Five important receptor or sense organs in dogfish

The main receptor or sense organs in dogfish include:

(1) Olfactory organs,

(2) Eyes,

(3) Ears,

(4) Neuromasts or lateral line organs and

(5) Ampullae of lorengini.

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(1) Olfactory organs :

These include a pair of olfactory or nasal sacs which lie ventrally in the snout, one on each side. They are characteristically large in elasmobranchs correlated with highly developed sense of smell as in dog.

After this characteristic, Scolodon is called dogfish. Each olfactory sac is housed inside the thin cartilaginous olfactory capsule of the skull.

Two rows of numerous closely set lamellae, called Schneiderian folds are held on the median connective tissue band, called median raphe.

These folds are covered by olfactory epithelium containing long receptor as well as supporting cells.

Receptor cells bear sensory hairs on free end and basal end is attached to the olfactory nerves. The olfactory sac opens to outside ventro-laterally by the external naris.

Three flaps of skin or nasal flaps over the naris direct water along a lateral in-current siphon and out through a median ex-current siphon.

(2) Eyes :

Scoliodon has a pair of large and well developed eyes or photoreceptors, housed in orbits of the cranium.

Internal structure of the eye:

The wall of eye in T.S. shows three layers similar to other vertebrates. Outermost sclerotic is cartilaginous and opaque.

Its frontal transparent part or cornea is flat and fused with an outer conjuctiva which is continuous with the lining of immovable eyelids.

Second layer choroid is richly vascular and pigmented. Its frontal part forms a circular curtain like iris around a vertical slit-like pupil which connot be dilated or contracted.

The innermost layer, retina consists mainly of elongated receptor cells or rods, which are connected with the fibers of the optic nerve.

In elasmobranchs, choroid contains cells with guanin crystals forming a light reflecting surface called tapetum lucidium.

In dim light it reflects light back upon retinal cells to increase their stimulation. In bright light it is covered by pigments so that light is not reflected.

A large crystalline lens lies immediately behind pupil held by a suspensory ligament with ciliary muscles.

A transparent saline fluid, the aqueous humor fills the small chamber in front of the lens. A jelly like mass, the vitreous humor fills the very large chamber behind the lens.

Working:

Eyes in dogfish are large but separated by a sufficient distance so that binocular vision is not possible.

Power of accommodation is poor because of non-contractile pupil and little change in shape of lens. However, lens can be shifted forward to catch more light.

Sharks can also achieve maximum light stimulation in dim conditions and prevent light scattering (blurring or halation) in bright condition, due to presence of tapetum of guanin plates. A shark is normally long sighted and also colour blind due to absence of cones in retina.

(3) Internal Ears :

In Scoliodon, the external or middle ear is absent, only an internal ear is present which is called membranous labyrinth. It is a delicate membranous ectodermal sac found embedded in the cartilaginous olfactory capsule one on either postero-lateral side of cranium.

Its main body or vestibule is laterally compressed and consists of dorsal utriculus and its posterior lobe, the sacculus.

A posterior outgrowth of sacculus is called lagena cochleae, which is the forerunner of an elaborate cochlea of higher vertebrates.

The anterior outgrowth of utriculus is known as recessus utriculi. Three slender tubes called semicircular canals arise from vestibule are at right angles to one another and each bears an ampulla at their ends.

The anterior vertical and horizontal canals arise from top of utriculus and open into its middle after forming their ampullae which lie anteriorly above the recessus utriculi.

The posterior vertical canal originates dorsally from sacculus and opens by its ampulla posteriorly into lagena cochleae.

The cavity of membranous labyrinth contains a fluid, the endolymph in which minute calcareous particles, the otoliths float.

From the endolymphatic cavity of sacculus arises a slender tube, the ductus endolymphaticus. It opens to outside on the top of cranium by its aperture lying in the parietal fossa.

The space between auditory capsule and membranous labyrinth is the paralymphatic space also containing a fluid, the paralymph. It opens outside through fenestra, which lies behind the small aperture of endolymphatic duct on the top of cranium.

Functions:

Besides audition, the internal ear of scoliodon has static function also and is called stato-accoustic organ. It is well innervated by auditory nerve.

(a) Static function:

Balance or equilibrium is the primary function. Movement of endolymph and otoliths stimulate sensory nerve endings in ampullae and vestibule, thus informing the animal about its position in water. The animal can detect changes in speed, direction and orientation and can adjust itself accordingly.

(b) Accoustic function:

Sacculus and lagena perhaps receive auditory stimuli and serve organs of hearing.

(4) Neuromast or Lateral Line System :

It is a system of sense organs concerned with life in water. Besides Fishes, it is also found in cyclostomes and aquatic stages of Amphibia.

It includes

(a) Lateral lines,

(b) Neuromast organs and

(c) Pit organs.

(a) Lateral lines:

A faint lateral line runs along either side of the trunk upto the tail. It contains a slender mucus-filled ectodermal canal sunk into dermis.

This canal opens to the surface by minute pores at regular intervals through a series of vertically running tubes.

Both the lateral line canals connect each other through a transverse commissural occipital canal on the dorsal surface. It extends into postorbital canal, which gives off numerous branches of canals.

(b) Neuromast Organs:

These are small groups of receptor cells and supporting ectodermal cells found in the lateral line canals.

These receptor cells bear stiff sensory hairs which project into the canal. They are supplied by the nerve branches of vagus nerves. They are very sensitive and work as rheoreceptors.

(c) Pit organs:

In the head region the dorsal and lateral surfaces bear numerous small ectodermal pits. Each pit organ consists of sensory hair cells, supporting cells.

They are innervated by nerve fibers of VII cranial nerves. They are considered to be isolated individual neuromasts or rheoreceptors. They are especially abundant in rays.

(5) Ampullae of Lorengini :

These are grape like structures found in bunches in the snout of dogfish, open on the dorsal and ventral surface by their minute individual apertures.

Each pore leads to an elongated mucus fiiled tubule and terminates below in 8 to 9 bulb-like chambers, the ampullary sacs, arranged radially around a central core, the centrum.

Two types of cells occur in ampulla or ampullae, pear shaped gland cells and pyramidal sensory hair cells.

Each ampulla or ampullary sac is supplied with a delicate nerve fiber from the branches of VII cranial nerve.

Ampullae of Lorenzini were previously regarded as neuromast organs. Lateon Sand (1938) observed that they are actually thermoreceptors and respond to slight changes in temperature.