THE SKELETON OF THE FISH
=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.
[Illustration: 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, chimæras, 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.
[Illustration: 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.]
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 himæra 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.
[Illustration: FIG. 27.--Lower jaw of _Amia calva_ (Linnæus), 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_, _Elopidæ_, 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 chimæras. 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.
[Illustration: 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
(_Catostomidæ_) the lower pharyngeal preserves its resemblance to a
gill-arch. In the carp family (_Cyprinidæ_) retaining this resemblance,
it possesses highly specialized teeth.
=Vertebral Column.=--The vertebral column is composed of a series of
vertebræ, 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.
[Illustration: FIG. 29.--Pharyngeal bone and teeth of European Chub,
_Leuciscus cephalus_ (Linnæus). (After Seelye.)]
[Illustration: FIG. 30.--Upper pharyngeals of a Parrot-fish, _Scarus
strongylocephalus_.]
[Illustration: FIG. 31.--Lower pharyngeals of a Parrot-fish, _Scarus
strongylocephalus_ (Bleeker).]
Below in the vertebræ of the posterior half of the body the
_hæmapophyses_ (69) unite to form the _hæmal spine_ (70), and through
the _hæmal canal_ thus formed passes a great artery. The vertebræ
having hæmal as well as neural spines are known as _caudal vertebræ_,
and occupy the posterior part of the body, usually that behind the
attachment of the _anal fin_ (78).
The anterior vertebræ known as _abdominal vertebræ_, bounding the
body-cavity, possess neural spines similar to those of the caudal
vertebræ. In place, however, of the hæmapophyses 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.
[Illustration: 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_.
[Illustration: FIG. 33.--_Roccus lineatus._ Vertebral column and
appendages, with a typical vertebra. (After Starks.)
64. Abdominal vertebræ.
65. Caudal vertebræ.
66. Centrum.
67. Neurapophysis.
68. Neural spine.
69. Hæmapophysis.
70. Hæmal spine.
71. Zygapophysis.
72. Parapophysis.
73. Ribs.
74. Epipleurals.
75. Interneural.
76. Dorsal fin.
77. Interhæmal.
78. Anal fin.
79. Hypural.
80. Caudal fin.]
In the striped bass the dorsal vertebræ are essentially similar in
form, but in some fishes, as the carp and the catfish, 4 or 5 anterior
vertebræ are greatly modified, coossified, and so arranged as to
connect the air-bladder with the organ of hearing. Fishes with vertebræ
thus altered are called _plectospondylous_.
In the garpike the vertebræ are convex anteriorly, concave behind,
being joined by ball-and-socket joints (opisthocoelian). In most
other fishes they are double concave (amplicoelian). In sharks the
vertebræ 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 Interhæmals.=--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 _interhæmals_ (77). In certain cases the
second interhæmal 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 interhæmal, are sometimes collectively called
_inter-spinals_. The flattened basal bone of the _caudal fin_ (80) is
known as _hypural_ (79).
[Illustration: 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 vertebræ
are progressively smaller behind and the hypural plate is obsolete or
nearly so, the vertebræ 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.
[Illustration: 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.
[Illustration: FIG. 36.--Sargassum-fish, _Pterophryne tumida_ (Osbeck).
One of the Anglers. Family _Antennariidæ_.]
[Illustration: 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).
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 41.--Part of skeleton of _Selene vomer_ (Linnæus).]
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 vertebræ 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.
[Illustration: FIG. 42.--Hyostylic skull of _Chiloscyllium indicum_, a
Scyliorhinoid Shark. (After Parker and Haswell.)]
[Illustration: FIG. 43.--Skull of _Heptranchias indicus_ (Gmelin), a
notidanoid shark. (After Parker and Haswell.)]
[Illustration: FIG. 44.--Basal bones of pectoral fin of Monkfish,
_Squatina_. (After Zittel.)]
=The Skeleton of Sharks.=--In the Elasmobranchs (sharks, rays,
chimæras) the tissues surrounding the notochord are segmented and
in most forms distinct vertebræ 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 vertebræ 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 chimæra 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 interhæmal cartilages, to which the soft fin-rays are
attached without articulation.
[Illustration: FIG. 45.--Pectoral fin of _Heterodontus philippi_. (From
nature.)]
[Illustration: 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 vertebræ, which are more or less fused
together.
[Illustration: 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 vertebræ are progressively smaller backward. If a caudal fin is
present, the last vertebræ 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 vertebræ
growing progressively smaller backward and not bent upward toward the
tip.
In the chimæras (_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.
[Illustration: FIG. 48.--Shoulder-girdle of a Toadfish, _Batrachoides
pacifici_ (Günther).]
[Illustration: 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.
[Illustration: 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.