THE SKELETON OF THE CODFISH. (_Gadus morrhua._)
I. EXOSKELETON.
The exoskeleton includes
(1) =Scales.= These are of the type known as =cycloid= and consist of
flat rounded plates composed of concentrically arranged laminae of
calcified matter, with the posterior margin entire. The anterior end
of each scale is imbedded in the skin and is overlapped by the
preceding scales.
(2) The =teeth=. These are small, pointed, calcified structures
arranged in large groups on the premaxillae, mandible, vomer, and
superior and inferior pharyngeal bones.
(3) The =fin-rays=. These are delicate, nearly straight bony rods
which support the fins.
II. ENDOSKELETON.
The endoskeleton of the Codfish, though partially cartilaginous, is
mainly ossified.
It is divisible into an =axial portion=, including the skull,
vertebral column, ribs, and skeleton of the median fins, and an
=appendicular portion=, including the skeleton of the paired fins and
their girdles.
1. THE AXIAL SKELETON.
A. THE VERTEBRAL COLUMN.
This consists of a series of some fifty-two vertebrae, all completely
ossified.
It is divisible into two regions only, viz. the =trunk= region, the
vertebrae of which bear movable ribs, and the =caudal= or =tail=
region, the vertebrae of which do not bear movable ribs.
=Trunk vertebrae.=
These are seventeen in number; the ninth may be described as typical
of them all. It consists of a short deeply biconcave =centrum= whose
two cavities communicate by a narrow central canal. From the dorsal
surface of the anterior half of the centrum arise two strong plates,
the dorsal or =neural processes=, which are directed obliquely
backwards and meet forming the dorsal or =neural arch=. This is
produced into a long backwardly-directed dorsal or =neural spine=.
From the lower part of the anterior edge of each neural arch arise a
pair of blunt triangular projections which overhang the posterior half
of the preceding centrum, and bear a pair of flattened surfaces which
correspond to the anterior or =prezygapophyses= of most vertebrae,
they differ however from ordinary prezygapophyses in the fact that
they look downwards and outwards. From the posterior end of the
centrum arise a pair of short blunt processes each of which bears an
upwardly- and inwardly-directed articulating surface corresponding to
a =postzygapophysis=.
The two halves of the ventral arch form a pair of large
=ventri-lateral processes= which arise from the anterior half of the
centrum and pass outwards and slightly backwards and downwards.
Behind these there arises on each vertebra a second outgrowth which is
small and flattened, and like the ventri-lateral process serves to
protect the air-bladder. The surface of the centrum is marked by more
or less wedge-shaped depressions, one in the mid-dorsal line, and two
on the ventral surface immediately mesiad to the bases of the
ventri-lateral process. There are also a number of smaller
depressions.
The space between one centrum and the next is in the fresh skeleton
filled up by the gelatinous remains of the =notochord=.
The first few vertebrae differ from the others in having very short
centra and no ventri-lateral processes.
The first vertebra comes into very close relation to the posterior
part of the skull, articulating with the exoccipitals. In the next few
vertebrae the centra gradually lengthen, and at the fourth or fifth
vertebra the ventri-lateral processes appear and gradually increase in
size as followed back. They likewise gradually come to arise at a
lower level on the centrum, and also become more and more downwardly
directed, till at the last trunk vertebra they nearly meet.
The =neural spines= of the anterior trunk vertebrae are much longer
than those of the posterior ones, that of the first vertebra being the
largest and longest of all, and articulating with the skull. The
spinal nerves pass out through wide notches or spaces between the
successive neural arches.
=Caudal vertebrae.=
The caudal vertebrae are about thirty-five in number, each consists of
a centrum with a slender backwardly-directed dorsal or neural arch,
similar to those of the posterior trunk vertebrae. The two halves of
the ventral or haemal arch however do not form outwardly-directed
ventri-lateral processes, but arise on the ventral surface of the
centrum, and passing downwards meet and enclose a space; they thus
form a complete canal, and are prolonged into a backwardly-directed
ventral or =haemal spine=. The anterior haemal arches are much larger
than the corresponding neural arches, but when followed back they
gradually decrease in size, till at about the twenty-fourth caudal
vertebra they are nearly as small as the neural arches. The last
caudal vertebra is succeeded by a much flattened =hypural= bone or
=urostyle=, which together with the posterior neural and haemal spines
supports the tail-fin.
B. THE RIBS.
The =ribs= are slender, more or less cylindrical bones attached to the
poster-dorsal faces of the ventri-lateral processes of all the trunk
vertebrae except the first and second. The earlier ones are thicker
and more curved; the later ones thinner and more nearly straight. The
ribs are homologous with the distal parts of the haemal arches of the
caudal vertebrae.
Associated with the ribs are a second series of rib-like bones, the
=intermuscular bones=. These are slender, curved bones which arise
from the ribs or from the ventri-lateral processes at a distance of
about an inch from the centra, and curve upwards, outwards and
backwards. In the anterior region where the ventri-lateral processes
are short they arise from the ribs, further back they arise from the
ventri-lateral processes.
C. THE UNPAIRED OR MEDIAN FINS.
These are six in number, three being =dorsal=, one =caudal= and two
=anal=.
The =dorsal= and =anal= fins each consist of two sets of structures,
the =fin-rays= and the =interspinous bones=. Each fin-ray forms a
delicate, nearly straight, bony rod which becomes thickened and
bifurcated at its proximal or vertebral end, while distally it is
transversely jointed and flexible, frequently also becoming more or
less flattened.
The first dorsal fin has thirteen rays, the second, sixteen to
nineteen, the third, seventeen to nineteen. The first anal fin has
about twenty-two, the second anal fourteen. In each fin the posterior
rays rapidly decrease in size when followed back.
The =interspinous bones= of the dorsal and anal fins alternate with
the neural and haemal spines respectively, and form short,
forwardly-projecting bones, each attached proximally to the base of
the corresponding fin-ray.
The =caudal fin= consists of a series of about forty-three rays which
radiate from the posterior end of the vertebral column, being
connected with the urostyle or hypural bone, and with the posterior
neural and haemal spines without the intervention of interspinous
bones. Like the other fin-rays those forming the caudal fin are
transversely jointed, and are widened and frayed out distally.
The tail-fin in the Cod is =homocercal=, i.e. it appears to be
symmetrically developed round the posterior end of the vertebral
column, though in reality a much greater proportion is attached below
the end of the vertebral column than above it. It is a masked
heterocercal tail.
THE SKULL.
Owing to the fact that very little cartilage remains in the skull of
the adult Codfish, its relation to the completely cartilaginous skull
of the Dogfish is not easily seen. Before describing it therefore, the
skull of the Salmon will be described, as it forms an intermediate
type.
THE SKULL OF THE SALMON[36].
The Salmon's skull consists of (1) the =chondrocranium=, which remains
partly cartilaginous and is partly converted into cartilage bone,
especially in the occipital region, (2) a large series of plate-like
membrane bones.
THE CHONDROCRANIUM.
This is an elongated structure, wide behind owing to the fusion of the
large auditory capsules with the cranium, and elongated and tapering
considerably in front; in the middle it is much contracted by the
large orbital cavities.
DORSAL SURFACE OF THE CRANIUM.
In the centre of the posterior end of the dorsal surface is the
=supra-occipital= (fig. 9, A, 1) with a prominent posterior ridge.
It is separated by two tracts of unossified cartilage from the large
series of bones connected with the =auditory organ=. The first of
these is the =epi-otic= (fig. 9, 2), which is separated by only a
narrow tract of cartilage from the supra-occipital, and is continuous
laterally with the large =pterotic= (fig. 9, A, 3) which overlaps in
front a smaller bone, the =sphenotic= (fig. 9, 4). Both epi-otic and
pterotic are drawn out into rather prominent backwardly-projecting
processes.
[Illustration FIG. 9. A. DORSAL AND B. VENTRAL VIEW OF THE CRANIUM OF
A SALMON (_Salmo salar_) from which most of the membrane bones have
been removed (after PARKER). Cartilage is dotted.
1. supra-occipital.
2. epi-otic.
3. pterotic.
4. sphenotic.
5. frontal.
6. median ethmoid.
7. parietal.
8. lateral ethmoid.
9. parasphenoid.
10. vomer.
11. exoccipital.
12. opisthotic.
13. alisphenoid.
14. orbitosphenoid.
16. foramen for passage of an artery.
17. pro-otic.
18. articular surface for hyomandibular.
II. VII. IX. X. foramina for the passage of cranial nerves.]
The greater part of the remainder of the dorsal surface is formed of
unossified cartilage which is pierced by three large vacuities or
=fontanelles=. The anterior fontanelle is unpaired, and lies far
forward near the anterior end of the long cartilaginous snout, the two
larger posterior ones lie just in front of the supra-occipital and
lead into the cranial cavity. In front of the orbit the skull widens
again, and is marked by two considerable =lateral ethmoid= (fig. 9, 8)
ossifications. In front of these are a pair of deep pits, the =nasal
fossae=, at the base of which are a pair of foramina through which the
olfactory nerves pass out; they communicate with a space, the =middle
narial cavity=, seen in a longitudinal section of the skull.
The long cartilaginous snout is more or less bifid in front,
especially in the male (fig. 9).
POSTERIOR END OF THE CRANIUM.
The =foramen magnum= forms a large round hole leading into the cranial
cavity, and is bounded laterally by the two =exoccipitals= and below
by them, and to a very slight extent by the =basi-occipital=, the
three bones together forming a concave =occipital condyle= by which
the vertebral column articulates with the skull.
The exoccipitals are connected laterally with a fourth pair of
auditory bones, the =opisthotics=, and just meet the epi-otics
dorsolaterally, while dorsally they are separated by a wide tract of
unossified cartilage from the supra-occipital.
The opisthotics are connected laterally with the pterotics.
SIDE OF THE CRANIUM.
At the posterior end is seen the =basi-occipital= in contact above
with the =exoccipital=, which is pierced by a prominent foramen for
the exit of the tenth nerve. In front of this lies a small foramen,
sometimes double, for the ninth nerve.
[Illustration FIG. 10. LATERAL VIEW OF THE CHONDROCRANIUM OF A SALMON
(_Salmo salar_) (after PARKER). A few membrane bones are also shown.
Cartilage is dotted.
1. supra-occipital.
2. epi-otic.
3. pterotic.
4. opisthotic.
5. exoccipital.
6. basi-occipital.
7. parasphenoid.
8. sphenotic.
9. alisphenoid.
10. orbitosphenoid.
11. lateral- or ectethmoid.
12. olfactory pit; the vomerine teeth are seen just below.
14. pro-otic.
15. basisphenoid.
16. foramen for the passage of an artery.
17. anterior fontanelle.
18. posterior fontanelle.
I. II. V. VII. IX. X. foramina for the passage of cranial nerves.]
In front of the exoccipital is the large =pro-otic= pierced by two
prominent foramina. Through the more dorsal of these (fig. 10, VII.)
the facial nerve passes out, while the more ventral (fig. 10, 16) is
for the passage of an artery. Dorsal to the exoccipital are the
=opisthotic= and =pterotic=, and dorsal to the pro-otic is the
=sphenotic=. The =pterotic= is marked by a prominent groove often
lined by cartilage, which is continued forwards along a tract of
cartilage between the pro-otic and sphenotic. With this groove the
hyomandibular articulates.
There are considerable ossifications in the sphenoidal region of the
side of the cranium. The anterior boundary of the posterior fontanelle
is formed by the large =alisphenoid=, which is continuous behind with
the pro-otic and sphenotic, and below with a slender =basisphenoid=.
Both in front of and behind the basisphenoid there are considerable
vacuities in the walls of the cranium; through the posterior of these
openings (fig. 10, V.) the main part of the trigeminal nerve passes
out, and through the anterior one, the optic (fig. 10, II.). The
alisphenoid is continuous in front with the =orbitosphenoid= (fig. 10,
10), which is pierced by the foramen for the exit of the first nerve
(fig. 10, I.), and in front of the orbitosphenoid there is a large
vacuity. The =lateral ethmoid= is seen in the side view as well as in
the dorsal view. Further forwards are seen the olfactory pits, and the
long cartilaginous snout.
A =ventral view= of the cartilaginous cranium shows much the same
points as the side view. The basisphenoid appears on the surface
immediately in front of the basi-occipital.
THE SKULL WITH MEMBRANE BONES.
The =dorsal surface=. The greater part of the dorsal surface in front
of the supra-occipital is overlaid by a pair of large rough _frontals_
(figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and
stretch over from the sphenotic to the lateral ethmoid, forming a roof
for the orbit. They meet in the middle line behind, but in front are
separated by a narrow tract of unossified cartilage, and are
overlapped by the _median ethmoid_ (figs. 9, A, 6, and 11, 6). At the
sides of the supra-occipital behind the frontals are a pair of small
_parietals_ (figs. 9, A, 7, and 11, 7).
[Illustration FIG. 11. LATERAL VIEW OF THE SKULL OF A SALMON (_Salmo
salar_) (after PARKER). Cartilage is dotted.
1. supra-occipital.
2. epi-otic.
3. pterotic.
4. sphenotic.
5. frontal.
6. median ethmoid.
7. parietal.
8. nasal.
9. lachrymal.
10. suborbital.
11. supra-orbital.
12. cartilaginous sclerotic.
13. ossification in sclerotic.
14. meso-pterygoid.
15. meta-pterygoid.
16. palatine.
17. jugal.
18. quadrate.
19. maxillae
20. premaxillae.
21. articular.
22. angular.
23. dentary.
24. hyomandibular.
25. symplectic.
26. epi-hyal.
27. cerato-hyal.
28. hypo-hyal.
29. glosso-hyal.
30. opercular.
31. sub-opercular.
32. infra-opercular.
33. pre-opercular.
34. supratemporal.
35. branchiostegal rays.
36. basi-branchiostegal.]
In a =ventral view= the cranium is seen to be chiefly covered by two
large membrane bones, the _parasphenoid_ (fig. 9, B, 9) behind, the
_vomer_ in front. A view of the =posterior end= differs from that of
the cartilaginous cranium only in the fact that the end of the
_parasphenoid_ appears lying ventral to the basi-occipital.
The =lateral view= differs very markedly from that of the
cartilaginous cranium, there being a great development of membrane
bone in connection with the jaws and branchial apparatus. Lying
dorsally are seen the _median ethmoid_, _frontal_, _parietal_, and
=supra-occipital= as before. Lying external to the middle of the
_median ethmoid_ is seen the small _nasal_ (fig. 11, 8), and below the
hinder part is the _lachrymal_. The _lachrymal_ (fig. 11, 9) forms the
first of a series of seven small bones which surround the orbit
forming the =orbital ring=. Of these the one lying immediately in the
mid-ventral line of the orbit is the _suborbital_, while the one lying
in the mid-dorsal line and attached to the frontal is the
_supra-orbital_ (fig. 11, 11). The orbit has a cartilaginous
_sclerotic_ in which are two small ossifications (fig. 11, 13)
laterally placed.
BONES OF THE UPPER JAW.
The =palato-pterygo-quadrate bar= is in a very different condition
from that of the dogfish, it is partially cartilaginous, partially
converted into cartilage bone, partially overlapped by membrane bone.
It is narrow in front but becomes much broader and deeper when
followed back. Its anterior end forms the =palatine= which bears
teeth, and in front is completely ossified, while behind the cartilage
is only sheathed by bone.
Just behind the palatine the outer part of the cartilage is ossified,
forming two small bones, the =pterygoid= and =meso-pterygoid=, while
behind them is a larger, somewhat square bone, the =meta-pterygoid=
(fig. 11, 15).
Below the meta-pterygoid is a tract of unossified cartilage, and then
comes the =quadrate= (fig. 11, 18).
The lower angle of the quadrate bears a cartilaginous =condyle= with
which the mandible articulates. In front of the palatine the
cartilaginous snout is overlapped by three membrane bones, the
_jugal_, _maxilla_ and _premaxillae_.
The _premaxillae_ (fig. 11, 20), the largest of these, overlaps the
maxilla behind; both bones bear teeth. The _jugal_ (fig. 11, 17) lies
above the maxilla and overlaps it in front.
THE LOWER JAW.
The =lower jaw= is a strong bar and is like the upper jaw, partly
cartilaginous, forming =Meckel's cartilage=, partly ossified, and
sheathed to a considerable extent in membrane bone.
The outer side and posterior end is ossified, forming the large
=articular= (fig. 11, 21), but the condyle is cartilaginous and the
anterior part of the articular forms merely a splint on the outer side
of Meckel's cartilage, which extends beyond it for a considerable
distance. The angle of the jaw just below the condyle is formed by a
small _angular_ (fig. 11, 22), and the anterior two-thirds of the jaw
is sheathed in the large tooth-bearing _dentary_ (fig. 11, 23).
THE HYOID ARCH.
The =hyoid arch= has a number of ossifications in it and is closely
connected with the mandibular arch.
The =hyomandibular= (fig. 11, 24) is a large bone which articulates
with a shallow groove lined by cartilage and formed partly in the
pterotic, partly in front of it. The hyomandibular is overlapped in
front by the meta-pterygoid, while below it tapers and is succeeded by
a small area of unossified cartilage followed by the forwardly
directed =symplectic= which fits into a groove in the quadrate.
The unossified tract between the hyomandibular and symplectic is
continuous in front with a strong bar, which remains partly
cartilaginous and is partly converted into cartilage bone. The
proximal part is ossified, forming the =epi-hyal=, the middle part
forms the =cerato-hyal= (fig. 11, 27), in front of which is the small
=hypo-hyal=. The hyoid arches of the two sides are united by the large
tooth-bearing =glosso-hyal= (fig. 11, 29). Attached to the lower
surface of the hyoid arch are a series of twelve flat _branchiostegal
rays_ (fig. 11, 35). Each overlaps the one in front of it, the
posterior one being the largest. The branchiostegal rays of the two
sides are united in front by an unpaired membrane bone, the
_basi-branchiostegal_ (fig. 11, 36).
=Opercular bones.= Behind the hyomandibular there is a large bony
plate, the =operculum=, formed of four large membrane bones. The
anterior of these, the _pre-opercular_ (fig. 11, 33), is crescentic in
shape, and with its upper end a small _supratemporal_ (fig. 11, 34) is
connected.
Behind the upper part of the pre-opercular is the largest of the
opercular bones, the _opercular proper_. Its lower edge overlaps the
sub-opercular, and both opercular and sub-opercular are overlapped by
the _infra-opercular_ (fig. 11, 32) in front. The infra-opercular is
in its turn overlapped by the _pre-opercular_.
BRANCHIAL ARCHES.
There are five branchial arches, the first four of which bear gill
rays. Each of the first three consists of a shorter upper portion
directed obliquely backwards and outwards, and a longer lower portion
forming a right angle with the upper and directed obliquely forwards
and inwards. The greater part of each arch is ossified.
The upper part of either of the first two consists of a short tapering
=pharyngo-branchial= directed inwards, and of a long =epi-branchial=
tipped with cartilage at both ends. The junction of the upper and
lower parts is formed by a cartilaginous hinge-joint between the
epi-branchial and cerato-branchial. The =cerato-branchial= is a long
bony rod separated by a short area of cartilage from the
=hypo-branchial=, which is succeeded by the =basibranchial= meeting
its fellow in the middle line. The =fourth arch= has a short
epi-branchial and no ossified pharyngo-branchial, while the fifth is
reduced to little more than the cerato-branchial, which bears a few
teeth on its inner edge. All the branchial arches have projecting from
their surfaces a number of little processes which act as strainers.
The first and fourth arches have one series of these, the second and
third have two.
THE SKULL OF THE CODFISH[37].
A full description having been already given of the Salmon's skull,
that of the Codfish will be described in a briefer manner. The skull
is very fully ossified, and the great number of plate-like bones
render it a very complicated structure.
THE CRANIUM.
At the posterior end of the dorsal surface is the large
=supra-occipital=, which is drawn out behind into the large blade-like
=occipital spine=. On each side of the supra-occipital are the small
irregular _parietals_, while in front of it the roof of the skull is
mainly formed by the very large unpaired _frontal_.
A complicated series of bones are developed in connection with the
=auditory capsule=, which forms a large projecting mass united with
the side of the cranium and drawn out behind into a pair of strong
processes, the =epi-otic= and =parotic= processes. Both these
processes are connected behind with a large V-shaped bone, the
_post-temporal_ (fig. 13, 1), which will be described when dealing
with the pectoral girdle. The =epi-otic process= is formed by the
=epi-otic=, which is continuous in front with the parietal. The
=parotic process= is formed by two larger bones, a more dorsal one,
the =pterotic=, and a more ventral and internal one, the =opisthotic=,
which is continuous in front with the large =pro-otic=. Intervening
between the pterotic and frontal is another rather large bone, the
=sphenotic=, this articulates below with the pro-otic. The pterotic
and sphenotic together give rise to a large concave surface by which
the hyomandibular articulates with the cranium. Several of the cranial
nerves pass out through the bones of the auditory capsule. The ninth
leaves by a foramen near the posterior border of the opisthotic, the
fifth and seventh by a notch in the anterior border of the pro-otic.
A number of bones are likewise developed in connection with the orbit
forming the =orbital ring=. Of these the most anterior, the
_lachrymal_, is much the largest, the others are five to seven in
number, the most ventral being the _suborbital_. The sclerotic coat of
the eye is cartilaginous.
Two pairs of bones and one unpaired bone are developed in connection
with the =olfactory capsules=, of these, the _nasals_ are narrow bones
lying next the lachrymals, but nearer the middle line; they overlap
the second pair of bones, the irregular =lateral ethmoids=. These meet
one another in the middle line, and are overlapped behind by the
frontal. They articulate laterally with the lachrymal and palatine,
and ventrally with the parasphenoid.
In a =posterior view= the foramen magnum and the four bones which
surround it and together form the occipital segment are well seen. On
the ventral side is the =basi-occipital=, terminated posteriorly by a
slightly concave surface which articulates with the centrum of the
first vertebra. The sides of the foramen magnum are formed by the
=exoccipitals=, a pair of very irregular bones, pierced by a pair of
prominent foramina for the exit of the tenth nerves. The exoccipitals
also bear a pair of surfaces for articulation with corresponding ones
on the neural arch of the first vertebra. The most dorsal of the four
bones is the supra-occipital.
On the ventral surface of the cranium in front of the basi-occipital
is seen the _parasphenoid_, a very long narrow bone which underlies
the greater part of the cranium. Behind, it articulates dorsally with
the basi-occipital and dorsolaterally with the pro-otics and
opisthotics, in front it articulates dorsally with the lateral ethmoid
and ventrally with the vomer. At the sides of the parasphenoid are the
small =alisphenoids= articulating above with the postfrontals, in
front with the frontals, and behind with the pro-otics.
The _vomer_ is an unpaired bone lying immediately in front of the
parasphenoid. In front it terminates with a thickened curved margin
bearing several rows of small teeth; behind it tapers out into a long
process which underlies the anterior part of the parasphenoid.
Immediately dorsal to the vomer is another median bone, the _median
ethmoid_; this is truncated in front and tapers out behind into a
process which fits into a groove on the ventral side of the frontal.
BONES IN CONNECTION WITH THE UPPER JAW.
These bear a close resemblance to those of the Salmon. The most
anterior bone is the _premaxillae_, a thick curved bone meeting its
fellow in the middle line. The point of junction of the two is drawn
out into a short process, and the oral surface is thickly covered with
small teeth. The dorsal ends of the premaxillae are seen in the fresh
skull to meet a large patch of cartilage. Behind the premaxillae is the
_maxilla_, a long rod-like toothless bone, somewhat expanded at the
upper end where it articulates with the premaxillae and vomer.
Articulating in front with the anterior end of the maxilla and with
the =lateral ethmoid= is a very irregular bone, the =palatine= (fig.
12, 1); it articulates behind with two flat bones, the =pterygoid= and
=meso-pterygoid=. The pterygoid is united behind with two more bones,
the =quadrate= (fig. 12, 4) and =meta-pterygoid=. The =quadrate= is a
rather stout irregular bone, bearing on its lower surface a prominent
saddle-shaped articulating surface for the mandible. The palatine,
pterygoid and quadrate bones are the ossified representatives of the
palato-pterygo-quadrate bar of the Dogfish.
[Illustration FIG. 12. MANDIBULAR AND HYOID ARCHES OF A COD (_Gadus
morrhua_) × 1/2 (Brit. Mus.).
1. palatine.
2. meso-pterygoid.
3. pterygoid.
4. quadrate.
5. symplectic.
6. meta-pterygoid.
7. hyomandibular.
8. angular.
9. articular.
10. dentary.
11. inter-hyal.
12. epi-hyal.
13. cerato-hyal.
14. hypo-hyal.
15. uro-hyal.
16. branchiostegal rays.]
The quadrate is united behind with the =symplectic= (fig. 12, 5), and
the meta-pterygoid with the symplectic and =hyomandibular=, both of
which bones will be described immediately in connection with the hyoid
arch.
THE LOWER JAW.
The =lower jaw or mandible= like that of the Salmon is partly
cartilaginous, forming =Meckel's cartilage=, partly formed of
cartilage bone, partly of membrane bone. Meckel's cartilage is of
course not seen in the dried skull.
The lower jaw includes one cartilage bone, the =articular= (fig. 12,
9), this is a large bone connected by a saddle-shaped surface with the
quadrate. Meckel's cartilage lies in a groove on its under surface,
and projects beyond it in front. The _angular_ is a small thick bone
united to the lower surface of the articular at its posterior end. The
_dentary_ (fig. 12, 10) is a large tooth-bearing bone meeting its
fellow in the middle line in front, while the articular fits into a
deep notch at its posterior end.
THE HYOID ARCH.
The =hyomandibular= (fig. 12, 7) is a large irregular bone,
articulating by a prominent rounded head with the sphenotic and
pterotic. It is united in front with the meta-pterygoid and
symplectic, and sends off behind a strong process which articulates
with the opercular. The =symplectic= is a long somewhat triangular
bone drawn out in front into a process which fits into a groove on the
inner surface of the quadrate. The distal portion of the hyoid arch is
strongly developed and consists of first the =inter-hyal= (fig. 12,
11), a short bony rod, which articulates dorsally with a patch of
cartilage intervening between the posterior part of the hyomandibular
and the symplectic. Below it is united with the apex of the triangular
=epi-hyal=, a bone suturally connected with the large =cerato-hyal=
(fig. 12, 13) which unites distally with two small =hypo-hyals=. To
the cerato-hyal are attached a series of seven strong curved
cylindrical rods, the _branchiostegal rays_. The first of these is the
smallest and they increase in size up to the last. The four dorsal
ones are attached to the outer surface of the cerato-hyal, the three
ventral ones to its inner surface. Interposed between the hypo-hyals
of the two sides is an unpaired somewhat triangular plate, the
uro-hyal or _basi-branchiostegal_ (fig. 12, 15).
THE BRANCHIAL ARCHES.
The =branchial arches= are five in number and consist of the following
parts on each side. The dorsal end is formed of the =supra-pharyngeal=
bone, a large irregular bone covered ventrally with teeth of a fair
size, and representing the fused =pharyngo-branchials= of the four
anterior arches. Its external surface is continuous with four small
=epi-branchials= which pass horizontally backwards and outwards. Their
distal ends meet four long =cerato-branchials= which are directed
forwards and inwards and form the principal part of the arches.
Each of the first three cerato-branchials articulates ventrally with a
=hypo-branchial=, and the hypo-branchials of the two sides are united
in the middle line by an unpaired =basibranchial=. The third
hypo-branchial is much flattened. The fourth cerato-branchial is
united by cartilage with the posterior surface of the third
hypo-branchial, which it meets near the middle line.
The fifth arch consists only of the cerato-branchial, a wide structure
covered with teeth and generally called the =inferior pharyngeal
bone=.
The skeleton of the =operculum= consists of the same four bones as in
the Salmon, namely the _opercular_, the _infra-opercular_, the
_pre-opercular_ and the _sub-opercular_. Of these the anterior bone,
the _pre-opercular_, is the largest, while the _infra-opercular_ is
the smallest. The _opercular_ has a facet for articulation with the
hyomandibular.
2. THE APPENDICULAR SKELETON.
THE PECTORAL GIRDLE.
This is of a highly specialised type. Membrane bones are greatly
developed, and the cartilage bones, the =scapula= and =coracoid=, are
much reduced in size and importance.
[Illustration FIG. 13. THE RIGHT HALF OF THE PECTORAL GIRDLE AND RIGHT
PECTORAL FIN OF A COD (_Gadus morrhua_) × 1/2 (Brit. Mus.).
1. post-temporal.
2. supra-clavicle.
3. clavicle.
4. coracoid.
5. scapula.
6. post-clavicle.
7. brachial ossicles.
8. dermal fin-rays.]
The largest bone in the shoulder girdle is the _clavicle_ (fig. 13,
3), which is irregularly crescent shaped, thick in front and tapering
off behind. To the outer side of its upper part is attached a thick
cylindrical bone, the _supra-clavicle_, which passes upwards and is
connected with a strong =V= shaped bone, the _post-temporal_. The apex
of the =V= meets the supra-clavicle, the inner limb articulates with
the epi-otic process, the outer with the parotic process. Projecting
downwards from the upper part of the clavicle is a long bony rod,
flattened proximally and cylindrical and pointed distally, this is
the _post-clavicle_ (fig. 13, 6).
The =scapula= (fig. 13, 5) is a small irregular plate of bone attached
to the inner side of the middle of the _clavicle_. The =coracoid=[38]
is a larger plate of similar character, irregularly triangular in
shape, attached to the inner side of the clavicle immediately below
the scapula. The scapula and coracoid bear the pectoral fin.
THE PECTORAL FINS.
Each of these consists of four small irregular bones, the =brachial
ossicles= (fig. 13, 7), bearing a series of about nineteen dermal
_fin-rays_. The brachial ossicles represent the reduced and modified
radiale and basalia of cartilaginous fish such as the dogfish. The
fin-rays (fig. 13, 8) which form the whole external portion of the fin
are long slender rods having essentially the same character as those
of the unpaired fins.
THE PELVIC GIRDLE.
The =pelvic girdle= in the Cod as in other Teleosteans is entirely
absent, its place being taken by the enlarged basi-pterygia of the
fins.
THE PELVIC FINS.
These have a very anomalous position in the Cod, being attached to the
throat in front of the pectoral girdle. Each consists of a basal
portion, the =basi-pterygium=, and of a number of dermal rays. The
basi-pterygium consists of an expanded ventral portion which meets its
fellow below in the middle line, and to which the rays are attached,
and of an inwardly-directed dorsal portion which also meets its fellow
and is imbedded in the flesh. The rays are six in number and are long
slender structures similar to those of the other fins.
FOOTNOTES:
[35] See T.J. Parker's _Zootomy_, London, 1884, p. 86.
[36] See W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
London, 1877, chap. 3.
[37] T.J. Parker, _Zootomy_, London, 1884, p. 91.
[38] According to G. Swirski, _Schultergurtel des Hechtes_, Dorpat,
1880, the true coracoid is aborted, and the so-called coracoid of
Teleosteans is really the precoracoid.