An Introduction to the Study of Fishes/Chapter 3

by Albert C. L. G. Günther

 

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CHAPTER III.

TERMINOLOGY AND TOPOGRAPHY OF THE SKELETON.

In order to readily comprehend the subsequent account of the modifications of the skeleton in the various sub-classes and groups of Fishes, the student has to acquaint himself with the terms used for the numerous bones of the fish skeleton, as well as with their relative position. The skeleton of any of the more common kinds of osseous fish may serve for this purpose; that of the Perch is chosen here.

The series of bones constituting the axis of the body, and destined to protect the spinal chord and some large longitudinal blood-vessels, is called the vertebral or spinal column; the single bones are the vertebræ. The skull consists of the bones surrounding the brain and organs of sense, and of a number of arches suspended from it, to support the commencement of the alimentary canal and the respiratory organs.

The vertebra (Fig. 22) consists of a body or centrum (c), with a concave anterior and posterior surface, and generally of several processes or apophyses, as—1. Two neurapophyses (na), which, on the dorsal side, rising upwards, form the neural arch over the canal, in which the spinal chord is lodged. 2. Two parapophyses (pa) usually projecting from the lower part of the sides of the body, or two hæmapophyses (ha) which actually coalesce to form on the ventral side the hæmal canal for a large trunk of the vascular system. 3. A neural spine (ns), which crowns the neurapophyses, or is interposed between their tips. 4. A hæmal spine (hs), having the same relation to the hæmapophyses.

Fig. 22.—Side and Front view of Fish-vertebra.

5. Two pleurapophyses or floating ribs, suspended from, or from the base of, the parapophyses. 6. In most fishes the neural arches are connected together by articular or oblique processes, zygapophyses (za), which are developed from the base of each neurapophysis.

The vertebræ are either abdominal or caudal vertebræ, the coalescence of the parapophyses into a complete hæmal ring, and the suspension of the anal fin generally forming a sufficiently well-marked boundary between abdominal and caudal regions (Fig. 23). In the Perch there are twenty-one abdominal and as many caudal vertebræ. The centrum of the first vertebra or atlas is very short, with the apophyses scarcely indicated, and lacking ribs like the succeeding vertebra. All the other abdominal vertebræ, with the exception of the last or two last, are provided with ribs, many of which are bifid (72). A series of flat spines (74), called interneurals, to which the spines and rays of the dorsal fins are articulated, are supported by the neural spines, the strength of the neurals and interneurals corresponding to that of the dermal spines (75). The caudal vertebræ, differ from the abdominal in having the hæmapophyseal elements converted into spines similar to the neurals, the anterior being likewise destined to support a series of interhæmals (79), to which the anal rays are articulated. The last and smallest caudal vertebra articulates with the hypural (70), a fan-like bone, which, together with the dilated hindmost neural and hæmal elements, supports the caudal rays.

Looking at a perch's skull from the side (Fig. 24), the most superficial bones will be found to be those of the jaws, a chain of thin bones round the lower half of the eye, and the opercles.

The anterior margin of the upper jaw is formed by the intermaxillary or premaxillary (17) which bears teeth, terminates in a pedicle above, to allow of a forward sliding motion of the jaw, and is dilated into a flat triangular process behind, on which leans the second bone of the upper jaw, the maxillary (18). This bone is toothless, articulates with the vomer and palatine bone, and is greatly dilated towards its distal extremity. Both the maxillary and intermaxillary lie and move parallel to each other, being connected by a narrow membrane; in many other fishes their relative position is very different.

The mandible or lower jaw consists of a right and left ramus; their union by a ligament in front is called symphysis. Each ramus is formed of several pieces; that which, by a sigmoid concavity articulates with the quadrate, is the articulary bone (35); it sends upwards a coronoid process, to which a ligament from the maxillary and the masticatory muscles are attached; and forwards a long-pointed process, to be sheathed in the deep notch of the anterior piece. A small separate piece (36) at the lower posterior angle of the mandible is termed angular. The largest piece (34) is tooth-bearing, and hence termed dentary; at its inner surface it is always deeply excavated, to receive a cylindrical cartilage, called Meckel's cartilage, the remains of an embryonic condition of the jaw, the articulary and angular being but ossified parts of it. In other Teleostei this number is still more increased by a splenial and other bones.

The infraorbital ring of bones (Fig. 23, 19) consists of several (four) pieces, of which the anterior is the largest, and distinguished as præorbital.

The so-called præoperculum (30) belongs rather to the bones of the suspensorium of the mandible, presently to be described, than to the opercles proper. It is narrow, strong, angularly bent, so as to consist of a vertical and horizontal limb, with an incompletely closed canal running along both limbs. As it is quite a superficial bone, and frequently armed with various spines, its form and configuration form an important item in the descriptive details of many fishes.

The principal piece of the gill-cover is the operculum (28), triangular in shape, situated behind, and movably united with, the vertical limb of the præoperculum. There is an articulary cavity at its upper anterior angle for its junction with the hyomandibular. The oblong lamella below the operculum is the suboperculum (32), and the one in front of this latter, below the horizontal limb of the præoperculum, is the interoperculum (33), which is connected by ligament with the angular piece of the lower jaw, and is also attached to the outer face of the hyoid, so that the gill-covers cannot open or shut without the hyoid apparatus executing a corresponding movement.

The chain of flat bones which, after the removal of the temporal muscles, appear arranged within the inner concavity of the præoperculum (Fig. 24), are comprised with the latter under the common name of mandibulary suspensorium. They connect the mandible with the cranium. The uppermost, the epitympanic or hyomandibular (23), is articulated by a double articulary head with the mastoid and posterior frontal. Another articulary head is destined for the opercular joint. The mesotympanic or symplectic (31) appears as a styliform prolongation of the lower part of the hyomandibular; is entirely cartilaginous in the young, but nearly entirely ossified in the adult. The position of this bone is noteworthy, because, directly inwards of its cartilaginous junction with the hyomandibular, there is situated the uppermost piece of the hyoid arch, the stylo-hyal. The next bone of the series is the pretympanic or metapterygoid (27), a flat bone forming a bridge towards the pterygoid, and not rarely absent in the teleosteous sub-class. Finally, the large triangular hypotympanic or quadrate (26) has a large condyle for the mandibulary joint.

The palatine arch (Fig. 26) connects the suspensorium with the anterior extremity of the skull, and is formed by three bones: the entopterygoid (25), an oblong and thin bone attached to the inner border of the palatine and pterygoid, and increasing the surface of the bony roof of the mouth towards the median line; it constitutes also the floor of the orbit. The pterygoid (24) (or os transversum) starts from the quadrate, and is joined by suture to the palatine, which is toothed, and reaches to the vomer and anterior frontal.

In the occipital region there are distinguished the basioccipital (5), readily recognised by the conical excavation corresponding and similar to that of the atlas, with which it is articulated through the intervention of a capsule filled with a gelatinous substance (the remains of the notochord); the exoccipitals (10), articulated, one on each side, to the basioccipital, and expanding on the upper surface of that bone, so as to meet and support the spinal column; a superficial thin lamella (13), suturally connected with the exoccipitals, not constant in fishes, and erroneously believed by Cuvier to be the petrosal (os petrosum) of higher animals; further, the paroccipitals (9), which are wedged in between the exoccipitals and supraoccipital. This last bone (8) forms the key of the arch over the occipital foramen, and raises a strong high crest from the whole length of its mesial line; a transverse supraoccipital ridge, coming from each side of the base of this spine runs outwards laterally to the external angles of the bone. The supraoccipital separates the parietals, and forms a suture with the frontals.

In front of the basioccipital the base of the skull is formed by the basisphenoid (parasphenoid of Huxley) (6). This very long and narrow bone extends from the basioccipital beyond the brain-capsule to between the orbits, where it forms the support of the fibro-membranous interorbital septum. Anteriorly it is connate with another long hammer-shaped bone (16), the vomer, the head of which marks the anterior end of the palate, and is beset with teeth. The alisphenoids (11) are short broad bones, rising from the basisphenoid; their posterior margins are suturally connected with the anterior of the basi- and ex-occipitals.

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The formation of the posterior part of the side of the skull is completed by the mastoid and parietal bones. The former (12) projects outwards and backwards farther than the paroccipital, forming the outer strong process of the side of the cranium. This process lodges on its upper surface one of the main ducts of the muciferous system, and affords the base of articulation to a part of the hyomandibular. Its extremity gives attachment to the strong tendon of the dorso-lateral muscles of the trunk. The parietals (7) are flat bones, of comparatively much smaller extent than in higher Vertebrates, and separated from each other by the anterior prolongation of the supra-occipital.

The anterior wall of the brain-capsule (or the posterior of the orbit) is formed by the orbitosphenoids (14), between which, superiorly, the olfactory nerves, and inferiorly, the optic, pass out of the cranium. In addition to this paired bone, the Perch and many other fishes possess another single bone (15),—the os sphenoideum anterius of Cuvier, ethmoid of Owen, and basisphenoid of Huxley; it is Y-shaped, each lateral branch being connected with an orbitosphenoid, whilst the lower branch rests upon the long basal bone.

A cartilage, the substance of which is thickest above the vomer, and which extends as a narrow stripe along the interorbital septum, represents the ethmoid of higher Vertebrata; the olfactory nerves run along, and finally perforate it.

There remain, finally, the bones distinguishable on the upper surface of the skull; the largest, extending from the nasal cavities to the occipital, are the frontal bones (1), which also form the upper margin of the orbit. The postfrontals (4) are small bones placed on the supero-posterior angle of the orbit, and serving as the point from which the infraorbital ring is suspended. The prefrontals (2), also small, occupy the anterior margin of the orbit. A pair of small tubiform bones (20), the turbinals, occupy the foremost part of the snout, in front of the frontals, and are separated from each other by intervening cartilage.

After removal of the gill-cover and mandibulary suspensorium, the hyoid arch, which encloses the branchial apparatus, and farther behind, the humeral arch are laid open to view (Fig. 25). These parts can be readily separated from the cranium proper.

The hyoid arch is suspended by a slender styliform bone, the stylohyal (29), from the hyomandibulars; it consists of three segments, the epihyal (37), ceratohyal (38), which is the longest and strongest piece, and the basihyal, which is formed by two juxtaposed pieces (39, 40). Between the latter there is a median styliform ossicle (41), extending forwards into the substance of the tongue, called glossohyal or os linguale; and below the junction of the two hyoid branches there is a vertical single bone (42), expanded along its lower edge, which, connected by ligament with the anterior extremity of the humeral arch, forms the isthmus separating the two gill-openings. This bone is called the urohyal. Articulated or attached by ligaments to the epihyal and ceratohyal are a number of sword-shaped bones or rays (43), the branchiostegals, between which the branchiostegal membrane is extended.

The branchial arches (Figs. 25 and 27) are enclosed within the hyoid arch, with which they are closely connected at the base. They are five in number, of which four bear gills, whilst the fifth (56) remains dwarfed, is beset with teeth, and called the lower pharyngeal bone. The arches adhere by their lower extremities to a chain of ossicles (53, 54, 55), basibranchials, and, curving as they ascend, nearly meet at the base of the cranium, to which they are attached by a layer of ligamentous and cellular tissue. Each of the first three branchial arches consists of four pieces movably connected with one another. The lowest is the hypobranchial (57), the next much longer one (58) the ceratobranchial, and, above this, a slender and a short irregularly-shaped epibranchial (61). In the fourth arch the hypobranchial is absent. The uppermost of these segments (62), especially of the fourth arch, are dilated, and more or less confluent; they are beset with fine teeth, and generally distinguished as the upper pharyngeal bones. Only the ceratobranchial is represented in the fifth arch or lower pharyngeal. On their outer convex side the branchial segments are grooved for the reception of large blood-vessels and nerves; on the inner side they support horny processes (63), called the gill-rakers, which do not form part of the skeleton.

The scapular or humeral arch is suspended from the skull by the (suprascapula) post-temporal (46), which, in the Perch, is attached by a triple prong to the occipital and mastoid bones. Then follows the (scapula) supraclavicula (47), and the arch is completed below by the union of the large (coracoid) clavicula (48) with its fellow. Two flat bones (51, 52), each with a vacuity, attached to the clavicle have been determined as the (radius and ulna) coracoid and scapula of higher vertebrates, and the two series of small bones (53) intervening between the forearm and the fin as carpals and metacarpals. A two-jointed appendage the (epicoracoid) post-clavicula, is attached to the clavicle: its upper piece (49) is broad and lamelliform, its lower (50) styliform and pointed.

The ventral fins are articulated to a pair of flat triangular bones, the pubic bones (80).

The bones of the skull of the fish have received so many different interpretations that no two accounts agree in their nomenclature, so that their study is a matter of considerable difficulty to the beginner. The following synonymic table will tend to overcome difficulties arising from this cause; it contains the terms used by Cuvier, those introduced by Owen, and finally the nomenclature of Stannius, Huxley, and Parker. Those adopted in the present work are printed in italics. The numbers refer to the figures in the accompanying woodcuts (Figs. 23-27).

		Cuvier.	Owen.		Stannius.		Huxley, Parker, etc.
1.		Frontal principal	Frontal		Os frontale
2.		Frontal antérieur	Prefrontal		Os frontale anterius		Lateral ethmoid (Parker)
3.		Ethmoid	Nasal		Os ethmoideum
4.		Frontal postérieur	Postfrontal		Os frontale posterius		Sphenotic (Parker)
5.		Basilaire	Basioccipital		Os basilare
6.		Sphénoide	Basisphenoid		Os sphenoideum basilare		Sometimes referred to as "Basal"
7.		Pariétal	Parietal		Os parietale
8.		Interpariétal or occipital supérieure	Supraoccipital		Os occipitale superius
9.		Occipital externe	Paroccipital		Os occipitale externum		Epioticum (Huxley)
10.		Occipital lateral	Exoccipital		Os occipitale laterale
11.		Grande aile du sphénoide	Alisphenoid		Ala temporalis		Prooticum (Huxley)
12.		Mastoidien	Mastoid		Os mastoideum + os extrascapulare	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Opisthoticum[1] + Squamosal (Huxley)
13.		Rocher	Petrosal and Osteal		Oberflächliche Knochen-lamelle
14.		Aile orbitaire	Orbitosphenoid		Ala orbitalis		Alisphenoid (Huxley)
15.		Sphenoide antérieur	Ethmoid and Ethmoturbinal		Os sphenoideum anterius		Basisphenoid (Huxley)
16.		Vomer	Vomer		Vomer
17.		Intermaxillaire	Inter- or Pre-maxillary		Os intermaxillare
18.		Maxillaire supérieur	Maxillary		Os maxillare
19.		Sousorbitaires	Infraorbital ring		Ossa infraorbitalia
20.		Nasal	Turbinal		Os terminale
22.		Palatine	Palatin		Os palatinum
23.		Temporal	Epitympanic		Os temporale		Hyomandibular (Huxley)
24.		Transverse	Pterygoid		Os transversum s. pterygoideum externum
25.		Ptérygoidien interne	Entopterygoid		Os pterygoideum		Mesopterygoid (Parker)
26.		Jugal	Hypotympanic		Os quadratojugale		Quadrate (Huxley)
27.		Tympanal	Pretympanic		Os tympanicum		Metapterygoid (Huxley)
28.		Operculaire	Operculum		Operculum
29.		Styloide	Stylohyal		Os styloideum
30.		Préopercule	Præoperculum		Præoperculum
31.		Symplectique	Mesotympanic		Os symplecticum
32.		Sousopercule	Suboperculum		Suboperculum
33.		Interopercule	Interoperculum		Interoperculum
34.		Dentaire	Dentary		Os dentale
35.		Articulaire	Articulary		Os articulare
36.		Angulaire	Angular		Os angulare
37.	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Grandes pièces latérales	Epihyal	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Segmente der Zungenbein-Schenkel
38.			Ceratohyal
39.	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Petites pièces latérales	Basihyal
40.
41.		Os lingual	Glossohyal		Os linguale s. entoglossum
42.		Queue de l'os hyoide	Urohyal				Basibranchiostegal (Parker)
43.		Rayon branchiostège	Branchiostegal		Radii branchiostegi
46.		Surscapulaire	Suprascapula		Omolita		Post-temporal (Parker)
47.		Scapulaire	Scapula		Scapula		Supraclavicula (Parker)
48.		Humeral	Coracoid		Clavicula		Clavicula (Parker)
49.	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Coracoid	Epicoracoid				Postclavicula (Parker)
50.
51.		Cubital	Radius	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Ossa carpi		Coracoid (Parker)
52.		Radial	Ulna				Scapula (Parker)
53.		Os du carpe	Carpals		Ossa metacarpi	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Basalia (Huxley),
							Brachials (Parker)
53 bis.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Chaine intermédiaire	Basibranchials		Copula
54.
55.
56.		Pharyngiens inférieurs	Lower Pharyngeals		Ossa pharyngea inferiora
57.		Pièce interne de partie inférieure de l'arceau branchiale	Hypobranchial	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Segmente der Kiemenbogen-Schenkel
58.		Piéce externe "	Ceratobranchial
59.		Stylet de prémière arceau branchiale	Upper epibranchial of first branchial arch
61.		Partie supérieure de l'arceau branchiale	Epibranchials
62.		Os pharyngian supérieur	Pharyngobranchial		Os pharyngeum superius		Upper pharyngeals
63.			Gill-rakers
65.		Rayons de la pectorale	Pectoral rays		Brustflossen-Strahlen
67, 68.		Vertèbres abdominales	Abdominal vertebræ		Bauchwirbel
69.		Vertèbres caudales	Caudal vertebræ		Schwanzwirbel
70.		Plaque triangulaire et verticale	[Aggregated interhæmals]		Verticale Platte		Hypural (Huxley)
71.			Caudal rays		Schwanzflossen Strahlen
72.		Côte	Rib		Rippen
73.		Appendices or stylets	Epipleural spines		Muskel-Gräthen
74.		Interépineux	Interneural spines		Ossa interspinalia s. obere Flossentræger
75.		Épines et rayons dorsales	Dorsal rays and spines		Rückenflossen-Strahlen u. Stacheln
76.			First interneural
78.			Rudimentary caudal rays
79.		Apophyses épineuses inférieures	Interhæmal spines		Untere Flossentræger
80.			Pubic		Becken
81.			Ventral spine		Bauchflossen-Stachel

1. Pterotic of Parker.