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   Chapter 4 THE SKELETON OF THE FISH

A Guide to the Study of Fishes, Volume 1 (of 2) By David Starr Jordan Characters: 38496

Updated: 2017-12-01 00:03


Specialization of the Skeleton.-In the lowest form of fish-like vertebrates (Branchiostoma), the skeleton consists merely of a cartilaginous rod or notochord extending through the body just below the spinal cord. In the lampreys, sharks, dipnoans, crossopterygians, and sturgeons the skeleton is still cartilaginous, but grows progressively more complex in their forms and relations. Among the typical fishes the skeleton becomes ossified and reaches a very high degree of complexity. Very great variations in the forms and relations of the different parts of the skeleton are found among the bony fishes, or teleostei. The high degree of specialization of these parts gives to the study of the bones great importance in the systematic arrangement of these fishes. In fact the true affinities of forms is better shown by the bones than by any other system of organs. In a general way the skeleton of the fish is homologous with that of man. The head in the one corresponds to the head in the other, the back-bone to the back-bone, and the paired fins, pectoral and ventral, to the arms and legs.

Homologies of Bones of Fishes.-But this homology does not extend to the details of structure. The bones of the arm of the specialized fish are not by any means identical with the humerus, coracoid, clavicle, radius, ulna, and carpus of the higher vertebrates. The vertebrate arm is not derived from the pectoral fin, but both from a cartilaginous shoulder-girdle with undifferentiated pectoral elements bearing fin-rays, in its details unlike an arm and unlike the pectoral fin of the specialized fish.

The assumption that each element in the shoulder-girdle and the pectoral fin of the fish must correspond in detail to the arm of man has led to great confusion in naming the different bones. Among the many bones of the fish's shoulder-girdle and pectoral fin, three or four different ones have successively borne the names of scapula, clavicle, coracoid, humerus, radius, and ulna. None of these terms, unless it be clavicle, ought by rights apply to the fish, for no bone of the fish is a true homologue of any of these as seen in man. The land vertebrates and the fishes have doubtless sprung from a common stock, but this stock, related to the crossopterygians of the present day, was unspecialized in the details of its skeleton, and from it the fishes and the higher vertebrates have developed the widely diverging lines.

Fig. 21.-Striped Bass, Roccus lineatus (Bloch). Potomac River.

Parts of the Skeleton.-The skeleton may be divided into the head, the vertebral column, and the limbs. The very lowest of the fish-like forms (Branchiostoma) has no differentiated head or skull, but in all the other forms the anterior part of the vertebral column is modified to form a cranium for the protection of the brain. In the lampreys there are no jaws or other appendages to the cranium.

In the sharks, dipnoans, crossopterygians, ganoids, and teleosts or bony fishes, jaws are developed as well as a variety of other bones around the mouth and throat. The jaw-bearing forms are sometimes known by the general name of gnathostomes. In the sharks and their relatives (rays, chim?ras, etc.) all the skeleton is composed of cartilage. In the more specialized bony fishes, besides these bones we find also series of membrane bones, more or less external to the skull and composed of ossified dermal tissues. Membrane bones are not found in the sharks and lampreys, but are developed in an elaborate coat of mail in some extinct forms.

Fig. 22.-Roccus lineatus. Lateral view of cranium.

1. Vomer. 3. Prefrontal. 5. Sphenotic. 7. Epiotic. 9. Pterotic. 11. Exoccipital. 13. Parasphenoid. 15. Prootic. 2. Ethmoid. 4. Frontal. 6. Parietal. 8. Supraoccipital. 10. Opisthotic. 12. Basioccipital. 14. Basisphenoid.

Fig. 23.-Roccus lineatus. Superior view of cranium.

1. Vomer. 3. Prefrontal. 5. Sphenotic. 7. Epiotic. 9. Pterotic. 11. Exoccipital. 2. Ethmoid. 4. Frontal. 6. Parietal. 8. Supraoccipital. 10. Opisthotic.

Fig. 24.-Roccus lineatus. Inferior view of cranium.

1. Vomer. 4. Frontal. 7. Epiotic. 9. Pterotic. 11. Exoccipital. 13. Parasphenoid. 16. Alisphenoid. 3. Prefrontal. 5. Sphenotic. 8. Supraoccipital. 10. Opisthotic. 12. Basioccipital. 15. Prootic.

Names of Bones of Fishes.-In the study of the names of the bones of fishes it will be more convenient to begin with a highly specialized form in which each of the various structures is present and in its normal position.

To this end we present a series of figures of a typical form, choosing, after Starks, the striped bass (Roccus lineatus) of the Atlantic coast of the United States. For this set of plates, drawn from nature by Mrs. Chloe Lesley Starks, we are indebted to the courtesy of Mr. Edwin Chapin Starks. The figures of the striped bass illustrate a noteworthy paper on "The Synonymy of the Fish Skeleton," published by the Washington Academy of Sciences in 1901.

Bones of the Cranium.-The vomer (1) is the anterior part of the roof of the mouth, armed with small teeth in the striped bass and in many other fishes, but often toothless. The ethmoid (2) lies behind the vomer on the upper surface of the skull, and the prefrontal (3) projects on either side and behind the ethmoid, the nostrils usually lying over or near it and near the nasal bone (51). Between the eyes above are the two frontal (4) bones joined by a suture. On the side behind the posterior angle of the frontal is the sphenotic (5) above the posterior part of the eye. Behind each frontal is the parietal (6). Behind the parietal and more or less turned inward over the ear-cavity is the epiotic (7). Between the parietals, and in most fishes rising into a thin crest, is the supraoccipital (8), which bounds the cranium above and behind, its posterior margin being usually a vertical knife-like edge. The pterotic (9) forms a sort of wing or free margin behind the epiotic and over the ear-cavity. The opisthotic (10) is a small, hard, irregular bone behind the pterotic. The exoccipital (11) forms a concave joint or condyle on each side of the basioccipital (12), by which the vertebral column is joined to the skull. The parasphenoid (13) forms a narrow ridge of the roof of the mouth, connecting the vomer with the basioccipital. In some fishes of primitive structure (Salmo, Beryx) there is another bone, called orbitosphenoid, on the middle line above and between the eyes. The basisphenoid (14) is a little bone above the myotome or tube in which runs the rectus muscle of the eye. It descends toward the parasphenoid and is attached to the prootic. The prootic (15) is an irregular bone below the ear region and lying in advance of the opisthotic. The alisphenoid (16) is a small bone in the roof of the mouth before the prootic. These sixteen bones (with a loose bone of specialized form, the otolith, within the ear-cavity) constitute the cranium. All are well developed in the striped bass and in most fishes. In some specialized forms they are much distorted, coossified, or otherwise altered, and their relations to each other may be more or less changed. In the lower forms they are not always fully differentiated, but in nearly all cases their homologies can be readily traced. In the sharks and lampreys the skull constitutes a continuous cartilaginous box without sutures. In the dipnoans and other forms having a bony casque the superficial bones outside the cranium may not correspond to the cartilaginous elements of the soft skull itself.

Fig. 25-Roccus lineatus. Posterior view of cranium.

6. Parietal.

7. Epiotic.

8. Supraoccipital.

9. Pterotic.

10. Opisthotic.

11. Exoccipital.

12. Basioccipital.

Bones of the Jaws.-The bones of the jaws are attached to the cranium by membranes only, not by sutures, except in a few peculiarly specialized forms.

The Upper Jaw.-The premaxillary (32) lies on either side and forms the front of the upper jaw. Its upper posterior tip or premaxillary spine projects backward almost at right angles with the rest of the bone into a groove on the ethmoid. There is often a fold in the skin by which this bone may be thrust out or protracted, as though drawn out of a sheath. When the spines of the premaxillary are very long the upper jaw may be thrust out for a considerable distance. The premaxillary is also often known as intermaxillary.

Lying behind the premaxillary, its anterior end attached within the angle of the premaxillary, is the maxillary (31), or supramaxillary, a flattened bone with expanded posterior tip. In the striped bass this bone is without teeth, but in many less specialized forms, as the salmon, it is provided with teeth and joined to the premaxillary in a different fashion. In any case its position readily distinguishes it. In some cases the maxillary is divided by one or more sutures, setting off from it one or more extra maxillary (supplemental maxillary) bones. This suture is absent in the striped bass, but distinct in the black bass, and more than one suture is found in the shad and herring. The roof of the mouth above is formed by a number of bones, which, as they often possess teeth, may be considered with the jaws. These are the palatine bones (21), one on either side flanking the vomer, the pterygoid (20), behind it and articulating with it, the mesopterygoid (22), on the roof of the mouth toward the median line, and the metapterygoid (23), lying behind this. Although often armed with teeth, these bones are to be considered of the general nature of the membrane bones. In some degraded types of fishes (eels, morays, congers) the premaxillary is indistinguishable, being united with the vomer and palatines.

Fig. 26.-Roccus lineatus. Face-bones, shoulder and pelvic girdles, and hyoid arch.

17. Hyomandibular.

18. Symplectic.

19. Quadrate.

20. Pterygoid.

21. Palatine.

22. Mesopterygoid.

23. Metapterygoid.

24. Preopercle.

25. Opercle.

26. Subopercle.

27. Interopercle.

28. Articular.

29. Angular.

30. Dentary.

31. Maxillary.

32. Premaxillary.

33. Interhyal.

34. Epihyal.

35. Ceratohyal

36. Basihyal.

37. Glossohyal.

38. Urohyal.

39. Branchiostegal.

49. Preorbital.

50. Suborbital.

51. Nasal.

52. Supratemporal.

53. Post-temporal.

54. Supraclavicle.

55. Clavicle.

56. Postclavicle.

57. Hypercoracoid.

58. Hypocoracoid.

60. Actinosts.

61. Pectoral fin.

62. Pelvic girdle.

63. Ventral fin.

The upper jaw of the shark is formed from the anterior portion of the palatine bones, which are not separate from the quadrate, the whole forming the palato-quadrate apparatus. In the him?ra and the dipnoans this apparatus is solidly united with the cranium. In these fishes the true upper jaw, formed of maxillary and premaxillary, is wanting.

Fig. 27.-Lower jaw of Amia calva (Linn?us), showing the gular plate.

The Lower Jaw.-The lower jaw or mandible is also complex, consisting of two divisions or rami, right and left, joined in front by a suture. The anterior part of each ramus is formed by the dentary bone (30), which carries the teeth. Behind this is the articular bone (28), which is connected by a joint to the quadrate bone (19). At the lower angle of the articular bone is the small angular bone (29). In many cases another small bone, which is called splenial, may be found attached to the inner surface of the articular bone. This little bone has been called coronoid, but it is doubtless not homologous with the coronoid bone of reptiles. In a few fishes, Amia, Elopid?, and certain fossil dipnoans, there is a bony gular plate, a membrane bone across the throat behind the chin on the lower jaw.

The Suspensorium of the Mandible.-The lower jaw is attached to the cranium by a chain of suspensory bones, which vary a good deal with different groups of fishes. The articular is jointed with the flat quadrate bone (19), which lies behind the pterygoid. A slender bone passes upward (18) under the preopercle and the metapterygoid, forming a connection above with a large flattish bone, the hyomandibular (17), which in turn joins the cranium. The slender bone which thus keys together the upper and lower elements, hyomandibular and quadrate, forming the suspensorium of the lower jaw, is known as symplectic (18). The hyomandibular is thought to be homologous with the stapes, or stirrup-bone, of the ear in higher animals. In this case the symplectic may be homologous with its small orbicular bone, and the malleus is a transformation of the articular. The incus, or anvil-bone, may be formed from part of Meckel's cartilage. All these homologies are however extremely hypothetical. The core of the lower jaw is formed of a cartilage called Meckel's cartilage, outside which the membrane bones, dentary, etc., are developed. This cartilage forms the lower jaw in sharks, true jaw-bones not being developed in these fishes. In lampreys and lancelets there is no lower jaw.

Membrane Bones of Face.-The membrane bones lie on the surface of the head, when they are usually covered by thin skin and have only a superficial connection with the cranium. Such bones, formed of ossified membrane, are not found in the earlier or less specialized fishes, the lancelets and lampreys, nor in the sharks, rays, and chim?ras. They are chiefly characteristic of the bony fishes, although in some of these they have undergone degradation.

The preorbital (49) lies before and below the eye, its edge more or less parallel with that of the maxillary. It may be broad or narrow. When broad it usually forms a sheath into which the maxillary slips. The nasal (51) lies before the preorbital, a small bone usually lying along the spine of the premaxillary. Behind and below the eye is a series of about three flat bones, the suborbitals (50), small in the striped bass, but sometimes considerably modified. In the great group of loricate fishes (sculpins, etc.), the third suborbital sends a bony process called the suborbital stay backward across the cheek toward the preopercle. The suborbital stay is present in the rosefish. In some cases, as in the gurnard, this stay covers the whole cheek with a bony coat of mail. In some fishes, but not in the striped bass, a small supraorbital bone exists over the eye, forming a sort of cap on an angle of the frontal bone.

The largest uppermost flat bone of the gill-covers is known as the opercle (25). Below it, joined by a suture, is the subopercle (26). Before it is the prominent ridge of the preopercle (24), which curves forward below and forms a more or less distinct angle, often armed with serrations or spines. In some cases this armature is very highly developed. The interopercle (27) lies below the preopercle and parallel with the lower limb.

Branchial Bones.-The bones of the branchial apparatus or gills are very numerous and complex, as well as subject to important variations. In many fishes some of these bones are coossified, and in other cases some are wanting. The tongue may be considered as belonging to this series, as the bones of the gills are attached to its axis below.

In the striped bass, as in most fishes, the tongue, gristly and immovable, is formed anteriorly by a bone called the glossohyal (37). Behind this are the basihyals (36), and still farther back, on the side, is the ceratohyal (35). To the basihyals is attached a bone extending downward and free behind the urohyal (38). Behind the ceratohyal and continuous with it is the epihyal (34), to which behind is attached the narrow interhyal (33). On the under surface of the ceratohyal and the epihyal are attached the branchiostegals (39). These are slender rays supporting a membrane beneath the gills, seven in number on each side in the striped bass, but much more numerous in some groups of fishes. The gill membranes connecting the branchiostegals are in the striped bass entirely separate from each other. In other fishes they may be broadly joined across the fleshy interspace between the gill-openings, known as the isthmus, or again they may be grown fast to the isthmus itself, so that the gill-openings of the two sides are widely separated.

The Gill-arches.-The gills are attached to four bony arches with a fifth of the same nature, but totally modified by the presence of teeth, and very rarely having on it any of the gill-fringes. The fifth arch thus modified to serve in mastication instead of respiration is known collectively as the lower pharyngeals (46). Opposite these are the upper pharyngeals (45).

The gill-arches are suspended to the cranium from above by the suspensory pharyngeal (44). Each arch contains three parts-the epibranchial (43), above, the ceratobranchial (42), forming the middle part, and the hypobranchial (41), the lower part articulating with the series of basibranchials (40) which lie behind the epihyal of the tongue. On the three bones forming the first gill-arch are attached numerous appendages called gill-rakers (47). These gill-rakers vary very greatly in number and form. In the striped bass they are few and spear-shaped. In the shad they are very many and almost as fine as hairs. In some fishes they form an effective strainer in separating the food, or perhaps in keeping extraneous matter from the gills. In some fishes they are short and lumpy, in others wanting altogether.

Fig. 28.-Roccus lineatus. Branchial arches. (After Starks.)

40. Basibranchial.

41. Hypobranchial.

42. Ceratobranchial.

43. Epibranchial.

44. Suspensory pharyngeal.

45. Upper pharyngeals.

46. Lower pharyngeals.

47. Gill-rakers.

The Pharyngeals.-The hindmost gill-arch, as above stated, is modified to form a sort of jaw. The tooth-bearing bones above, 2 to 4 pairs, are known as upper pharyngeals (45), those below, single pair, as lower pharyngeals (46). Of these the lower pharyngeals are most highly specialized and the most useful in classification. These are usually formed much as in the striped bass. Occasionally they are much enlarged, with large teeth for grinding. In many families the lower pharyngeals are grown together in one large bone. In the suckers (Catostomid?) the lower pharyngeal preserves its resemblance to a gill-arch. In the carp family (Cyprinid?) retaining this resemblance, it possesses highly specialized teeth.

Vertebral Column.-The vertebral column is composed of a series of vertebr?, 24 in number in the striped bass and in many of the higher fishes, but varying in different groups from 16 to 18 to upwards of 400, the higher numbers being evidence of unspecialized or more usually degenerate structure.

Each vertebra consists of a double concave body or centrum (66). Above it are two small projections often turned backward, zygapophyses (71), and two larger ones, neurapophyses (67), which join above to form the neural spine (68) and thus form the neural canal, through which passes the spinal cord from end to end of the body.

Fig. 29.-Pharyngeal bone and teeth of European Chub, Leuciscus cephalus (Linn?us). (After Seelye.)

Fig. 30.-Upper pharyngeals of a Parrot-fish, Scarus strongylocephalus.

Fig. 31.-Lower pharyngeals of a Parro

t-fish, Scarus strongylocephalus (Bleeker).

Below in the vertebr? of the posterior half of the body the h?mapophyses (69) unite to form the h?mal spine (70), and through the h?mal canal thus formed passes a great artery. The vertebr? having h?mal as well as neural spines are known as caudal vertebr?, and occupy the posterior part of the body, usually that behind the attachment of the anal fin (78).

The anterior vertebr? known as abdominal vertebr?, bounding the body-cavity, possess neural spines similar to those of the caudal vertebr?. In place, however, of the h?mapophyses are projections known as parapophyses (72), which do not meet below, but extend outward, forming the upper part of the wall of the abdominal cavity.

Fig. 32.-Pharyngeals of Italian Parrot-fish, Sparisoma cretense (L.). a, upper; b, lower.

To the parapophyses, or near them, the ribs (73) are rather loosely attached and each rib may have one or more accessory branches (74) called epipleurals.

Fig. 33.-Roccus lineatus. Vertebral column and appendages, with a typical vertebra. (After Starks.)

64. Abdominal vertebr?.

65. Caudal vertebr?.

66. Centrum.

67. Neurapophysis.

68. Neural spine.

69. H?mapophysis.

70. H?mal spine.

71. Zygapophysis.

72. Parapophysis.

73. Ribs.

74. Epipleurals.

75. Interneural.

76. Dorsal fin.

77. Interh?mal.

78. Anal fin.

79. Hypural.

80. Caudal fin.

In the striped bass the dorsal vertebr? are essentially similar in form, but in some fishes, as the carp and the catfish, 4 or 5 anterior vertebr? are greatly modified, coossified, and so arranged as to connect the air-bladder with the organ of hearing. Fishes with vertebr? thus altered are called plectospondylous.

In the garpike the vertebr? are convex anteriorly, concave behind, being joined by ball-and-socket joints (opisthoc?lian). In most other fishes they are double concave (amplic?lian). In sharks the vertebr? are imperfectly ossified, a number of terms, asterospondylous, cyclospondylous, tectospondylous, being applied to the different stages of ossification, these terms referring to the different modes of arrangement of the calcareous material within the vertebra.

The Interneurals and Interh?mals.-The vertical fins are connected with the skeletons by bones placed loosely in the flesh and not joined by ligament or suture. Below the dorsal fin (76) lies a series of these bones, dagger-shaped, with the point downward. These are called interneurals (75) and to these the spines and soft rays of the fin are articulated.

In like fashion the spines and rays of the anal fin (18) are jointed at base to bones called interh?mals (77). In certain cases the second interh?mal is much enlarged, made hollow and quill-shaped, and in its concave upper end the tip of the air-bladder is received. This structure is seen in the plume-fishes (Calamus). These two groups of bones, interneural and interh?mal, are sometimes collectively called inter-spinals. The flattened basal bone of the caudal fin (80) is known as hypural (79).

Fig. 34.-Basal bone of dorsal fin, Holoptychius leptopterus (Agassiz). (After Woodward.)

The tail of the striped bass, ending in a broad plate which supports the caudal, is said to be homocercal. In more primitive forms the tail is turned upward more or less, the fin being largely thrown to its lower side. Such a tail as in the sturgeon is said to be heterocercal. In the isocercal tail of the codfish and its relatives the vertebr? are progressively smaller behind and the hypural plate is obsolete or nearly so, the vertebr? remaining in the line of the axis of the body and dividing the caudal fin equally. The simplest form of tail, called diphycercal, is extended horizontally, tapering backward, the fin equally divided above and below, without hypural plate. In any form of the tail, it may through degeneration be attenuate or whip-like, a form called leptocercal.

The Pectoral Limb.-The four limbs of the fish are represented by the paired fins. The anterior limb is represented by the pectoral fin and its basal elements with the shoulder-girdle, which in the bony fishes reaches a higher degree of complexity than in any other vertebrates. It is in connection with the shoulder-girdle that the greatest confusion in names has occurred. This is due to an attempt to homologize its parts with the shoulder-girdle (scapula, coracoid, and clavicle) of higher vertebrates. But it is not evident that a bony fish possesses a real scapula, coracoid, or even clavicle. The parts of its shoulder-girdle are derived by one line of descent from the undifferentiated elements of the cartilaginous shoulder-girdle of ancestral crossopterygian or dipnoan forms. From a similar ancestry by another line of differentiation has come the amphibian and reptilian shoulder-girdle and its derivative, the girdle of birds and mammals.

The Shoulder-girdle.-In the higher fishes the uppermost bone of the shoulder-girdle is called the post-temporal (suprascapula) (53). In the striped bass and in most fishes this bone is jointed to the temporal region of the cranium. Sometimes, as in the trigger-fishes, it is grown fast to the skull, but it usually rests lightly with the three points of its upper end. In sharks and skates the shoulder-girdle, which is formed of a continuous cartilage, does not touch the skull. In the eels and their allies, it has, by degradation, lost its connection and the post-temporal rests in the flesh behind the cranium.

The post-temporal sometimes projects behind through the skin and may bear spines or serrations. In front of the post-temporal and a little to the outside of it is the small supratemporal (52) also usually connecting the shoulder-girdle with the skull. Below the post-temporal, extending downward and backward, is the flattish supraclavicle (posterotemporal) (54). To this is joined the long clavicle (proscapula) (55), which runs forward and downward in the bony fishes, meeting its fellow on the opposite side in a manner suggesting the wishbone of a fowl. Behind the base of the clavicle, the sword-shaped post-clavicle (56) extends downward through the muscles behind the base of the pectoral fin. In some fishes, as the stickleback and the trumpet-fish, a pair of flattish or elongate bones called interclavicles (infraclavicles) lie between and behind the lower part of the clavicle. These are not found in most fishes and are wanting in the striped bass. They are probably in all cases merely extensions of the hypocoracoid.

Fig. 35.-Inner view of shoulder-girdle of the Buffalo-fish, Ictiobus bubalus Rafinesque, showing the mesocoracoid (59). (After Starks.)

Two flat bones side by side lie at the base of the pectoral fin, their anterior edges against the upper part of the clavicle. These are the hypercoracoid (57), above, and hypocoracoid (58), below. These have been variously called scapula, coracoid, humerus, radius, and ulna, but being found in the higher fishes only and not in the higher vertebrates, they should receive names not used for other structures. The hypercoracoid is usually pierced by a round foramen or fenestra, but in some fishes (cods, weavers) the fenestra is between the two bones. Attached to the hypercoracoid in the striped bass are four little bones shaped like an hour-glass. These are the actinosts (60) (carpals or pterygials), which support the rays of the pectoral fin (61). In most bony fishes these are placed much as in the striped bass, but in certain specialized or aberrant forms their form and position are greatly altered.

In the anglers (Pediculati) the "carpals" are much elongated, forming a kind of arm, by which the fish can execute a motion not unlike walking.

In the Alaska blackfish (Dallia pectoralis) the two coracoids are represented by a thin, cartilaginous plate, imperfectly divided, and there are no actinosts. In almost all bony fishes, however, these bones are well differentiated and distinct. In most of the soft-rayed fishes an additional V-shaped bone or arch exists on the inner surface of the shoulder-girdle near the insertion of the hypercoracoid. This is known as the mesocoracoid (59). It is not found in the striped bass, but is found in the carp, catfish, salmon, and all their allies.

Fig. 36.-Sargassum-fish, Pterophryne tumida (Osbeck). One of the Anglers. Family Antennariid?.

Fig. 37.-Shoulder-girdle of Sebastolobus alascanus Gilbert. (After Starks.)

POT. Post-temporal.

CL. Clavicle.

PCL. Postclavicle.

HYC. Hypercoracoid.

HYPC. Hypocoracoid.

The Posterior Limbs.-The posterior limb or ventral fin (63) is articulated to a single bone on either side, the pelvic girdle (62).

Fig. 38.-Cranium of Sebastolobus alascanus Gilbert. (After Starks.)

V. Vomer.

N. Nasal.

E. Ethmoid.

PF. Prefrontal.

FR. Frontal.

PAS. Parasphenoid.

ALS. Alisphenoid.

P. Parietal.

BA. Basisphenoid.

PRO. Prootic.

BO. Basioccipital.

SO. Supraoccipital.

EO. Exoccipital.

EPO. Epiotic.

SPO. Sphenotic.

PTO. Pterotic.

In the shark the pelvic girdle is rather largely developed, but in the more specialized fishes it loses its importance. In the less specialized of the bony fishes the pelvis is attached at a distance from the head among the muscles of the side, and free from the shoulder-girdle and other parts of the skeleton. The ventral fins are then said to be abdominal. When very close to the clavicle, but not connected with it, as in the mullet, the fin is still said to be abdominal or subabdominal. In the striped bass the pelvis is joined by ligament between the clavicles, near their tip. The ventral fins thus connected, as seen in most spiny-rayed fishes, are said to be thoracic. In certain forms the pelvis is thrown still farther forward and attached at the throat or even to the chin. When the ventral fins are thus inserted before the shoulder-girdle, they are said to be jugular. Most of the fishes with spines in the fins have thoracic ventrals. In the fishes with jugular ventrals these fins have begun a process of degeneration by which the spines or soft rays or both are lost or atrophied.

Fig. 39.-Lower jaw and palate of Sebastolobus alascanus. (After Starks.)

PA. Palatine.

MSPT. Mesopterygoid.

PT. Pterygoid.

MPT. Metapterygoid.

D. Dentary.

AR. Articular.

AN. Angular.

Q. Quadrate.

SY. Symplectic.

HM. Hyomandibular.

POP. Preopercle.

IOP. Interopercle.

SOP. Subopercle.

OP. Opercle.

Degeneration.-By degeneration or degradation in biology is meant merely a reduction to a lower degree of complexity or specialization in structure. If in the process of development of the individual some particular organ loses its complexity it is said to be degenerate. If in the geological history of a type the same change takes place the same term is used. Degeneration in this sense is, like specialization, a phase of adaptation. It does not imply disease, feebleness, or mutilation, or any tendency toward extinction. It is also necessary to distinguish clearly phases of primitive simplicity from the apparent simplicity resulting from degeneration.

The Skeleton in Primitive Fishes.-To learn the names of bones we can deal most satisfactorily with the higher fishes, those in which the bony framework has attained completion. But to understand the origin and relation of parts we must begin with the lowest types, tracing the different stages in the development of each part of the system.

Fig. 40.-Maxillary and premaxillary of Sebastolobus alascanus. M, maxillary; PM, premaxillary.

In the lancelets (Leptocardii), the vertebral column consists simply of a gelatinous notochord extending from one end of the fish to the other, and pointed at both ends, no skull being developed. The notochord never shows traces of segmentation, although cartilaginous rods above it are thought to forecast apophyses. In these forms there is no trace of jaws, limbs, or ribs.

Fig. 41.-Part of skeleton of Selene vomer (Linn?us).

In the embryo of the bony fish a similar notochord precedes the segmentation and ossification of the vertebral column. In most of the extinct types of fishes a notochord more or less modified persisted through life, the vertebr? being strung upon it spool fashion in various stages of development. In the Cyclostomi (lampreys and hagfishes) the limbs and lower jaw are still wanting, but a distinct skull is developed. The notochord is still present, but its anterior pointed end is wedged into the base of a cranial capsule, partly membranous, partly cartilaginous. There is no trace of segmentation in the notochord itself in these or any other fishes, but neutral arches are foreshadowed in a series of cartilages on each side of the spinal chord. The top of the head is protected by broad plates. There are ring-like cartilages supporting the mouth and other cartilages in connection with the tongue and gill structures.

Fig. 42.-Hyostylic skull of Chiloscyllium indicum, a Scyliorhinoid Shark. (After Parker and Haswell.)

Fig. 43.-Skull of Heptranchias indicus (Gmelin), a notidanoid shark. (After Parker and Haswell.)

Fig. 44.-Basal bones of pectoral fin of Monkfish, Squatina. (After Zittel.)

The Skeleton of Sharks.-In the Elasmobranchs (sharks, rays, chim?ras) the tissues surrounding the notochord are segmented and in most forms distinct vertebr? are developed. Each of these has a conical cavity before and behind, with a central canal through which the notochord is continued. The form and degree of ossification of these vertebr? differ materially in the different groups. The skull in all these fishes is cartilaginous, forming a continuous undivided box containing the brain and lodging the organs of sense. To the skull in the shark is attached a suspensorium of one or two pieces supporting the mandible and the hyoid structures. In the chim?ra the mandible is articulated directly with the skull, the hyomandibular and quadrate elements being fused with the cranium. The skull in such case is said to be autostylic, that is, with self-attached mandible. In the shark it is said to be hyostylic, the hyomandibular intervening. The upper jaw in the shark consists not of maxillary and premaxillary but of palatine elements, and the two halves of the lower jaw are representatives of Meckel's cartilage, which is the cartilaginous centre of the dentary bone in the bony fishes. These jaw-bones in the higher fishes are in the nature of membrane bones, and in the sharks and their relatives all such bones are undeveloped. The hyoid structures are in the shark relatively simple, as are also the gill-arches, which vary in number. The vertical fins are supported by interneural and interh?mal cartilages, to which the soft fin-rays are attached without articulation.

Fig. 45.-Pectoral fin of Heterodontus philippi. (From nature.)

Fig. 46.-Pectoral fin of Heptranchias indicus (Gmelin). (After Dean.)

The shoulder-girdle is made of a single cartilage, touching the back-bone at a distance behind the head. To this cartilage three smaller ones are attached, forming the base of the pectoral fin. These are called mesopterygium, propterygium, and metapterygium, the first named being in the middle and more distinctly basal. These three segments are subject to much variation. Sometimes one of them is wanting; sometimes two are grown together. Behind these the fin-rays are attached. In most of the skates the shoulder-girdle is more closely connected with the anterior vertebr?, which are more or less fused together.

Fig. 47.-Shoulder-girdle of a Flounder, Paralichthys californicus (Ayres).

The pelvis, remote from the head, is formed, in the shark, of a single or paired cartilage with smaller elements at the base of the fin-rays. In the males a cartilaginous generative organ, known as the clasper, is attached to the pelvis and the ventral fins. In the Elasmobranchs the tail vertebr? are progressively smaller backward. If a caudal fin is present, the last vertebr? are directed upward (heterocercal) and the greater part of the fin is below the axis. In other forms (sting-rays) the tail degenerates into a whip-like organ (leptocercal), often without fins. In certain primitive sharks (Ichthyotomi), as well as in the Dipnoi and Crossopterygii, the tail is diphycercal, the vertebr? growing progressively smaller backward and not bent upward toward the tip.

In the chim?ras (Holocephali) the notochord persists and is surrounded by a series of calcified rings. The palate with the suspensorium is coalesced with the skull, and the teeth are grown together into bony plates.

Fig. 48.-Shoulder-girdle of a Toadfish, Batrachoides pacifici (Günther).

Fig. 49.-Shoulder-girdle of a Garfish, Tylosurus fodiator (Jordan and Gilbert).

The Archipterygium.-The Dipnoans, Crossopterygians, and Ganoids represent various phases of transition from the ancient cartilaginous types to the modern bony fishes.

In the Ichthyotomous sharks, Dipnoans, and Crossopterygians the segments of the pectoral limb are arranged axially, or one beyond another. This type of fin has been called archipterygium by Gegenbaur, on the theory that it represents the condition shown on the first appearance of the pectoral fin. This theory is now seriously questioned, but it will be convenient to retain the name for the pectoral fin with segmented axis fringed on one or both sides by soft rays.

Fig. 50.-Shoulder-girdle of a Hake, Merluccius productus (Ayres).

The archipterygium of the Dipnoan genus Neoceratodus is thus described by Dr. Günther ("Guide to the Study of Fishes," p. 73): "The pectoral limb is covered with small scales along the middle from the root to the extremity, and is surrounded by a rayed fringe similar to the rays of the vertical fins. 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 a broad basal cartilage, generally single, sometimes showing traces of a triple division. Along the middle of the fin runs a jointed axis gradually becoming smaller and thinner towards the extremity. Each joint bears on each side a three-, two-, or one-jointed branch."

In the genus Lepidosiren, also a Dipnoan, the pectoral limb has the same axial structure, but is without fin-rays, although in the breeding season the posterior limb or ventral fin in the male is covered with a brush of fine filaments. This structure, according to Prof. J. G. Kerr,[3] is probably without definite function, but belongs to the "category of modifications so often associated with the breeding season (cf. the newts' crest) commonly called ornamental, but which are perhaps more plausibly looked upon as expressions of the intense vital activity of the organisms correlated with its period of reproductive activity." Professor Kerr, however, thinks it not unlikely that this brush of filaments with its rich blood-supply may serve in the function of respiration, a suggestion first made by Professor Lankester.

FOOTNOTES:

[3] Philos. Trans., Lond., 1900.

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