentation is complete, each vertebra having a deep conical excavation in front and behind, with a central
Fig. 31.—Heterocercal Tail of Centrina salviani.
a. Vertebræ; b, Neurapophyses; c, Hæmapophyses.
canal through which the notochord is continued; but the degree in which the primitive cartilage is replaced by concentric or radiating lamellæ of bone varies greatly in the various genera, and according to the age of the individuals. In the Rays all the vertebræ are completely ossified, and the anterior ones confluent into one continuous mass.
In the majority of Chondropterygians the extremity of the vertebral column shows a decidedly heterocercal condition (Fig. 31), and only a few, like Squatina and some Rays, possess a diphycercal tail.
The advance in the development of the skeleton of the Chondropterygians beyond the primitive condition of the previous sub-classes, manifests itself further by the presence of neural and hæmal elements, which extend to the foremost part of the axial column, but of which the hæmal form a closed arch in the caudal region only, whilst on the trunk they appear merely as a lateral longitudinal ridge.
The neural and hæmal apophyses are either merely attached to the axis, as in Chondropterygians with persistent notochord, the Rays and some Sharks; or their basal
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| Fig. 32. | Fig. 33. | Fig. 34. |
| Fig. 32.—Lateral view. Fig. 33.—Longitudinal section. Fig. 34.—Transverse section of Caudal vertebra of Basking Shark (Selache maxima). (After Hasse.) a, Centrum; b, Neurapophysis; c, Intercrural cartilage; d, Hæmapophysis; e, Spinal canal; f, Intervertebral cavity; g, Central canal for persistent portion of notochord; h, Hæmal canals for blood-vessels. | ||
portions penetrate like wedges into the substance of the centrum, so that, in a transverse section, in consequence of the difference in their texture, they appear in the form of an ×.[1] The interspaces between the neurapophyses of the vertebræ are not filled by fibrous membrane, as in other fishes, but by separate cartilages, laminæ or cartilagines intercrurales, to which frequently a series of terminal pieces is superadded, which must be regarded as the first appearance of the interneural spines of the Teleostei and many Ganoids. Similar terminal pieces are sometimes observed on the hæmal arches. Ribs are either absent or but imperfectly represented (Carcharias).
The substance of the skull of the Chondropterygians is cartilage, interrupted especially on its upper surface by more or less extensive fibro-membranous fontanelles. Superficially it is covered by a more or less thick chagreen-like osseous deposit. The articulation with the vertebral column is effected by a pair of lateral condyles. In the Sharks, besides, a central conical excavation corresponds to that of the centrum of the foremost vertebral segment, whilst in the Rays this central excavation of the skull receives a condyle of the axis of the spinous column.
The cranium itself is a continuous undivided cartilage, in which the limits of the orbit are well marked by an anterior and posterior protuberance. The ethmoidal region sends horizontal plates over the nasal sacs, the apertures of which retain their embryonic situation upon the under surface of the skull. In the majority of Chondropterygians these plates are conically produced, forming the base of the soft projecting snout; and in some forms, especially in the long-snouted Rays and the Saw-fishes (Pristis) this prolongation appears in the form of three or more tubiform rods.
As separate cartilages there are appended to the skull a suspensorium, a palatine, mandible, hyoid, and rudimentary maxillary elements.
The suspensorium is movably attached to the side of the skull. It generally consists of one piece only, but in some Rays of two. In the Rays it is articulated with the mandible only, their hyoid possessing a distinct point of attachment to the skull. In the Sharks the hyoid is suspended from the lower end of the suspensorium together with the mandible.
What is generally called the upper jaw of a Shark is, as Cuvier has already stated, not the maxillary, but palatine. It consists of two simple lateral halves, each of which articulates with the corresponding half of the lower jaw, which is formed by the simple representative of Meckel's cartilage.
Some cartilages of various sizes are generally developed on each side of the palatine, and one on each side of the mandible. They are called labial cartilages, and seem to represent maxillary elements.
The hyoid consists generally of a pair of long and strong lateral pieces, and a single mesial piece. From the former cartilaginous filaments (representing branchiostegals) pass directly outwards. Branchial arches, varying in number, and similar to the hyoid, succeed it. They are suspended from the side of the foremost part of the spinous column, and, like the hyoid, bear a number of filaments.
The vertical fins are supported by interneural and interhæmal cartilages, each of which consists of two and more pieces, and to which the fin-rays are attached without articulation.
The scapular arch of the Sharks is formed by a single coracoid cartilage bent from the dorsal region downwards and forwards. In some genera (Scyllium, Squatina) a small separate scapular cartilage is attached to the dorsal extremities of the coracoid; but in none of the Elasmobranchs is the scapular arch suspended from the skull or vertebral column; it is merely sunk, and fixed in the substance of the muscles. Behind, at the point of its greatest curvature, three carpal cartilages are joined to the coracoid, which Gegenbaur has distinguished as propterygium, mesopterygium, and metapterygium, the former occupying the front, the latter the hind margin of the fin. Several more or less regular transverse series of styliform cartilages follow. They represent the phalanges, to which the horny filaments which are imbedded in the skin of the fin are attached.
In the Rays, with the exception of Torpedo, the scapular arch is intimately connected with the confluent anterior portion of the vertebral column. The anterior and posterior carpal cartilages are followed by a series of similar pieces, which extend like an arch forwards to the rostral portion of the skull, and backwards to the pubic region. Extremely numerous phalangeal elements, longest in the middle, are supported by the carpals, and form the skeleton of the lateral expansion of the so-called disk of the Ray's body, which thus, in fact, is nothing but the enormously enlarged pectoral fin.
The pubic is represented by a single median transverse cartilage, with which a tarsal cartilage articulates. The latter supports the fin-rays. To the end of this cartilage is also attached, in the male Chondropterygians, a peculiar accessory generative organ or clasper.
The Holocephali differ from the other Chondropterygians in several important points of the structure of their skeleton, and approach unmistakably certain Ganoids. That their spinal column is persistently notochordal has been mentioned already. Their palatal apparatus, with the suspensorium, coalesces with the skull, the mandible articulating with a short apophysis of the cranial cartilage. The mandible is simple, without anterior symphysis. The spine with which the dorsal fin is armed articulates with a neural apophysis, and is not immovably attached to it, as in the Sharks. The pubic consists of two lateral halves, with a short, rounded, tarsal cartilage.
The skeleton of the Ganoid Fishes offers extreme variations with regard to the degree in which ossifications replace the primordial cartilage. Whilst some exhibit scarcely any advance beyond the Plagiostomes with persistent cartilage, others approach, as regards the development and specialisation of the several parts of their osseous framework, the Teleosteans so closely that their Ganoid nature can be demonstrated by, or inferred from, other considerations only. All Ganoids possess a separate gill-cover.[2]
The diversity in the development of the Ganoid skeleton is well exemplified by the few representatives of the order in the existing Fish-fauna. Lowest in the scale (in this respect) are those with a persistent notochord, and an autostylic skull, that is, a skull without separate suspensorium—the fishes constituting the sub-order Dipnoi, of which the existing representatives are Lepidosiren, Protopterus, and Ceratodus, and the extinct (as far as demonstrated at present) Dipterus, Chirodus (and Phaneropleuron ?). In these fishes the notochord is persistent, passing uninterruptedly into the cartilaginous base of the skull. Only now and then a distinct vertical segmentation occurs in the caudal portion of the column, but it does not extend to the notochord itself, but indicates only the limits between the superadded apophyseal elements, each neural being confluent with the opposite hæmal. Some Dipnoi are diphy-, others hetero-cercal.
Neural and hæmal elements and ribs are well developed. In Ceratodus each neurapophysis consists of a basal cartilaginous portion, forming an arch over the myelon, and of a superadded second portion. The latter is separated from the former by a distinct line of demarcation, and its two branches are more styliform, cartilaginous at the ends and in the centre, but with an osseous sheath, and coalesced at the top, forming a gable over an elastic fibrous band which runs along and parallel to the longitudinal axis of the column (Ligamentum longitudinale superius). To the top of this gable is joined a single long cylindrical neural spine. From the eleventh apophyseal segment a distinct interneural spine, of the same structure as the neural, begins to be developed, and farther on a second interneural is superadded. Towards the extremity of the column these various pieces are gradually reduced in size and number, finally only a low cartilaginous band (the rudiments of the neurapophysis) remaining. The hæmapophyses are in form, size, and structure, very similar to the neurapophyses; and all these long bones, including the ribs, have that in common, that they consist of a solid rod of cartilage enclosed in a bony sheath, which, after the disappearance or decomposition of the cartilage, appears as a hollow tube. Such bones are extremely common throughout the order of Ganoids, and their remains have led to the designation of a family as Cœlacanthi (κοιλος, hollow; and ἀκανθος, spine).
The primordial cranium of the Dipnoi is cartilaginous, but with more or less extensive ossifications in its occipital, basal, or lateral portions, and with large tegumentary bones, the arrangement of which varies in the different genera. There is no separate suspensorium for the lower jaw. A strong process descends from the cranial cartilage, and offers by means of a double condyle (Fig. 35 s) attachment to corresponding articulary surfaces of the lower jaw. 
Fig. 35.—Palatal view of Skull of Ceratodus.
Maxillary and intermaxillary elements are not developed, but, perhaps, represented in Ceratodus by some inconstant mdimentary labial cartilages situated behind the posterior nasal opening. Facial cartilages and an infraorbital ring are developed at least in Ceratodus. The presence of a pair of small teeth in front indicates the vomerine portion (v) which remains cartilage, whilst the posterior pair of teeth are implanted in a pterygo-palatine ossification (l), which sometimes is paired, sometimes continuous. The base of the skull is constantly covered by a large basal ossification (o).
The hyoid is well developed, sometimes reduced to a pair of ceratohyals, sometimes with a basihyal and glossyhyal. The skeleton of the branchial apparatus approaches the Teleosteous type, less so in Lepidosiren than in Ceratodus, in which five branchial arches are developed, but with the lateral and mesial pieces reduced in number.
A large operculum, and a smaller sub- or inter-operculum are present.
The scapular arch consists of a single median transverse cartilage, and a pair of lateral cartilages which bear the articular condyle for the pectoral limb. The latter cartilages form the base of a large membrane-bone, and the whole arch is suspended from the skull by means of an osseous supraclavicle.
The fore-limb of the Dipnoi (Fig. 36) differs externally greatly from the pectoral fin of other Ganoid fishes. It is covered with small scales along the middle, from the root to its extremity, and surrounded by a rayed fringe similar to the vertical fin. A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by an oblong cartilage, followed by a broad basal cartilage (a), generally single, sometimes showing traces of a triple division. Along the middle of the fin runs a jointed axis (b), the joints gradually becoming smaller and thinner towards the extremity; each joint bears on each side a three, two, or one-jointed branch (c, d). This axial arrangement of the pectoral skeleton, which evidently represents one of its first and lowest conditions, has been termed Archipterygium by Gegenbaur. It is found in Ceratodus and other genera, but in Lepidosiren the jointed axis only has been preserved, with the addition of rudimentary rays in Protopterus.
Fig. 36.—Fore-limb of Ceratodus.
The pubic consists of a single flattened subquadrangular cartilage, produced into a long single anterior process. Posteriorly it terminates on each side in a condyle, to which the basal cartilage of the ventral paddle is joined. The endoskeleton of the paddle is almost identical with that of the pectoral.
The Ganoid fishes with persistent notochord, but with a hyostylic skull (that is, a skull with a separate suspensorium) consist of the sub-order Chondrostei, of which the existing representatives are the Sturgeons (Acipenser, Scaphirhynchus, Polyodon), and the extinct the Chondrosteidæ, Palæoniscidæ, and (according to Traquair) Platysomidæ.
Their spinal column does not differ essentially from that of the Dipnoi. Segmentation is represented only as far as the neural and hæmal elements are concerned. All are eminently heterocercal. Ribs are present in most, but replaced by ligaments in Polyodon.
The primordial cranium of the Sturgeons consists of persistent cartilage without ossifications in its substance, but superficial bones are still more developed and specialised than in the Dipnoi; so it is, at least, in the true Sturgeons, but less so in Polyodon (Fig. 37). The upper and lateral parts of the skull are covered by well-developed membrane bones, which,
Fig. 37.—Skull of Polyodon (after Traquair).
n, Nasal cavity; sq, squamosal; mh, hyomandibular; sy, symplectic; pa, palato-pterygoid; m, Meckelian cartilage; mx, maxillary; d, dentary; h, hyoid; op, opercle; br, branchiostegal; s.cl, supra-clavicular; p.cl, post-clavicular; cl, clavicle; i.cl, infra-clavicular.
from this sub-order, upwards in the series, will be found to exist throughout the remaining forms of fishes. They are bones, the origin of which is not in cartilage but in membranous connective tissue. The lower surface of the skull is covered by an extremely large basal bone, which extends from the vomerine region on to the anterior part of the spinal column. The nasal excavation in the skull is rather lateral than inferior. The ethmoidal region is generally much produced, forming the base of the long projecting snout. The suspensorium is movably attached to the side of the skull, and consists of two pieces, a hyomandibular and a symplectic, which now appears for the first time as a separate piece, and to which the hyoid is attached. The palato-maxillary apparatus is more complex than in the Sharks and Dipnoi; a palato-pterygoid consists of two mesially-connected rami in Polyodon, and of a complex cartilaginous disk in Acipenser, being articulated in both to the Meckelian cartilage. In addition, the Sturgeons possess one or two pairs of osseous rods, which, in Polyodon at least, represent the maxillary, and therefore must be the representatives of the labial cartilages of the Sharks. The Meckelian cartilage is more or less covered by tegumentary bones.
In the gill-cover, besides the operculum, a sub- and inter-operculum may be distinguished in Acipenser.
Fig. 38.—Fore-limb of Acipenser.
The hyoid consists of three pieces, of which the posterior bears a broad branchiostegal in Polyodon.
In the scapulary arch the primordial cartilaginous elements scarcely differ from those of the Dipnoi. The membrane-bones are much expanded, and offer a continuous series suspended from the skull. Their division in the median ventral line is complete.
The pectoral is supported by a cartilaginous framework (Fig. 38) similar to that of Ceratodus, but much more shortened and reduced in its periphery, the branches being absent altogether on one side of the axis. This modification of the fin is analogous to the heterocercal condition of the end of the spinous column. To the inner corner of a basal cartilage (a) a short axis (b) is joined, which on its outer side bears a few branches (d) only, the remaining branches (c) being fixed to the basal cartilage. The dermal fin-rays are opposed to the extremities of the branches, as in the Dipnoi.
The pubic consists of a paired cartilage, to which tarsal pieces supporting the fin-rays are attached.
The other living Ganoid fishes have the spinous column entirely or nearly entirely ossified, and have been comprised under the common name Holostei. However, they form three very distinct types; several attempts have been made to coordinate with them the fossil forms, but this task is beset with extreme difficulties, and its solution hitherto has not proved to be satisfactory.
The Polypteroidei have their spinous column formed by distinct osseous amphicœlous vertebræ, that is, vertebræ with concave anterior and posterior surfaces. It is nearly diphycercal; a slight degree of heterocercy obtains, inasmuch as the last vertebra is succeeded by a very thin cartilaginous filament which penetrates between the halves of one of the middle rays of the terminal fin. The rays above this cartilaginous filament are articulated to interneurals, those below lack interhæmals, and are attached either to the hæmals or vertebral centres. The neural arches, though ossified, do not coalesce with the centrum, and form one canal only, for the myelon. There are no intermediate elements between the neural spines. Interneurals developed, but simple, articulating with the dermoneurals. The abdominal vertebræ have parapophyses developed with epipleural spines. Only the caudal vertebræ have hæmal spines, which, like the interhæmals, agree in every essential respect with the opposite neurals. Ribs are inserted, not on the parapophyses, but on the centre, immediately below the parapophyses.
The skull of Polypterus (Fig. 39) shows a great advance towards the Teleosteous type, the number of separable bones
Fig. 39.—Skull of Polypterus. (After Traquair.)
being greatly increased. They are arranged much in the same fashion as in Teleostei. But a great portion of the primordial cranium remains cartilaginous. The membrane-bones which cover the upper and lower surfaces of the brain-case are so much developed as to cause the underlying cartilage to disappear, so that a large vacuity or fontanelle exists in the substance of the upper as well as lower cartilaginous wall. Of ossifications belonging to the primordial skull must be noticed the single occipital with a mastoid on each side. They are separated by persistent cartilage from the sphenoids and postfrontals; the former, which are the largest ossification of the primordial cranium, enclose the anterior half of the brain cavity. Finally, the nasal portion contains a median ethmoid and a pair of præfrontal bones.
Only a very small portion of the bones described are visible externally, nearly the whole of the primordial cranium being covered by the membrane-bones. Of these are seen on the upper surface a pair of parietals, frontals, "nasals," and turbinals; on the lower surface a large cross-shaped basal, anteriorly bordered on each side by a pterygoid, parallel to a palatine which forms a suture with the double vomer. The suspensorium has in front a metapterygoid and quadrate bone, and an operculum and suboperculum are attached to it behind.
Præmaxillaries and maxillaries are now fully developed, but immovably attached to the skull. The lower jaw is ossified, and consists of an articulary, angular, dentary, and splenial. Of labial cartilages a rudiment at the angle of the mouth has remained persistent.
The side of the skull, in front of the operculum, is covered by a large irregularly-shaped bone (T) (corresponding to the "tympanic lamina" of Ceratodus, Fig. 35, q), held by some to be the præoperculum; along its upper circumference lies a series of small ossicles, of which two may be distinguished as spiraculars, as they form a valve for the protection of the spiracular orifice of these fishes. An infraorbital ring is represented by a præ- and post-orbital only.
Each hyoid consists of three pieces, none of which bear branchiostegals, the single median piece being osseous in front and cartilaginous behind. Four branchial arches are developed, the foremost consisting of three, the second and third of two, and the last of a single piece. There is no lower pharyngeal. Between the rami of the lower jaw the throat is protected by a pair of large osseous laminæ (gular plates), which have been considered to represent the urohyal of osseous fishes.
The scapulary arch is almost entirely formed by the well-developed membrane-bones, which in the ventral line are suturally united. The pectoral fin is supported by three bones, pro-, meso-, and metapterygium, of which the dilated middle one alone bears rays, and is excluded from the articulation with the shoulder-girdle.
The pubic consists of paired bone, to which tarsal bones supporting the fin-rays are attached.
In the Lepidosteoidei the vertebræ are completely ossified, and opisthocœlous, having a convexity in front and a concavity behind, as in some Amphibians. Though the end of the body externally appears nearly diphycercal, the termination of the vertebral column is, in fact, distinctly heterocercal (Fig. 40). Its extremity remains cartilaginous, is turned upwards, and lies immediately below the scutes which cover the upper margin of the caudal fin. It is preceded by a few rudimentary vertebræ which gradually pass into the fully developed normal vertebræ. The caudal fin is suspended from hæmapophyses only, and does not extend to the neural side of the vertebral column. The neural arches coalesce with the centrum; interneurals simple. The abdominal vertebræ have parapophyses, to which the ribs are attached. Only the caudal vertebræ have hæmal spines.
Fig. 40.—Heterocercal Tail of Lepidosteus.
n, Vertebral column; h, hæmal spines; dn, fulcra; dh, lower fulcra.
In the skull of Lepidosteus the cartilage of the endocranium is still more replaced by ossifications than in Polypterus; those ossifications, moreover, being represented by a greater number of discrete bones; especially the membrane-bones are greatly multiplied: the occipital, for instance, consists of three pieces the vomer is double as in Polypterus; the maxillary consists of a series of pieces firmly united by suture. The symplectic reaches the lower jaw, so that the articulary is provided with a double joint, viz. for the symplectic and quadrate; the component parts of the lower jaw are as numerous as in reptiles, a dentary, splenial, articulary, angular, supra-angular, and coronary being distinct. The sides of the head are covered with numerous bones, and a præoperculum is developed in front of the gill-cover which, again, consists of an operculum and suboperculum.
Each hyoid consists of three pieces, of which the middle is the longest, the upper bearing the largest of the three branchiostegals which Lepidosteus possesses; a long and large glossohyal is intercalated between the lower ends of the hyoids. There are five branchial arches, the hindmost of which is modified into a lower pharyngeal; upper pharyngeals are likewise present as in the majority of Teleosteous fishes. No gular plate.
Of the scapulary arch the two halves are separated by a suture in the median line; the membrane-bones are well developed, only a remnant of the primordial cartilage remaining; the supraclavicle is very similar to that of Teleosteous fishes, less so the post-temporal. The base to which the limb is attached is a single osseous plate, supporting on its posterior margin semi-ossified rods in small number, which bear the pectoral rays.
The pubic consists of paired bone, the anterior ends of which overlap each other, the extremity of the right pubis being dorsad to that of the left. The elements representing a tarsus are quite rudimentary and reduced in number (two or three).
The vertebral column of the Amioidei shows unmistakable characters of the Palæichthyic type. The arrangement of its component parts is extremely simple. The centra of the amphicœlous vertebæ are well ossified, but the neural and hæmal arches do not coalesce with the centra, from which they are separated by a thin layer of cartilage. Singularly, not every vertebra has apophyses: in the caudal portion of Amia the vertebræ are alternately provided with them and lack them. The heterocercal condition of the spinous column is well marked: as in the other Holostei the hindmost vertebræ are turned upwards, become smaller and smaller in size, and lose their neural arches, the hæmals remaining developed to the end. Finally, the column terminates in a thin cartilaginous band, which is received between the lateral halves of the fifth or sixth upper caudal ray. Interneurals and interhæmals simple. Only the abdominal vertebræ have parapophyses, with which the ribs are articulated.
The configuration of the skull, and the development and arrangement of its component parts, approaches so much the Teleosteous type that, perhaps, there are greater differences in skulls of truly Teleosteous fishes than between the skulls of Amia and many Physostomi. Externally the cranium is entirely ossified; and the remains of the cartilaginous primordial cranium (which, however, has no vacuity in its roof) can only be seen in a section, and are of much less extent than in many Physostomous fishes. The immovable intermaxillary, the double vomer, the plurality of ossifications representing the articulary, the double articulary cavity of the mandible for junction with the quadrate and symplectic bones, remind us still of similar conditions in the skull of Lepidosteus, but the mobility and formation of the maxillary, the arrangement of the gill-covers, the development of the opercles, the suspensorium, the palate, the insertion of a number of branchiostegals on the long middle hyoid piece, the composition of the branchial framework (with upper and lower pharyngeals), are as in the Teleosteous type. A gular plate replaces the urohyal.
The scapular arch is composed entirely of the membrane-bones found in the Teleostei, and the two sides are loosely united by ligament. The base to which the limb is attached is cartilaginous; short semi-ossified rods are arranged along its hinder margin and bear the pectoral rays.
The skeleton of the hind-limb agrees entirely with that of Lepidosteus.
- [T. W. Bridge, The Cranial Osteology of Amia calva; in Journ. Anat. and Physiol, vol. xi.]
In the Teleosteous fishes the spinous column consists of completely ossified amphicœlous vertebræ; its termination is homocercal—that is, the caudal fin appears to be more or less symmetrical, the last vertebra occupying a central position in the base of the fin, and being coalesced with a flat osseous lamella, the hypural (Fig. 23, 70), on the hind margin of which the fin-rays are fixed. The hypural is but a union of modified hæmapophyses which are directed backwards, and the actual termination of the notochord is bent upwards, and lies along the upper edge of the hypural, hidden below the last rudimentary neural elements. In some Teleosteans, as the Salmonidæ, the last vertebræ are conspicuously bent upwards: in fact, strictly speaking, this homocercal condition is but one of the various degrees of heterocercy, different from that of many Ganoids in this respect only, that the caudal fin itself has assumed a higher degree of symmetry.
The neural and hæmal arches generally coalesce with the centrum, but there are many exceptions, inasmuch as some portion of the arches of a species, or all of them, show the original division.
The vertebræ are generally united with one another by zygapophyses, and frequently similar additional articulations exist at the lower parts of the centra. Parapophyses and ribs are very general, but the latter are inserted on the centra and the base of the processes, and never on their extremities. The point of insertion of the rib, more especially on the anterior vertebræ, may be still higher—viz. at the base of the neural arch, as in Cotylis and allied genera, and even on the top of the neurapophysis, as in Batrachus.
There is a great amount of variation as regards the degree in which the primordial cranium persists; it is always more or less replaced by bone; frequently it disappears entirely, but in some fishes, like the Salmonidæ or Esocidæ, the cartilage persists to the same or even to a greater extent than in the Ganoidei holostei. Added to the bones preformed in cartilage are a great number of membrane-bones. The different kinds of these membrane-bones occur with greater or less constancy throughout this sub-class; they often coalesce with, and are no more separable from, the neighbouring or underlying cartilage-bones. All these bones have been topographically enumerated in Chapter IV.
Many attempts have been made to classify the bones of the Teleosteous skull, according to their supposed relation to each other, or with the view to demonstrate the unity of plan on which the skull has been built; but in all either the one or the other of the following two principles has been followed:—
A. The "vertebral doctrine" starts from the undeniable fact that the skull is originally composed of several segments, each of which is merely the modification of a vertebra. The component parts of such a cranial segment are considered to be homologous to those of a vertebra. Three, four, or five cranial vertebræ have been distinguished, all the various bones of the fully-developed and ossified skull being referred, without distinction as to their origin, to one or the other of those vertebral segments. The idea of the typical unity of the osseous framework of Vertebrates has been worked out with the greatest originality and knowledge of detail, by Owen, who demonstrates that the fish-skull is composed of four vertebræ.
The bones of the fish-skull are, according to him, primarily divisible into those of the neuroskeleton, splanchnoskeleton, and dermoskeleton.
The bones of the neuro- or proper endo-skeleton are arranged in a series of four horizontally succeeding segments: the occipital, parietal, frontal, and nasal vertebræ; each segment consisting of an upper (neural) and a lower (hæmal) arch, with a common centre, and with diverging appendages.
The neural arches of the four vertebræ, in their succession from the occiput towards the snout, are:—
1. Epencephalic arch, composed of the occipitals.
2. Mesencephalic arch, composed of basisphenoid, alisphenoid, parietal, and mastoid.
3. Prosencephalic arch, composed of presphenoid, orbitosphenoid, frontal, and postfrontal.
4. Rhinencephalic arch, composed of vomer, prefrontal, and nasal.
The hæmal arches in the same order of succession are:—
1. Scapular or scapulo-coracoid arch, composed of suprascapula, scapula, aud coracoid; its appendage consists of the ulna, radius and carpal.
2. Hyoid or stylo-hyoid arch, composed of stylo-hyal, epihyal, ceratohyal, basihyal, glossohyal, and urohyal; its appendage is the branchiostegals.
3. Mandibular or tympano-mandibidar arch, composed of epi-, meso-, pre-, and hypo-tympanic, and the bones of the lower jaw; its appendage consists of the præoperculum and the other opercles.
4. Maxillary or palato-maxillary arch, composed of palatine, maxillary, and premaxillary; its appendage consists of the pterygoid and entopterygoid.
Parts of the splanchnoskeleton are held to be the ear-capsule or petrosal and the otolite, the eye-capsule or sclerotic, the nose-capsule or "ethmoid" and turbinal; the branchial arches.
The bones of the dermoskeleton are the supratemporals, supraorbitals, suborbitals, and labials.
B. In the second method of classifying the bones of the skull prominence is given to the facts of their different origin as ascertained by a study of their development. The parts developed from the primordial skull, or the cartilaginous case protecting the nervous centre are distinguished from those which enclose and support the commencement of the alimentary canal and the respiratory apparatus, and which, consisting of several arches, are comprised under the common name of visceral skeleton of the skull. Further, a distinction is made between the bones preformed in cartilage and those originating in tegumentary or membranous tissue. It is admitted that the primordial cranium is a coalition of several segments, the number of which is determined by that of the visceral arches, these representing the hæmal arches of the vertebral column; but the membrane-bones are excluded from a consideration of the vertebral division of the primordial skull, as elements originally independent of it, although these additions have entered into special relations to the cartilage-bones.
With these views the bones of the Teleosteous skull are classified thus:—
1. Cartilage-bones of the primordial skull.—The basi-occipital (5 in Figs. 23–26) has retained the form of a vertebral centrum; it is generally concave behind, the concavity containing remains of the notochord; rarely a rounded articulary head of the first vertebra fits into it, as in Symbranchus, and still more rarely it is provided with such an articulary head (Fistularia); frequently it shows two excavations on its inner surface for the reception of the saccus vestibuli. The exoccipitals (10) are situated on the side of the basi-occipital, and contribute the greater portion of the periphery of the foramen magnum; frequently they articulate with the first vertebra, or meet in the upper median line, so as to exclude the supraoccipital from the foramen magnum. The supraoccipital (8) is intercalated between the exoccipitals, and forms a most prominent part by the median crest, which sometimes extends far forwards on the upper side of the skull, and offers attachment to the dorsal portion of the large lateral muscle of the trunk. When the interior portions of this bone remain cartilaginous, some part of the semicircular canals may be lodged in it.
The region of the skull which succeeds the bones described encloses at least the greater portion of the labyrinth, and its component parts have been named with reference to it by some anatomists.[3] The alisphenoids (11) (Prooticum) form sutures posteriorly with the basi- and ex-occipitals, and meet each other in the median line at the bottom of the cerebral cavity; they contribute to the formation of a hollow in which the hypophysis cerebri and the saccus vasculosus are received; in conjunction with the exoccipital it forms another hollow for the reception of the vestibulum; generally it is perforated by the Trigeminal and Facial nerves. The paroccipitals (9) (Epioticum) lodge a portion of the posterior vertical semicircular canal, and form a projection of the skull on each side of the occipital crest, to which a terminal branch of the scapular arch is attached. The Mastoid (12 + 13) (Opisthoticum) occupies the postero-external projection of the head; it encloses a part of the external semicircular canal; is generally coalesced with a membrane-bone, the superficial squamosal, which emits a process for the suspension of the scapular arch, and is frequently, as in the Perch, divided into two separate bones.
The anterior portion of the skull varies greatly as regards form, which is chiefly dependent on the extent of the cerebral cavity; if the latter is advanced far forwards, the lateral walls of the primordial cranium are protected by more developed ossifications than if the cerebral cavity is shortened by the presence of a wide and deep orbit. In the latter case parts which normally form the side of the skull are situated in front of the brain-case, between it and the orbit, and generally reduced in extent, often replaced by membranes; especially the interorbital septum may be reduced to membrane. The most constant ossifications of this part of the skull are the orbitosphenoids (14), which join the upper anterior margin of the alisphenoids. They vary much with regard to their development—they are small in Gadoids; larger in the Perch, Pike, Salmonoids, Macrodon, and the Clupeoids; and very large in Cyprinoids and Siluroids, in which they contribute to the formation of the side of the brain-case. The single Y-shaped Sphenoideum anterius (15) is as frequently absent as present; it forms the anterior margin of the fossa for the hypophysis. Finally, the post-frontal (4) belongs also to this group of cartilage-bones.
The centre of the foremost part of the skull is occupied by the ethmoid (3), which shows great variations as regards its extent and the degree of ossification; it may extend backwards into the interorbital septum, and reach the orbitosphenoids, or may be confined to the extremity of the skull; it may remain entirely cartilaginous, or ossify into a lamina which separates the two orbits and encloses an anterior prolongation of the brain-case, along which the olfactory nerves pass: modifications occurring again in higher vertebrates. A paired ossification attached to the fore-part of the ethmoid is the prefrontals (2), which form the base of the nasal fossa.
2. Membrane-bones attached to the primordial skull.—To this group belong the parietals (7) and frontals (1). The squamosal (12) has been mentioned above in connection with the mastoid. The supraorbital is always small, and frequently absent. The lower surface of the skull is protected by the basisphenoid (parasphenoid) (6) and the vomer (16), both of which, especially the latter, may be armed with teeth.[4]
3. Cartilage bones of the alimentary portion of the visceral skeleton of the skull.—The suspensorium consists of three cartilage-bones, and affords a base for the opercular apparatus as well as a point of attachment to the hyoid, whilst in front it is connected with the palato-pterygo-palatine arch. They are the hyomandibular (23), symplectic (31), and quadrate (26), connected by means of the metapterygoid (27) with the ecto- (24) and ento-pterygoid (25), the foremost bone of the arch being the palatine (22). All these bones have been sufficiently described above (p. 55), and it remains only to be mentioned that the bones of the palatine arch are but rarely absent, as for instance in Murænophis; and that the symplectic does not extend to the articulary of the mandible, as in Amia and Lepidosteus, though its suspensory relation to the Meckelian cartilage is still indicated by a ligament which connects the two pieces. Of the mandibulary bones the articulary (35) is distinctly part of Meckel's cartilage. Frequently another portion of cartilage below the articulary remains persistent, or is replaced by a separate membrane-bone, the angular.
4. Membrane-bones of the alimentary portion of the visceral skeleton of the skull.—The suspensorium has one tegumentary bone attached to it, viz. the præoperculum (30); it is but rarely absent, for instance in Murænophis. The premaxillary (17) and maxillary (18) of the Teleostei appear to be also membrane-bones, although they are clearly analogous to the upper labial cartilages of the Sharks. The premaxillaries sometimes coalesce into a single piece (as in Diodon, Mormyrus), or they are firmly united with the maxillaries (as in all Gymnodonts, Serrasalmo, etc.) The relative position and connection of these two bones differs much, and is a valuable character in the discrimination of the various families. In some, the front margin of the jaw is formed by the premaxillary only, the two bones having a parallel position, as it has been described in the Perch (p. 53); in others, the premaxillary is shortened, allowing the maxillary to enter, and to complete, the margin of the upper jaw; and finally, in many no part of the maxillary is situated behind the premaxillary, but the entire bone is attached to the end of the premaxillary, forming its continuation. In the last case the maxillary may be quite abortive. The mobility of the upper jaw is greatest in those fishes in which the premaxillary alone forms its margin. The form of the premaxillary is subject to great variation: the beak of Belone, Xiphias is formed by the prolonged and coalesced premaxillaries. The maxillary consists sometimes of one piece, sometimes of two or three. The principal membrane-bone of the mandible is the dentary (34), to which is added the angular (36) and rarely a smaller one, the splenial or os operculare, which is situated at the inside of the articulary.
5. Cartilage-bones of the respiratory portion of the visceral skeleton of the skull.—With few exceptions all the ossifications of the hyoid and branchial arches, as described above (p. 58), belong to this group.
6. Membrane-bones of the respiratory portion of the visceral skeleton of the skull.—They are the following: the opercular pieces, viz. operculum (28), suboperculum (32), and interoperculum (33). The last of these is the least constant; it may be entirely absent, and represented by a ligament extending from the mandible to the hyoid. The urohyal (42) which separates the musculi sternohyoidei, and serves for an increased surface of their insertion; and finally the branchiostegals (43), which vary greatly in number, but are always fixed to the cerato- and epi-hyals.
7. Dermal bones of the skull.—To this category are referred some bones which are ossifications of, and belong to, the cutis. They are the turbinals (20), the suborbitals (19), and the supratemporals. They vary much with regard to the degree in which they are developed, and are rarely entirely absent. Nearly always they are wholly or partly transformed into tubes or hollows, in which the muciferous canals with their numerous nerves are lodged. Those in the temporal and scapulary regions are not always developed; on the other hand, the series of those ossicles may be continued on to the trunk, accompanying the lateral line. In many fishes those of the infraorbital ring are much dilated, protecting the entire space between the orbit and the rim of the præoperculum; in others, especially those which have the angle of the præoperculum armed with a powerful spine, the infraorbital ring emits a process towards the spine, which thus serves as a stay or support of this weapon (Scorpænidæ, Cottidæ).
The pectoral arch of the Teleosteous fishes exhibits but a remnant of a primordial cartilage, which is replaced by two ossifications,[5] the coracoid (51) and scapula (52); they offer posteriorly attachment to two series of short rods, of which the proximal are nearly always ossified, whilst the distal frequently remain small cartilaginous nodules hidden in the base of the pectoral rays. The bones, by which this portion is connected with the skull, are membrane-bones, viz. the clavicle (49), with the postclavicle (49 + 50), the supraclavicle (47), and post-temporal (46). The order of their arrangement in the Perch has been described above (p. 59). However, many Teleosteous fish lack pectoral fins, and in them the pectoral arch is frequently more or less reduced or rudimentary, as in many species of Murænidæ In others the membrane-bones are exceedingly strong, contributing to the outer protective armour of the fish, and then the clavicles are generally suturally connected in the median line. The postclavicula and the supraclavicula may be absent. Only exceptionally the shoulder-girdle is not suspended from the skull, but from the anterior portion of the spinous column (Symbranchidæ, Murænidæ, Notacanthidæ). The number of basal elements of each of the two series never exceeds five, but may be less; and the distal series is absent in Siluroids.
The pubic bones of the Teleosteous fishes undergo many modifications of form in the various families, but they are essentially of the same simple type as in the Perch.
- ↑ C. Hasse has studied the modifications of the texture of the vertebræ and the structure of the Chondropterygian skeleton generally, and shown that they correspond in the main to the natural groups of the system, and, consequently, that they offer a valuable guide in the determination of fossil remains.
- ↑ The Ganoids formed at former epochs the largest and most important order of fishes, many of the fossil forms being known from very imperfect remains only. It is quite possible that not a few of the latter, in which nothing whatever of the (probably very soft) endoskeleton has been preserved, should have to be assigned to some other order lower in the scale of organisation than the Ganoids (for instance, the Cephalaspidæ).
- ↑ As first proposed by Huxley.
- ↑ Stannius (pp. 60, 65) doubts the pure origin of these two bones from membranous tissue, and is inclined to consider them as "the extreme end of the abortive axial system."
- ↑ Parker's nomenclature is adopted here.