The main receptor or sense organs of Scoliodon include (i) olfactory organs, (ii) eyes, (iii) ears, (iv) neuromasts or lateral line organs, and (v) ampullae of Lorenzini.
1. Olfactory Organs
- A pair of nasal or olfactory sacs are the main olfactory organs.
- They are characteristically correlated with a highly developed sense of smell for the perception of chemical substances dissolved in water.
- They are situated ventrally in the snout one on either side.
- Each olfactory sac is large and oval pouch.
- They are ectodermal blind pouch covered by a thin membrane.
- They are located inside the thin cartilaginous olfactory capsule of the skull.
- Each olfactory sac contains receptor and supporting cells.
- Sensory hairs project from the free ends of receptor cells.
- The basal ends of receptor cells are continued into fibres of olfactory nerve going to the olfactory lobe of the brain.
- Each olfactory sac opens to the outside ventro-laterally by the external nares.
2. Eyes
- They are also called photoreceptor organs.
- Scoliodon has a pair of large eyes.
- The eyes are located inside the orbits, one on either side of the head.
A. Eye Muscle
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- The eyeball is elliptical in shape.
- The eyeball remains attached to the inner wall of the orbit by 6 eye muscles and a cartilaginous stalk called optic pedicle.
- There are two groups of eye muscles. They are oblique muscles and recti muscles.
- There are 2 oblique muscles and they are 1. inferior oblique muscle and 2. superior oblique muscle.
- The inferior oblique muscle is attached to the antero-ventral side of the eyeball.
- The superior oblique muscle is attached to the antero-dorsal side of the eyeball.
- The action of oblique muscles twists the eyeball.
- There are 4 recti muscles and they are 1. inferior rectus muscle, 2. superior rectus muscle, 3. anterior rectus muscle, and 4. posterior rectus muscle.
- The inferior rectus muscle is attached to the postero-ventral side of the eyeball.
- The superior rectus muscle is attached to the postero-dorsal side of the eyeball.
- The inferior and superior rectus muscles serve to rotate the eyeball in the vertical plane.
- The anterior (or internal) and posterior (or external) rectus muscles are inserted anteriorly and posteriorly.
- The anterior and posterior rectus muscles serve to rotate the eyeball in the horizontal plane.
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B. Internal Structure
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- The outermost layer of the eyeball is the sclerotic layer.
- The sclerotic layer is divided into two parts. The anterior part is the cornea and the posterior part is the white of the eye-ball.
- Cornea is transparent and it permits the light rays to enter into the eye. It is covered with thin and transparent conjunctiva.
- Choroid layer is located underneath the sclerotic layer.
- Choroid is a richly vascularized and pigmented layer.
- At the anterior end of the choroid, just behind the cornea, the choroid forms a pigmented circular disc called the iris.
- The iris has an aperture at the center named pupil. The whole eyeball is lightproof except the pupil.
- A large crystalline lens lies immediately behind the pupil. The pupil can not contract and dilate.
- Suspensory ligaments are located just behind the iris and they keep the lens in position.
- The innermost layer of the eyeball is the retinal layer.
- This is the only light-sensitive part of the eyeball.
- The retina consists mainly of elongated receptor cells, or rods, which are connected with the fibres of the optic nerve. Scoliodon has no cone cells.
- A transparent saline fluid, the aqueous humour, fills the small chamber in front of the lens.
- A jelly-like mass, the vitreous humour, fills the very large chamber behind the lens.
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C. Mechanism of Sight/Vision
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- Eyes in sharks are large but far separated, so binocular vision is not possible.
- The power of accommodation is poor because of the non-contractile pupils and little change in the shape of the lens.
- However, the lens can be shifted forward to catch more light.
- Since the retina is without cones, the fish is colour-blind and the eye cannot discriminate minute details.
- But the eye is adapted for near vision in dim light and fish can see its prey.
- Light rays pass through the cornea, pupil, and lens and reach the retina.
- The image formed by the retina is transmitted to the brain with the help of optic nerves.
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3. Internal Ear
- In Scoliodon, there are no external or middle ears.
- Only internal ear is present called the membranous labyrinth.
- It is a delicate membranous ectodermal sac.
- It is found embedded in the cartilaginous olfactory capsule, one on either postero-lateral side of the cranium.
- The membranous labyrinth is innervated by the auditory nerve.
- Internal ears or membranous labyrinths of Scoliodon are also called stato-acoustic organs because of their two important functions. The functions are balancing and hearing.
4. Neuromast or lateral Line System
- It is a system of sense organs concerned with life in water.
- In Scoliodon it includes (a) lateral lines, (b) neuromast organs, and (c) pit organs.
a) Lateral Lines
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- A lateral line runs along either lateral side of trunk and tail.
- It contains a mucus-filled ectodermal canal.
- It opens to the surface by minute pores.
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b) Neuromast Organs
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- These are small groups of receptor and supporting ectodermal cells located in the lateral line canals.
- Receptor or sensory cells bear tiny stiff sensory hairs.
- They are supplied with nerve fibres of facial, glossopharyngeal or vagus nerves.
- Neuromast organs are rheoreceptors or current receptors.
- They can perceive very low-frequency vibrations.
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c) Pit Organs
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- Small ectodermal pits are found scattered on the dorsal and lateral surfaces of head of Scoliodon.
- Each pit organ consists of sensory hair cells with supporting cells.
- They are innervated by nerve fibres of the VII cranial nerve.
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5. Ampullae of Lorenzini
- These are situated in grape-like bunches in the snout of Scoliodon.
- They open on the dorsal and ventral surfaces of the head by their minute apertures.
- Each ampulla or ampullary sac is supplied with a nerve fibre from the branches of the VII cranial nerve.
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