THE SKELETON OF THE DOGFISH[34].
_Scyllium canicula._
I. EXOSKELETON.
The exoskeleton of the dogfish is mainly composed of placoid scales,
each of which consists of a little bony base imbedded in the skin,
bearing a small backwardly-directed spine formed of dentine capped
with enamel. The scales are larger on the dorsal than on the ventral
surface, and on the jaws they are specially large and regularly
arranged in rows, there forming the teeth. The margins of the jaws or
lips are without scales.
A second exoskeletal structure is found in the fins, all of which,
both paired and unpaired, have, in addition to their cartilaginous
endoskeleton, large numbers of long slender horny fibres, the
fin-rays, which are of exoskeletal origin.
II. ENDOSKELETON.
The endoskeleton of the dogfish consists almost entirely of cartilage,
which however may become calcified in places, e.g. the centrum of each
vertebra is lined by a layer of calcified tissue.
The endoskeleton is divisible into an =axial= portion consisting of
the vertebral column, skull, and skeleton of the median fins, and an
=appendicular= portion consisting of the skeleton of the paired fins
and their girdles.
1. THE AXIAL SKELETON.
A. THE VERTEBRAL COLUMN AND RIBS.
The vertebral column consists of a series of some hundred and thirty
vertebrae, each of which is united with its predecessor and successor
in such a way as to allow a large amount of flexibility.
These vertebrae are developed round an unsegmented rod, the
=notochord=, which forms the axial support of the embryo. The
notochord remains continuous throughout the whole vertebral column,
but is greatly constricted opposite the middle of each vertebra, and
thus rendered moniliform. The vertebrae are divided into two groups,
an anterior group of trunk vertebrae, and a posterior group of caudal
or tail vertebrae.
A typical vertebra consists of a middle portion, the =centrum=, a
dorsal portion, the =dorsal= or =neural arch=, which surrounds the
spinal cord, and a ventral portion, the =ventral= or =haemal arch=,
which similarly encloses a space.
The tail vertebrae of the dogfish have this typical arrangement, the
trunk vertebrae have the haemal arches modified.
Each =centrum= is a short cylinder of cartilage surrounding an
hourglass-shaped cavity occupied by the notochord. The =neural arches=
are composed of three separate elements, the =vertebral neural plates=
(basidorsalia), =intervertebral neural plates= (interdorsalia), and
=neural spines= (supradorsalia).
The =vertebral neural plates= are in the adult fused with their
respective centra, and are notched behind for the exit of the ventral
(motor) roots of the spinal nerves. The =intervertebral neural plates=
are polygonal pieces alternating with the vertebral neural plates;
they are notched behind, but at a more dorsal level than are the
vertebral neural plates, for the exit of the dorsal or sensory roots
of the spinal nerves.
The =neural spines= are small patches of cartilage filling up the gaps
between the dorsal ends of the neural plates.
The =haemal arches= (basiventralia) differ much in the trunk and tail
portions of the vertebral column. In the trunk portion the centra are
flattened below, and the two halves of the haemal arch diverge from
one another as blunt =ventri-lateral processes= to
which short cartilaginous rods, the =ribs=, are attached. Further back
at about vertebra 37, the two halves of the haemal arch project
downwards and meet forming a complete arch. Further back still,
towards the hind end of the tail, the haemal arches bear median
=haemal spines= (ventrispinalia).
B. THE SKULL.
The skull of the dogfish remains cartilaginous throughout the life of
the animal, and has consequently a far more simple structure than have
the skulls of higher animals, in which complication has been produced
by the development of bone.
The skull consists of the following parts:--
(1) a dorsal portion, the =cranium=, which lodges the brain, and to
the sides of which the capsules of the auditory and olfactory sense
organs are united. The cranium may be compared to an unsegmented
continuation of the vertebral column;
(2) a number of ventral structures, disconnected or only loosely
connected with the cranium. These together constitute the =visceral
skeleton= forming the jaws and supporting the gills.
(1) THE CRANIUM.
The =Cranium= is an oblong box, with a flattened floor and a more
irregular roof. Its sides are expanded in front owing to the
olfactory capsules, and behind owing to the auditory capsules, while
in the middle they are deeply hollowed to form the orbits.
(_a_) On the dorsal surface of the cranium the following points should
be noticed. First at the anterior end, the large thin-walled =nasal=
or =olfactory capsules= (fig. 6, 1), each of which is drawn out into a
narrow cartilaginous process.
The olfactory capsules have no ventral walls, and are separated from
one another by the =internasal septum=, which is drawn out into a
third slender process. These three processes together constitute the
=rostrum= (fig. 6, 2).
Behind the olfactory capsules comes a large, nearly circular, hole,
the =anterior fontanelle=, slightly behind which are the two
=ophthalmic foramina=. The dorsal and ventral boundaries of the orbits
are respectively formed by the prominent =supra-orbital= and
=suborbital ridges=. Behind are the =auditory capsules= (fig. 6, 8),
each of which is marked by a pair of prominent ridges, converging
towards the middle line to a pair of apertures. These apertures
communicate with two canals, the =aqueductus vestibuli=, which lead
into the internal ear. The two ridges lodge respectively the =anterior
and posterior vertical semicircular canals= of the ear.
(_b_) The principal structures to be noted in a side view of the
cranium are contained in the =orbit= or eye-cavity. Near the base of
the orbit at its anterior end is seen the small =orbitonasal foramen=
(fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is
the large =ophthalmic foramen= (fig. 6, 5) so prominent in a dorsal
view of the skull; through it the ophthalmic branches of the fifth and
seventh nerves pass. Slightly further back near the ventral surface is
the large =optic foramen= (fig. 6, II.) for the passage of the second
nerve. Vertically above the optic foramen, near the dorsal surface, is
the very small =foramen for the fourth nerve= (fig. 6, IV.). Behind
and a little above the optic foramen is another small aperture, the
=foramen for the third nerve=. Behind and slightly below this is the
large =foramen for the sixth and main branches of the fifth and
seventh nerves= (fig. 6, V.). In front of and slightly below this
foramen are seen two other small apertures; the more anterior and
ventral of these (fig. 6, 4) is for the passage of a vessel connecting
the efferent artery of the hyoid gill with the internal carotid artery
inside the skull, the more posterior and dorsal is for the
=interorbital canal= (fig. 6, 3) which unites the two orbital sinuses.
Above and very slightly in front of the large foramen for the sixth
and main parts of the fifth and seventh nerves, are two small foramina
(fig. 6, Va., and VIIa.), through which the =ophthalmic branches of
the fifth and seventh nerves= enter the orbit. Behind and slightly
below the large foramen just mentioned is a small hole through which
the external carotid enters the orbit (fig. 6, 9).
[Illustration Fig. 6. LATERAL VIEW OF THE SKULL OF A DOGFISH
(_Scyllium canicula_) × 2/3.]
1. nasal capsule.
2. rostrum.
3. interorbital canal.
4. foramen for hyoidean artery.
5. foramen for the exit of the
ophthalmic branches of
Vth and VIIth nerves.
6. foramen through which the
external carotid leaves the
orbit.
7. orbitonasal foramen.
8. auditory capsule.
9. foramen through which the
external carotid enters the
orbit.
10. ethmo-palatine ligament.
11. palato-pterygo-quadrate bar.
12. Meckel's cartilage.
13. hyomandibular.
14. cerato-hyal.
15. pharyngo-branchial.
16. epi-branchial.
17. cerato-branchial.
18. gill filaments, nearly all have
been cut off short for the
sake of clearness.
19. extra-branchial.
20. pre-spiracular ligament.
II. III. IV. V. Va. VIIa. foramina
for passage of cranial nerves.
Behind the orbit is the =auditory capsule=. This is marked below by a
prominent =surface for the articulation of the hyomandibular=, above
which is the deep =postorbital groove= for the passage of a
blood-vessel, connecting the orbital and anterior cardinal sinuses.
(_c_) Passing to the posterior end of the cranium: in the centre is
seen the large =foramen magnum= through which the brain and spinal
cord communicate. The =notochord= enters the skull just below this
foramen, and on each side of the notochord is a projection, the
=occipital condyle=, by which the first vertebra articulates with the
skull.
External to the condyles are the prominent =pneumogastric foramina=
for the passage of the tenth nerves, and further to the sides, just
beyond the posterior vertical semicircular canals, are a pair of deep
pits in which lie the =foramina for the ninth nerves= (fig. 6, IX).
(_d_) The broad and flat ventral surface of the cranium is continued
in front as the =internasal septum= and terminated laterally by the
=suborbital ridges=. At a little behind the middle it is traversed by
two shallow grooves along which the internal carotid arteries run. At
the divergent ends of these grooves are seen two small apertures
through which the external carotids enter the orbit (fig. 6, 9), and
at the point where they meet is a single small aperture through which
the internal carotid enters the cranium.
(2) THE VISCERAL SKELETON.
The =Visceral skeleton= forms a series of seven cartilaginous arches
or hoops, surrounding the anterior part of the alimentary canal, and
enclosing a wide but rather shallow space.
(_a_) The first or =mandibular arch= is the largest of the series, and
forms the upper and lower jaws. Each half of the upper jaw or
=palato-pterygo-quadrate= bar is formed by a thick cartilaginous rod
which meets its fellow in the middle line in front, the two being
united by ligament. Each half is connected to the cranium just in
front of the orbit by the =ethmo-palatine ligament= (fig. 6, 10), and
at its hind end articulates with one of the halves of the lower jaw.
Each half of the lower jaw or =Meckel's cartilage= (fig. 6, 12) is a
cartilaginous bar, wide behind but narrow in front, where it is united
to its fellow by a median ligament. Imbedded in the tissue external to
the upper jaw are a pair of =labial cartilages=, and a similar but
smaller pair are imbedded in the tissue external to the lower jaw.
The jaws are developed from a structure whose dorsal and ventral
portions subsequently become of very different importance. The ventral
portion forms both upper and lower jaws, the former being developed as
an outgrowth from the latter. The dorsal portion forms only the
=pre-spiracular ligament= (fig. 6, 20), a strong fibrous band
containing a nodule of cartilage, and running from the anterior part
of the auditory capsule to the point where the jaws are connected with
the hyomandibular.
(_b_) The =hyoid arch= consists of a pair of cartilaginous rods which
are attached at their dorsal ends to the cranium, and are united
ventrally by a broad median plate of cartilage, the =basi-hyal=. Each
rod is divided into a dorsal portion, the =hyomandibular= and a
ventral portion, the =cerato-hyal=. The =hyomandibular= (fig. 6, 13)
is a short stout rod of cartilage projecting outwards, and somewhat
backwards and downwards from the cranium, with which it articulates
behind the orbit and below the postorbital groove. Its distal end
articulates with a rather long slender bar, the =cerato-hyal= (fig. 6,
14), which is in its turn attached to the side of the =basi-hyal=. The
=basi-hyal= is a broad plate, rounded in front and drawn out behind
into two processes to which the two halves of the first branchial arch
are attached. The posterior surfaces of both hyomandibular and
cerato-hyal bear slender cartilaginous processes, the =gill rays=. The
hyoid arch forms the main =suspensorium= or means by which the jaws
are attached to the cranium. This attachment is chiefly brought about
by a series of short ligaments which connect the posterior ends of
both upper and lower jaws with the hyomandibular, but there is also a
ligament connecting the lower jaw with the cerato-hyal. The attachment
of the jaws to the cranium is also partially effected by the
pre-spiracular and ethmo-palatine ligaments.
(_c_) Each of the five =branchial arches= is a hoop, incomplete above
and formed of four or more pieces of cartilage. The most dorsal
elements, the =pharyngo-branchials=, are flattened, pointed plates
whose free inner ends run obliquely backwards, and terminate below the
vertebral column. They are connected at their outer ends with the
short broad =epi-branchials= (fig. 6, 16) which lie at the sides of
the pharynx. From the epi-branchials arise the long =cerato
branchials= (fig. 6, 17) which run forwards and inwards along the
ventral wall of the pharynx. The first four cerato-branchials are
connected with small rods, the =hypo-branchials=, which run backwards
to meet one another in the middle line. The last two pairs of
hypo-branchials and the fifth cerato-branchials are connected with a
broad median plate, the =basibranchial=. Along the outer sides of the
second, third and fourth cerato-branchials are found elongated curved
rods, the =extra-branchials= (fig. 6, 19). The epi-branchials and
cerato-branchials bear gill rays along their posterior borders.
C. THE SKELETON OF THE MEDIAN FINS.
The =dorsal fins= have a skeleton consisting of a series of short
cartilaginous rods, the =basals= or basalia, which slope obliquely
backwards. Their bases are imbedded in the muscles of the back, while
their free ends bear a number of small polygonal cartilaginous plates,
the =radials= or radiale. Associated with this cartilaginous skeleton
are a number of long slender horny fibres, the fin-rays, which have
been already referred to in connection with the exoskeleton. The
skeleton of the other median fins mainly consists of these fibres, the
cartilaginous portion being reduced or absent.
2. THE APPENDICULAR SKELETON.
This includes the skeleton of the two pairs of limbs and of their
respective girdles.
THE PECTORAL GIRDLE forms a crescent-shaped hoop of cartilage,
incomplete above and lying just behind the visceral skeleton. The
mid-ventral part of the hoop is the thinnest portion, and is drawn out
in front into a short rounded process which is cupped dorsally and
supports part of the floor of the pericardium (fig. 7, 1). On each
side of this flattened mid-ventral portion the arch becomes very thick
and bears on its outer border a surface with which the three basal
cartilages of the fin articulate. The dorsal ends or scapular portions
of the girdle form a pair of gradually tapering horns.
THE PECTORAL FIN articulates with the pectoral girdle by means of
three basalia or basal cartilages, the =propterygium=, =meso
pterygium= and =meta-pterygium=. The most anterior and the smallest of
these is the =propterygium= (fig. 7, 5), while the most posterior
one, the =meta-pterygium= (fig. 7, 3), is much the largest. Along the
outer borders of the three basalia are arranged a series of close set
cartilaginous pieces, the =radiale=. The propterygium supports only a
single radial, which is however much larger than any of the others.
The meso-pterygium also supports only a single radial which divides
distally.
[Illustration FIG. 7. SEMIDORSAL VIEW OF THE PECTORAL GIRDLE AND FINS
OF A DOGFISH (_Scyllium canicula_) × 2/3.
The gaps between the radiale are blackened.
1. hollow in the mid-ventral part
of the pectoral girdle which
supports the pericardium.
2. dorsal (scapular portion) of
pectoral girdle.
3. meta-pterygium.
4. meso-pterygium.
5. propterygium.
6. propterygial radial.
7. meso-pterygial radial.
8. meta-pterygial radial.
9. outline of the distal part of
the fin which is supported
by horny fin-rays.]
The meta-pterygium bears about twelve long narrow radials, the first
nine of which are traversed by a transverse joint at about two-thirds
of the way from their origin. Succeeding the radials are a series of
small polygonal pieces of cartilage arranged in one or more rows and
attached to the ends of the radials, and finally the fin is completed
by the dermal fin-rays.
[Illustration FIG. 8. DORSAL VIEW OF THE PELVIC GIRDLE AND FINS OF A
MALE DOGFISH (_Scyllium canicula_).
1. pelvic girdle.
2. basi-pterygium.
3. clasper.
4. radiale.]
THE PELVIC GIRDLE is much smaller than the pectoral. It is formed of a
stout nearly straight bar of cartilage placed transversely across the
ventral region of the body. The bar has no dorsal or lateral
extensions, and is terminated by short blunt processes. It bears on
its posterior surface a pair of facets with which the pelvic fins
articulate.
THE PELVIC FIN is smaller and more simply constructed than is the
pectoral. It consists of a long, somewhat curved rod, the
=basi-pterygium= (fig. 8, 2), running directly backwards on the inner
side of the fin, and articulating in front with the pelvic girdle.
From its outer side arise a series of about fourteen parallel
cartilaginous radials which bear smaller polygonal pieces. The
anterior one or two of these radials may articulate independently with
the pelvic girdle. In the adult male dogfish the distal end of the
basi-pterygium bears a stout rod nearly as long as itself, and grooved
on the dorsal surface. This is the skeleton of the =clasper= (fig. 8,
3).
FOOTNOTE:
[34] See Marshall and Hurst's _Practical Zoology_, 4th ed. London,
1895, p. 214.