There seems to be no easy way of cleaning the skeleton of dogfish which
have been preserved in formalin or alcohol, the only procedure being to
cut, pick, and scrape the flesh away from the skeleton. Time and
patience are required, but if these are allowed there is no reason why
all the parts of the skeleton cannot be thoroughly studied. Specimens
which have been preserved in brine are more easily skeletonized.
The skeleton is entirely composed of cartilage which, in large species
of elasmobranchs and in old individuals of small species, becomes
impregnated with lime salts, in some cases to such an extent as to
resemble soft bone.
The parts of the skeleton are frequently grouped under two heads: the
_axial skeleton_, comprising the skull and vertebral column; and the
_appendicular skeleton_, including the pectoral and pelvic girdles and
the skeleton of the fins.
VERTEBRAL COLUMN. The vertebral column is divided into two regions,
thoracic and caudal, distinguished by the slightly different character
of the vertebrae. Remove the muscle and connective tissue from the
vertebral column for a short distance anterior to the first dorsal fin.
Care is required not to cut away small cartilages occupying the
positions of ribs. Now remove from the body about two inches of the
portion of the column exposed with any cartilaginous parts which may be
attached to the vertebrae. The vertical column is made up of segments,
called _vertebrae_. Each vertebra consists of a large ventral mass, the
centrum, and an arch, the neural arch, roofing over the dorsal surface
of the centrum; the arch is composed of several small plates of
cartilage. The opening enclosed by each centrum and its neutral arch is
the vertebral foramen; the joined vertebral foramina form the neural
canal, which is occupied by the spinal cord.
Separate one of the vertebrae from the rest. The centrum is deeply
concave at each end; such a centrum is termed amphicoelous. At the
middle of the centrum the concavities meet and thus a canal is formed
through it. This canal and the spaces between the ends of adjoining
vertebrae are filled by the remains of the notochord, a rather pulpy
structure extending from end to end of the vertebral column.
The concave faces of the vertebrae consist of much firmer cartilage than
the remaining portions, sometimes even calcified. Make a transverse
section through the middle of a centrum and observe the relations of the
parts.
On each side of the centrum, near the ventral edge, is a plate-like
projection, the transverse process. Attached to the extremity of this is
a slender cartilaginous _rib_.
Each _neural arch_ is made up of two distinguishable sets of plates. The
first consists of a pair of broad _neural plates_ extending upward from
each side of the centrum and uniting with each other dorsally. Between
the neural plates of two successive vertebrae is a pair (one on each
side) of _intercalary plates_ which also unite over the neural canal.
The intercalary plates are over the joint between the centra. Neural and
intercalary plates together make the lateral and dorsal walls of the
neural canal. The relations of these plates can sometimes be seen best
when the neural arch is cleaned, then cut away from the centrum, and
looked through toward the light.
In the lower part of each neural plate is a small foramen which allows
the passage of the ventral root of the spinal nerve. A foramen for the
dorsal root is found at about the middle of the intercalary plate.
Clean and remove some of the caudal vertebrae from the region just back
of the cloaca. In general they have nearly the same structure and
relations as the thoracic vertebrae, but have no transverse processes
and the plates of the neural arches are not so distinct. There is also
added a ventral arch similar in form to the neural arch. This is the
_haemal arch_, in which lie the caudal aorta and vein. Its roof is the
surface of the centrum, the sides are formed by pairs of plates which
correspond in number to the centra, and unite with each other ventrally.
Between the successive plates are openings for the passage of branches
of the artery and vein.
In this region foramina for the roots of the spinal nerves are found
only in every other pair of neural and intercalary plates. Toward the
tip of the vertebral column the relation of the neural and intercalary
plates to the centra becomes very irregular.
In Eugaleus the roof of the neural arch is formed by a row of small,
diamond-shaped plates which fit in between the other two sets. As these
plates correspond morphologically to the neural spines of higher
vertebrates, they may receive that name here. It is probable that the
dorsal portion of the arch in Squalus is composed of similar neural
spine elements which have become fused with the neural and intercalary
plates of each side.
SKULL. The skull is entirely cartilaginous, and comprises three
principal divisions: (1) the _cranium_, an undivided mass of cartilage
lodging the brain and the organs of smell, sight, and hearing; (2) the
_jaws_; (3) the _visceral arches_, or skeletons of the gill arches.
(1) The cranium. A blunt prolongation of the anterior extremity of the
cranium forms the _rostrum_, which supports the soft tissues of the
snout. At each side of the base of the rostrum the cranium widens
abruptly. On the anterior face of the widened portion and below the
posterior angles of the rostrum is a pair of protruding _olfactory
capsules_, in which the olfactory sacs are enclosed. An oval aperture in
the posterior wall of each capsule opens into the braincase and permits
the passage of the olfactory nerve through the cranium.
Back of the olfactory capsules are large lateral cavities, the _orbits_.
The dorsal edge of the orbit makes an overhanging ledge, known as the
_supra-orbital crest_. The projecting anterior and posterior angles of
the orbit are distinguished as the prae- and postorbital processes.
The portions of the cranium back of the orbit and at the sides of the
braincase form large lateral projections (_auditory capsules_)
containing the organs of hearing.
At the center of the nearly vertical posterior surface of the cranium is
a large opening, the _foramen magnum_, through which the spinal cord
passes.
At either side of and below the foramen magnum is a smooth articulatory
surface (_occipital condyle_) articulating with the centrum of the first
vertebra.
The flattened ventral surface of the posterior part of the cranium forms
the roof of the mouth, or palate.
In the mid-dorsal line of the cranium, between the prae-orbital
processes, is a small aperture opening into the brain cavity, the
_epiphysial foramen_. It is closed during life by a tough, fibrous
membrane. The stalk of the epiphysis extends to the under surface of
this membrane.
Between the auditory capsules is a deep depression in the roof of the
cranium in the floor of which can be seen the two small pores through
which the ducti endolymphatici pass into the capsules. Close behind them
are two larger openings for the perilymph ducts.
A pair of foramina passes through the inner edge of the prae-orbital
process; these permit the passage of the ophthalmic branches of the
trigeminal and facial nerves to the dorsal surface of the snout. Near
the bottom of the inner wall of the orbit is the foramen of the optic
nerve. In the postero-ventral angle of the orbit is the large
trigemino-facial foramen for the exit of branches of the trigeminal and
facial nerves; in front of it is the small oculo-motor foramen. The
extremely small foramen of the trochlear nerve is almost directly above
the optic foramen, near the top of the inner wall of the orbit. Close
below the trigemino-facial foramen is the small passage for the abducens
nerve. Below the abducens foramen is the transbasal canal. Behind and
below the trigemino-facial foramen are two foramina, through which pass
the hyomandibular branches of the facial nerve. The foramen of the vagus
nerve is close to the foramen magnum, upon the posterior surface of the
cranium. The foramen of the glossopharyngeal nerve is lateral to that of
the vagus, near the postero-lateral angle of the cranium.
The cranium of Eugaleus is much like that of Squalus, except that the
rostrum is formed by three rods, two dorsal and one ventral, which arise
from the front of the braincase and converge anteriorally until they
meet and fuse. The olfactory capsules are much larger and of heavier
cartilage than in Squalus. The auditory region similarly is more
prominent.
(2) The jaws. The jaws in reality are the first pair of visceral or gill
arches, and in spite of the modification which has taken place this
relation can be seen easily in the adult shark. The upper jaw consists
of a pair of _palato-quadrate_ cartilages, united medially by ligament,
and bearing the upper series of teeth. A large hooked palatine process
extends from each palato-quadrate cartilage upward along the inner wall
of the orbit. The lower jaw likewise consists of a pair of _Meckel’s
cartilages_, united medially (the union is called the symphysis), and
bearing the lower series of teeth. A pair of small labial cartilages,
which support the edges of the labial pockets, lie at each corner of the
mouth.
(3) Visceral arches. The first of the visceral arches is much larger and
heavier than the rest. It is known as the _hyoid arch_. Each side of the
arch consists of two rods of cartilage: (1) the _hyomandibular
cartilage_, which articulates with a distinct facet on the lateral
surface of the auditory capsule, and extends from here downward,
outward, and backward; (2) the _ceratohyal_ cartilage, which is movably
articulated to the hyomandibular and extends downward, forward and
inward. The ventral ends of the ceratohyals are united by a median,
plate-like _basihyal_.
The palato-quadrate and Meckelian cartilages are suspended from the
hyomandibular by several strong ligaments, the direct attachments of the
jaws to the cranium being of soft connective tissue only. Both the
hyomandibular and ceratohyal cartilages bear slender rods (_branchial
rays_) on their posterior edges, which support the anterior wall of the
first gill pouch. Note the position of the spiracle between the
mandibular and hyoid arches. The anterior wall of the spiracle is
strengthened by two small, flat, vertical cartilages, probably
homologous with the branchial rays of the gill arches.
The remaining five visceral arches differ little in their construction.
Dorsally, each has a flat, sickle-shaped _pharyngo-branchial_
cartilage attached to the vertebral column by fibrous bands. The
pharyngo-branchials of the last two arches are fused. Ventrad to each
pharyngo-branchial is an _epibranchial_ cartilage. The next segment of
each arch is formed by the _ceratobranchial_ cartilage. All the
epibranchials and ceratobranchials except those of the fifth arch bear
slender branchial rays. The ventral ends of the ceratobranchials
articulate with each other, the first being attached to the ceratohyal
by ligament. The second, third, and fourth arches have another more
ventral series of cartilages, the _hypobranchials_. The lower ends of
the hypobranchials are attached to a large median plate, the
_basibranchial_. The fourth ceratobranchial joins the third
hypobranchial, while the ceratobranchials of the fifth arch are attached
to the basibranchial directly. The basibranchial is composed of two
segments closely united by ligament; the anterior one narrow, the
posterior broad and flat in front, tapering to a sharp point behind.
Short teeth of cartilage, called gill rakers, project into the pharynx
from the inner edges of the arches.
A dorsal and a ventral series of _extra-branchial_ cartilages, thin,
slender plates, lie on the external side of each gill arch.
PECTORAL GIRDLE AND FIN. Remove from the body the pectoral girdle, with
the fins attached, and carefully scrape off the muscles from the
cartilaginous parts. It will be found that the support of the fin is
partly of cartilaginous plates and rods, partly of horny fibres (_dermal
fin-rays_) which overlie the extremities of the cartilages and extend to
the edges of the fin. These fibres are in two layers, one beneath the
skin of each side. They are formed in the dermis. A similar arrangement
of horny fibres is found in all the other fins.
The pectoral girdle passes across the ventral surface of the body and
upward on each side to the level of the vertebral column. The stout
ventral bar presents numerous facets for the origin and insertion of
muscles. The articular surfaces for the pectoral fins are well up on the
sides of the girdle. The slender dorsal end of each side of the girdle
consists of a separate bar of cartilage, movably articulated to the
lower portion. The ascending limb of the girdle, from the fin
articulations to the base of the cartilage just mentioned, is called the
_scapular portion_; the small bar is the _supra-scapular_; the ventral
bar between the fin articulations is the _coracoid portion_.
The cartilaginous skeleton of the pectoral fin consists primarily of a
row of three _basal_ cartilages, all articulating proximally with the
girdle. The middle basal is much the largest. Distal to the basals are
three rows of rod-like _radial_ cartilages, the proximal row being
articulated to the basals.
PELVIC GIRDLE AND FIN. Remove the pelvic girdle from the body with the
pelvic fins attached, and clean away the muscles.
The pelvic girdle consists of an almost straight bar of cartilage,
slightly thicker at its middle than at its ends, which lies transversely
in the ventral wall of the abdomen. To each end is attached a long
_basal_ cartilage which lies in the fin, close to and parallel with its
inner margin. A proximal series of slender _radial_ cartilages is
attached to the lateral side of the basal; a distal series of very short
radials lies outside of the first series, while the portion of the fin
beyond these is supported by the dermal fin-rays.
FIRST DORSAL FIN. Remove the mass of muscles on both sides of the base
of the fin down to the vertebral column. The principal cartilages of the
fin lie in the median connective tissue septum which separates the
dorsal musculature of the two sides of the body. The basal cartilages of
the fin are attached to the vertebral column by means of this septum. It
is best to remove the underlying portion of the column with the fin. The
cartilages can then be scraped perfectly clean. The skeleton of the fin
is composed of three rows of cartilages: (1) a basal row consisting of
one very large, flat plate and two or three smaller ones posterior to
it; (2) an intermediate row of several plates of nearly equal size; and
(3) a distal row of several very small plates. The intermediate and
distal rows extend beyond the body musculature into the base of the fin.
The remainder of the fin is supported by the dermal rays. In front of
the cartilages which have been mentioned is the strong spine of dentine
(see p. 5), with its free portion sheathed by an enamel-like covering.
SECOND DORSAL FIN. Remove this from the body in the same manner as the
first dorsal. Its structure follows the same general plan, the
differences being minor ones of shape, size, and number of plates.
Several thin cartilaginous plates are sometimes formed in the median
septum in front of the spine.
CAUDAL FIN. Only one side of the caudal fin should be cleaned, as when
both sides are cleaned there is danger of breaking the delicate
cartilages. The cartilaginous skeleton of the caudal fin consists of a
row of slender rods along the dorsal side of the vertebral column,
extending to its tip. There are no cartilaginous elements in the fin
ventral to the vertebral column. By far the greater part of the caudal
fin is supported by the two layers of horny fin-rays only.