The endoskeleton of dogfish is entirely made of cartilage.
The axial skeleton includes
(1) Skull and
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(2) Vertebral column
While the appendicular skeleton comprises of
(1) Pectoral and
(2) Pelvic girdles and
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(3) Skeleton of fins.
Skull :
Skull or chondrocranium of dogfish consists of
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(1) Cranium and capsules for sense organs and
(2) Visceral skeleton that supports jaws, tongue, and gill apparatus.
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(1) Cranium and sense capsule:
Cranium or brain box encloses and protects the brain. It is composed of a single piece of cartilage to which the olfactory and auditory sense capsules are intimately fused.
It is a simple structure perforated by several foramina for nerves and blood vessels. Its arched roof or dorsal side remains incomplete leaving two spaces or fontanelles.
Cranium is roughly divisible into 4 regions
(a) Occipital
(b) Auditory
(c) Orbital and
(d) Ethmoidal
(a) Occipital region:
This is a posterior part of the cranium having a very large median hole, the foramen magnum through which brain is continued into spinal cord.
On either side of foramen magnum is a small rounded occipital condyle for articulation with first vertebra.
Each condyle bears a foramen on its outer side for Xth or vagus nerve. Dorsally the occipital region shows a slight mid longitudinal ridge or occipital crest.
(b) Auditory region:
It lies anterior to occipital region and consists of two lateral optic or auditory capsules enclosing ears fused with the middle part of cranium.
On the dorsal side of cranium a depression, the parietal fossa contains two pairs of apertures, anterior pair for endolymphatic ducts and posterior pair for perilymphatic foramina.
Three ridges on each auditory capsule indicate the internal position of three semicircular canals of membranous labyrinth. Below the lateral postorbital groove is the articular surface for the hyomandibula of 2nd or hyoid arch.
(a) Orbital, region:
This region lies in middle of the cranium in front of auditory region. On each lateral side it carries a large hole called orbit which encloses the eye during the life.
Each orbit is bordered dorsally by a supra orbital ridge, anteriorly by a shell slender pre-orbitai process and posteriorly by a longer and stouter post orbital process. Within the orbit the cranial wall is pierced by a number of foramina for nerves.
(b) Ethmoidal region:
This region lies in the anterior part of cranium, consists two nasal or olfactory capsules and rostrum.
The lateral olfactory capsules are thin walled; open ventrally, each lodging an olfactory sac. Their cavities are separated from each other by a common thin, median vertical plate, the mesethmoid or internasal septum.
The cranial roof presents a large depression, the anterior fontanelle covered by a connective tissue membrane. In front of the fontanelle project three rod-like rostral cartilages, one ventro median and two dorso-lateral. The three meet anteriorly and form the skeleton of snout or rostrum.
(2) Visceral skeleton:
It forms the jaws and the skeleton of pharynx and includes a series of 7 pairs of U-shaped, visceral arches that encircle the buccal cavity and pharynx.
(a) First or mandibular arch:
It is found on either side and is made up of two segments. The dorsal segment called palato-pterygo-quadrate forms the upper jaw which is attached to cranium by a ligament.
The lower segment called Meckel’s cartilage forms the lower jaw. The two jaws bear teeth and are joined by a hinge joint at the angle of mouth.
(b) Second or hyoid arch:
Second visceral or hyoid arch consists of three segments. The ventral median and single basihyal is common to both sides.
It supports buccal floor and tongue. The lateral ceratohyal also lies in buccal floor behind the Meckel’s cartilage of first arch.
The dorsal segment or hyomandibula is short but stout and connects both the jaws posteriorly. Its upper end articulates with cranium.
Thus jaws are not connected directly to cranium in dogfish. Such an arrangement or suspensorium in which hyomandibula serve to suspend jaws from cranium is called hyostylic.
(c) Branchial arches:
Behind the hyoid arch, the remaining 5 visceral arches are known as branchial arches as they support the pharyngeal gills and serve in their respiratory movements.
Each branchial arch on either side consists of 4 segments connected by joints. These are a dorsal pharyngobranchial, lateral epibranchial and ceratobranchial and a ventral hypobranchial.
All the branchial arches are connected to a common mid-ventral dagger-shaped cartilage, representing the fused basibranchial segments of all arches.
Only the epibranchials and ceratobranchials bear gill rays which support gills. Besides, there are 4 pairs of extra-branchial cartilages lying at right angles to the first 4 pairs of branchial arches.
Vertebral column:
The vertebral column of adult dogfish consists of about 130 vertebrae arranged one behind the other.
They develop around the persistent notochord present as an elastic axis of vacuolated cells. There are only two regions in the vertebral column-trunk region and caudal region.
(1) Trunk Vertebra:
A typical trunk vertebra consists of a thick centrum deeply concave at both ends. Such centra are called amphicoelous.
In a transverse section the centra show calcified and uncalcified areas and such a centra or vertebra is called asterospondylous.
The centrum bear dorsally a neural arch enclosing a neural canal. All the neural canals of vertebra form a tube like structure that encloses the spinal cord.
The foramen of dorsal and ventral root of spinal nerves lies at the upper and lower margins of the neural arch respectively. To each ventro lateral border of centrum is attached a broad plate like transverse process.
(2) Caudal Vertebra:
It is same as the trunk vertebra but differs in one aspect. Its transverse processes fuse together to form haemal arch enclosing a haemal canal below the centrum. The caudal artery and vein run through the haemal canal.
Pectoral girdle and fins
(1) Pectoral girdle:
It is embedded in the latero-ventral body wall, just posterior to the gills above the heart and protects it.
It is a U-shaped structure made of right and left halves united mid-ventrally. Each half is made of a dorsal rod like scapular portion and a flattened coracoid portion. A distinct suprascapular cartilage marks the dorsal end.
Pectoral girdle provides attachment surface, to the body myotomes, to muscles working for swallowing and breathing, It also supports and protects the heart.
(2) Pectoral fin skeleton:
The basal part of each pectoral fin is made by 3 basal cartilages called propterygium mescpterygium and metapterygium.
These articulate with three little facets forming the glenoid surface at the junction of scapular and coracoid portions of pectoral girdle.
From the basal cartilage extend numerous radial cartilages or somactidia each made up of 2 to 3 segments.
The distal somactidia bear two sets of long numerous, thread like stiff and horny fin rays or ceratotrichia which support the peripheral membranous part or vane of the fin.
Pelvic girdle and fins
(1) Pelvic girdle:
Pelvic girdle is also not attached to the vertebral column. It consists of a simple flattened and transverse cartilaginous rod, called ischiopubic bar.
It lies embedded in the ventral abdominal wall in front of cloaca. It has a small blunt iliac process on either end, projecting above the acetabular facet for articulation of the basal cartilage of pelvic fin.
(2) Pelvic fin skeleton:
Skeleton of a pelvic fin includes a single large and curved basal cartilage, the basipterygium, 15 or more slender radials or somactidia and numerous distal fin rays or ceratortichia.
(3) Claspers:
In the male dogfish a clasper is attached to the distal end of basipterygium. Its skeleton includes an elongated tubular axial cartilage grooved dorsally.
The anterior opening of groove or apopyle communicates with cloaca and a glandular sac or siphon.
The posterior opening of the groove, the hypopyle opens on a sharp pointed style enclosed within a dorsal and a ventral sheating plate.
Skeleton of median fins is similar to that of paired fins.
Myotomes and locomotion :
Parietal muscles of body wall beneath the skin form the bulk of body musculature. In a skinned dogfish these will be seen arranged in a series of zigzag muscle bands or segments called myotomes or myomeres.
They are seperated from one another by tough connective tissue partitions or septa called myocommata when myotomes contract, they pull the myocommata which cause the bending of backbone and the rest of the body. Paired and unpaired fins help in swimming and balancing the body during swimming.