Shark: Excretory System, Reproductive System, Nervous System, and, Sense Organs
The excretory system is closely related to the reproductive system. Males and females differ significantly in several key ways. A pair of opisthonephric kidneys, which are long, thin, highly vascularized strips of tissue that filter nitrogenous wastes from the blood, chiefly urea, make up the excretory system. The archinephric ducts are responsible for kidney drainage. The more caudal section of the archinephric duct also carries urea produced by the kidneys in males, whereas the cranial portion of the duct serves primarily a reproductive purpose. The female has smaller and shorter archinephric ducts. The two tiny auxiliary urinary ducts in males that drain into the urogenital sinus receive urine from the archinephric ducts. The male auxiliary urinary ducts and the archinephric ducts discharge into the urinary papilla, a little projection into the cloaca. In males, the urogenital papilla is another name for the urinary papilla. Materials from the digestive, reproductive, and excretory systems are delivered to the cloaca. Excretory waste leaves the cloaca and travels outside. A duct connects the rectal or digitiform gland to the big intestine. This tubular organ helps the dogfish maintain a healthy osmotic equilibrium in its bodily fluids by excreting extra sodium chloride.
The testes, which are thin, soft organs situated dorsal to the liver lobes, are the male shark’s gonads. During the breeding season, the testes are particularly active and generate sperm. A narrow mesentary called the mesorchium supports the testicles. This mesentary connects the testes to several blood vessels and nerves. The numerous tiny efferent ductules transport sperm from the testes to the epididymis and are also seen in the mesorchium. Each kidney’s anterior part contains the sperm-conducting tubule-containing epididymis. There is hardly any excretory activity in this region of the kidney. Leydig’s gland is the region of the kidney that lies immediately caudal to the testes and below the epididymis. A thick, milky seminal fluid is the byproduct. Sperm are transported by the massive, tightly coiling vas deferens from the epididymis. The seminal vesicle is the largest caudal section of each vas deferens. Sperm and seminal fluid travel into the sperm sac, which is located at the caudal end of each seminal vesicle. A urogenital sinus is created when the caudal ends of the seminal vesicles and the sperm sacs on either side unite. The urogenital papilla is penetrated by the two urogenital sinuses. Just cranial to the cloaca, male sharks also have two syphons. The sacs fill with seawater when the claspers are bent. Water is forced out of the sacs during copulation as a result of the sacs’ constriction. The female oviduct is reached after the water and sperm have passed through a groove in the clasper. In female sharks, the mesovarium, a thin layer of connective tissue, supports a pair of soft, cream-colored ovaries that are located dorsal to the liver. Eggs are produced by ovaries. Mature eggs are discharged into the bodily cavity during ovulation and enter the oviducts. The eggs travel to the uterus in lengthy tubes called oviducts. The oviducts connect within the falciform ligament and share an ostium, which is where eggs enter the body. Typically, the oviducts are used to fertilize shark eggs. The nidamental gland is an expanded region close to each oviduct’s anterior end. A thin, protein-rich shell that protects the eggs is secreted by the nidamental gland. The uterus is created by enlarging the caudal part of the oviduct. The dogfish is ovoviviparous. Embryos are created from fertilized eggs in the uterus. Every egg has a unique yolk. The young are delivered through the cloaca once the embryos have reached complete development, which could take up to two years.
The peripheral nervous system, which is made up of cranial and spinal nerves, and the central nervous system, which is made up of the brain and spinal cord, are the two primary divisions of the neurological system of sharks. Separate studies will be done on the sensory organs. The chondrocranium encloses and safeguards the shark’s brain. The meninx, a thin, highly vascular membrane, also encloses and shields the brain. The forebrain, also known as the prosencephalon, the midbrain, sometimes known as the mesencephalon, and the hindbrain, also known as the rhombencephalon, make up the shark’s brain.
There are two divisions in the prosencephalon. The telencephalon is located farther anterior. The olfactory bulbs and olfactory tracts are located in the rostral region of the telencephalon. These are linked to the brain’s cerebral hemispheres and receive and transmit sensory input from the olfactory epithelium, giving sharks the ability to smell. The back of the telencephalon is made up of the paired cerebral hemispheres. The diencephalon, the second part of the forebrain located anterior to the optic lobes and narrow and depressed, is posterior to the telencephalon. The tela choroidea, a thin membrane, covers the diencephalon’s roof. The anterior choroid plexus, which secretes cerebrospinal fluid into the ventricles, is formed by vascular folds that stretch from the tela choroidea into the third ventricle of the cerebral hemispheres. The paraphysis, which is a component of the telencephalon, is formed by the anterior portion of the tela choroidea. The pineal gland or epiphysis is located in the caudal region of the diencephalon. The unknown is the function of this shark gland. The epithalamus is located on the diencephalon’s roof closest to the cerebral hemispheres. The thalamus is formed by the ventral extension of the diencephalon’s lateral walls. The hypothalamus is the diencephalon’s floor.
The two substantial optic lobes, which are located in the mesencephalon, define the midbrain. The optic lobes interpret impulses sent by the optic nerves. The metencephalon and the myelencephalon are the two divisions of the rhombencephalon or hindbrain. The metencephalon is the frontal region of the hindbrain. The cerebellum is located in the metencephalon’s dorsal region. The cerebellum partially covers the optic lobes. The cerebellum is in charge of regulating muscle responses. Particularly in animals that are very active and constantly moving, the cerebellum is enormous. The cerebellar ventricle is the cerebellum’s internal space. A pair of auricular lobes, which serve as foci of equilibrium, is located at the back of the cerebellum. The posterior part of the hindbrain, also known as the myelencephalon or medulla oblongata, is located directly behind the cerebellum. The spinal cord is where the extended medulla tapers. The fourth ventricle, a sizable cavity, is located within the medulla. The medulla serves as a nerve conduit connecting the brain and spinal cord. It also includes centers that manage circulation.
The telencephalon and the olfactory tract are evident in a ventral view of the brain. The optic chiasma is visible caudally. Before entering the brain, the optic nerves intersect at the optic chiasma. The hypothalamus and the pituitary gland, or hypophysis, are caudal to the optic chiasma. The production of various hormones is carried out by the pituitary gland, an endocrine gland. A dorsal hollow nerve cord is one of the most distinguishing features of all animals. A midsagittal slice of the brain reveals evidence of this trait, in the ventricles. The telencephalon contains the first and second ventricles. These join the third ventricle, which is located inside the diencephalon, through a small passage called the foramen of Monro. Through the Sylvius aqueduct, the third ventricle joins the fourth ventricle. The medulla’s cavity makes up the fourth ventricle. The cerebrospinal fluid is present in the ventricles.
The cranial nerves and spinal nerves are both a part of the peripheral nervous system. Eleven pairs of cranial nerves exit the spine without first going through the spinal cord, connecting directly to the brain. The short terminal nerve (number 0), which starts with the olfactory nerve and spreads into the nasal areas, is the most anterior. (The terminal nerve’s identification of 0 reflects the fact that it was found after the numbering scheme for the other 10 cranial nerves had already been established.) Numerous tiny, independent sensory fibers make up the olfactory nerve (I). The nasal epithelium is where the olfactory nerve begins, and the olfactory bulb is where it terminates. The retina of the eye is where the optic nerve (II), a broad white nerve tract, begins. The ventral surface of the diencephalon is where this sensory nerve enters. The optic chiasma, or the junction of the two optic nerves, can be seen in a ventral view of the brain. Each one enters the brain’s side across from the eye where it started. The oculomotor nerve (III), a branching somatic motor nerve that innervates four of the six main muscles of the eye, also leaves the ventral surface of the brain. It leaves the mesencephalon ventral surface and finishes in the four muscles. The superior oblique muscle of the eye receives impulses from the trochlear nerve (IV), a motor nerve that leaves the mesencephalon’s dorsal surface. A sizable mixed nerve with both sensory and motor capabilities is the trigeminal nerve (V). It separates into four branches after being joined to the lateral side of the medulla by nerves VII and VIII. The superficial ophthalmic nerve, a sensory nerve that exits in the skin of the rostral part of the head, is created when one of these branches, the superficial ophthalmic branch, merges with a branch of the facial nerve. A motor neuron called the abducens nerve (VI) emerges from the ventral surface of the medulla. The lateral rectus muscle of the eye receives impulses from it. It is a mixed nerve, the facial nerve (VII). The trigeminal nerve and facial nerve unite when they leave the brain after the facial nerve emerges from the front of the medulla. The hyoid arch, spiracle, and lateral line organs on the rostrum are all innervated by the facial nerve. Along with nerves V and VII, the statoacoustic nerve (VIII) is a sensory nerve that leaves the medulla and conveys impulses from the inner ear. Just behind the statoacoustic nerve in the medulla is a mixed nerve known as the glossopharyngeal nerve (IX). The first-gill arch receives impulses from the glossopharyngeal nerve and also receives them there. The vagus nerve (X), the final cranial nerve, is a mixed nerve that projects branches to the lateral line, the remaining gills, the head, and the abdominal organs. It emerges from the posterior end of the medulla.
The central nervous system includes the spinal cord as well. The spinal cord begins in the medulla and extends throughout the vertebral column of the body. The spinal cord’s cross-section reveals a lot about its composition. An H-shaped region of grey matter, made up of nerve cell bodies and synaptic connections, is surrounded by a section of white matter made up of myelinated nerve fibers. A dorsal horn and a ventral horn are present in the grey matter region. The grey matter’s core is cut by the central canal. The many spinal nerves that protrude between each pair of vertebrae are created by the spinal cord. The spinal nerves are mixed nerves that transmit impulses from the brain to the numerous organs and body regions as well as impulses back to the brain. Each spinal nerve contains two roots, a dorsal root, and a ventral root, which come together outside the vertebrae to form a single spinal nerve, according to a depiction of the spinal cord. While the ventral root exclusively contains motor neurons, the dorsal root also contains sense neurons. The sensory nerve cell bodies are found in the dorsal root ganglion, which is located on the dorsal root just before it connects with the ventral root.
The Sense Organs
The ampullae of Lorenzini, the lateral line organ, the olfactory organ, the ear, and the eye are the shark’s five primary sense organs. There are many pores throughout the head’s surface. These are the entrances to the Lorenzini ampullae, which are crucial for locating prey and are used to sense weak electrical fields. They may also help with predator detection and avoidance. Each pore opens into a tiny tube, as shown in this diagram, which then connects to the ampulla, which resembles a bulb. Sensory cells in the ampullae can pick up very little variations in water temperature, salinity, and electrical currents. Delicate nerve fibres from facial nerve branching are used to innervate these cells. The lateral line organ extends from the shark’s nose to its tail down its side. Numerous lateral line branches form linked canals on the cranium. Numerous pores that lead to a canal located under the skin can be seen on a schematic of the lateral line. The canal is lined with sensory receptors called neuromasts at various points. The ciliated cells known as neuromasts are bent by currents and other water motions, assisting in direction and movement. The olfactory organ is in charge of taking in external cues that are then translated into odours in the brain. The external nares are where the olfactory organs open. For the passage of water, each naris has an incurrent and an excurrent aperture. A flap of tissue divides the apertures. The external nares open into the olfactory sacs, which house the olfactory lamellae, as can be seen in this area of the rostrum. The olfactory epithelium covers the lamellae. Fibers from the olfactory tract, which connects to the brain’s olfactory lobe, innervate the olfactory epithelium. The chondrocranium’s cartilage houses the ear, a delicate tissue. The ear’s roles include hearing and balance maintenance. The endolymphatic pores, which are situated medially on the dorsal surface of the head, are the entrances to the ears. These holes connect to the membranous labyrinth through two endolymphatic channels. Three semicircular canals make up the inner ear, which is filled with endolymph, a fluid that flows with head movement. A swelling region termed an ampulla, found in each semicircular canal, is home to cristae, or clusters of sensory cells. The cristae locate movements in the endolymph and transmit impulses to the statoacoustic nerve’s nerve ends. The inner ear’s sacculus is its sizable centre chamber. Sand and calcareous deposits called otoliths, as well as sensory receptors termed maculae that resemble the cristae, are found in the sacculus. When the shark moves, the otoliths contact the maculae, which causes countermoves that maintain equilibrium.
The shark’s eye and the eyes of other vertebrates are very similar. The orbits, where the eyes are housed, are on either side of the head. The shark’s eyelids are inflexible. The eye is moved by six oculomotor muscles. The superior rectus, superior oblique, lateral rectus, inferior oblique, inferior rectus, and medial rectus are all visible from the back of the eye. The huge optic nerve and the optic pedicel, a cartilaginous rod that supports the eye, are both visible towards the back of the eye. A depiction of the eye’s cross-section exposes several structures. The sclera is located on the exterior. The front of the eye’s sclera, which is thin and transparent, is called the cornea. The choroid is the intermediate layer, which is darkly pigmented and highly vascularized. The iris is the choroid’s pigmented anterior section. The pupil, an aperture that controls how much light enters the eye, is located in the centre of the iris. The lens is concealed by the iris. To focus light, the shark’s lens does not alter form. Instead, it alters direction. The elastic suspensory ligament holds the lens in place. An anterior chamber filled with fluid is located between the lens and the iris. The membrane containing light receptors on the back of the eye is called the retina (the rods and cone cells). The optic nerve comprises the converging nerve fibres from the rods and cones. The vitreous chamber, which contains the viscous vitreous humour, is located between the retina and the lens. The vitreous humour supports the preservation of the eye’s shape. Contrary to popular opinion, sharks are not low-level creatures. Instead, it is an extremely advanced, highly specialized vertebrate descended from bony fish forebears. The shark’s internal anatomy shows that it is a close relative of the higher vertebrates, and its external morphology shows some important adaptations for its life as a marine predator.