In this article we will discuss about:- 1. Habitat and Morphology of Bhetki Fish 2. Skeletal System of Bhetki Fish 3. Muscular System 4. Body Cavity 5. Alimentary System 6. Vascular System 7. Respiratory System 8. Nervous System 9. Receptor Organs 10. Endocrine System 11. Excretory System 12. Reproductive System 13. Life-History.
- Habitat and Morphology of Bhetki Fish
- Skeletal System of Bhetki Fish
- Muscular System of Bhetki Fish
- Body Cavity of Bhetki Fish
- Alimentary System of Bhetki Fish
- Vascular System of Bhetki Fish
- Respiratory System in Bhetki Fish
- Nervous System of Bhetki Fish
- Receptor Organs in Bhetki Fish
- Endocrine System of Bhetki Fish
- Excretory System of Bhekti Fish
- Reproductive System in Bhetki Fish
- Life-History of Bhetki Fish
1. Habitat and Morphology of Bhetki Fish:
The Bhetki is a bony fish found in saline estuaries. It occasionally visits fresh water streams. The animal is one of the relished food fish and is abundantly available during the winter season.
The spindle-shaped body is compressed from side to side and perfectly streamlined, so as to facilitate swift movement through water. The outer surface of the body is covered with a soft mucus- producing epidermis. Beneath this are dermal scales which are arranged in rows in an imbricate fashion like the tiles on a roof.
The free posterior margin of each scale is provided with sharp spine-like outgrowths or ctenes. This type of spine-bearing scale is known as the ctenoid scale. A second type, known as the cycloid scale, is found in other common fishes such as Rohu (Labeo rokita) and Catla (Cotla catla).
A cycloid scale has no spinous outgrowths and its free posterior margin is smooth. The scales constitute the exoskeleton for protecting the body from external violence. The dermis, in which the scales are embedded, is composed of connective tissue, pigment cells and sensory receptors. It is thicker than the epidermis and the two together form the skin of the fish.
The body of the fish is divisible into three regions:
(ii) Trunk, and
The head extends from the tip of the snout to the posterior margin of the operculum (gill-cover), the trunk from this point to the anal opening, and the remainder constitute d post-anal tall (Fig. 121). There is an eye on each side of the head.
The eyes axe without eyelids and covered by a protective transparent membrane. In front of each eye, there is a pair of nostrils which do not open into the buccal cavity and hence serve no purpose in breathing.
The large month is terminal; it is surrounded by well-developed jaws which bear fine teeth. There is a bony operculum or gill-cover on each side of the head. Attached to the margin of the operculum, there is a flap-like branchiostegal membrane supported by bony rods.
The operculum and the branchiostegal membrane are movable structures which form the outer boundary of the gill-chamber, one on each side of the body. Each chamber contains four comb-like gills. Behind the operculum is an oblique slit, the opening of the gill-chamber, which can be tightly closed by the branchiostegal membrane. This opening serves for the exit of the respiratory water.
There is a dark lateral line on either side of the body, extending backwards through the trunk and tail regions. This is composed of a row of minute pores which communicate with deep- seated sensory cells, thus forming the lateral line sense organs by which the fish can understand vibrations of low frequency in its watery environment.
There is a depression, called vent, on the ventral border of the fish at the junction of the trunk and the tail. Into this open the anus in front, urinary opening behind, and the two gonopores in between them at the two sides.
The tail is supported by powerful muscles and constitutes an efficient oar for propelling the fish forwards. The fish bears certain appendages known as the fins. The fins are membranous extensions of the skin and are supported by fin-rays.
The fins are either:
(i) Median unpaired fins, or
(ii) Lateral paired fins.
The median fins are:
(a) Anterior and posterior dorsal fins placed on the dorsal surface of the fish and united at their bases: the anterior dorsal fin is supported by seven spinous rays of which the third one is the largest, and the posterior dorsal fin is supported by twelve rays of which the first one is spinous and the rest art soft;
(b) The caudal fin, at the end of the tail, is a fan-shaped expansion supported by 10 to 20 bony rays;
(c) Anal or ventral fin, placed on the ventral border of the post-anal tail, is supported by about a dozen fin-rays of which the first three are spinous; of the three spinous rays the third one is the largest.
The paired fins are:
(a) Pectoral fins, one on each side of the body, and placed behind the opercula; each pectoral fin is supported by 14 bony rays;
(b) Pelvic fins are placed on the ventral side of the body below and behind the pectoral fins; each bears a spinous rod in front followed by four bony rays.
The fins serve chiefly for steering, balancing and checking of speed, whereas the muscular tail is the effective organ of locomotion.
2. Skeletal System of Bhetki Fish:
The dermal scales and the fin-rays constitute the exoskeleton. The endoskeleton consists of the skull, vertebral column, ribs, pectoral girdle, and the small plate-like pterygiophores supporting the fin- rays.
(A) The skull of Bhetki is a complex structure and is composed of the cranium or brain-case housing the brain, the paired sense capsules (olfactory, optic and otic), the jaws, the hyoid, and the branchial arches supporting the tongue and gills.
The skull is tightly fixed to the vertebral column so that the fish cannot turn its head from side to side. The roof of the cranium is formed posteriorly by the parietals and anteriorly by the frontals.
The posterior part of the cranium is composed of four bones:
(i) A basi- occipital with a single occipital condyle,
(ii) A supra-occipital forming the occipital crest and
(iii) Two ex-occipitals, one on each side of the foramen magnum through which the brain becomes continuous with the spinal cord.
Ventrally, and in front of the basi- occipital there is:
(i) A small Y-shaped basi-sphenoid,
(ii) A largely sphenoid on each side of the basi-sphenoid and
(iii) A pair of Orbito Spbenoids. The orbits housing the eyes, are surrounded by a ring of scale-like bones the anterior of which is called the lacrimal. The nasal capsule includes the ethmoid which lies anterior to the frontals. Its roof is formed by the nasals which partially overlap the ethmoid, and its floor is formed by the vomers which lie in front of the para-sphenoid.
The otic capsule, containing the ear, is composed of five bones:
Epiotic, opisthotic, pro-otic, pterotic and sphere otic.
The upper jaw is composed of an outer series of membrane bones and an inner series of cartilage bones.
The true jaw, on each side, is formed by the outer series of three bones:
(i) Premaxilla and
(ii) Maxilla, bearing teeth; and
(iii) A scale-like jugal attached to the posterior end of the maxilla.
The inner series is the palatoquadrate bar, which consists of
(i) A tooth-bearing palatine articulated to the ethmoid;
(ii) A pterygoid in the middle, represented by ecto-, meso-, and meta-pterygoid bones;
(iii) A quadrate posteriorly which articulates with the lower jaw.
The visceral arches consist of seven pairs of cartilaginous bars which support the jaws, tongue and gills. All of them are more or less ossified in later life.
The mandibular arch (first arch) is composed of two parts on each side:
(i) The dorsally placed palatoquadrate which has been already described, and
(ii) The ventrally placed Meckel’s cartilage which forms the lower jaw.
Each lower jaw is composed of Meckel’s cartilage, which is en-sheathed by three membrane bones:
(i) An anterior tooth bearing dentary,
(ii) A posterior articular which articulates with the quadrate of the upper jaw and,
(iii) A small angular at the posterior end of the articular.
The hyoid arch (second arch) is in part a support for the tongue and the gill-cover. Each hyoid consists of a hyomandibular in front and a symplectic behind. Ventral to these two, there are several cartilaginous rods—epihyal, interhyal, ceratohyal and basihyal, which support the tongue as well as the floor of the buccopharynx.
Behind the symplectic and connected with the hyoid, there are certain bones supporting the gill-cover and branchiostegal membrane.
(iii) Sub- opercular and
There are seven branchiostegal rays supporting the branchiostegal membrane on each side. The third, fourth, fifth and sixth arches constitute the branchial arches which support the gills on their outer curvatures, and spiny gill rakers on their inner curvatures.
The small seventh arch does not bear any gill but only pharyngeal teeth are present on its inner curvature.
(B) The vertebral column of Bhetki is composed of numerous amphicoelour vertebrae, and is divided into two regions, precaudal and caudal.
A typical precaudal vertebra from the trunk region consists of:
(i) A spool-like centrum, concave at both ends (amphicoelous);
(ii) A dorsally placed neural arch to lodge the spinal cord;
(iii) A slender neural spine in the mid-dorsal part of the neural arch and
(iv) Paired transverse processes or parapophytes projecting out from the sides of the centrum.
Articulated to the distal ends of the transverse processes, there are ventral ribs curving downwards to enclose the viscera. In addition to these, there are paired zygapophyses or articular processes by which successive vertebrae articulate with one another.
In a caudal vertebra from the tail region, the centrum bears a ventrally placed haemal arch crowned by a slender haemal spine in addition to the dorsally placed neural arch and neural spine. The haemal arch protects the caudal vessels. Ventral ribs are not found in caudal vertebrae.
(C) The pectoral girdle consists of several bones which support the pectoral fins on either side. Each half of the girdle is composed of a dorsal scapula and a ventral coracoid which are articulated to a large clavicle or cleithrum. The clavicle slopes downwards to meet its fellow of the opposite side in the mid-ventral line.
From the dorsal end of the clavicle, a slender post-clavicle extends backwards and a stout supra-clavicle extends upwards. A forked post-temporal connects the supra-clavicle with the back of the skull.
Attached to the posterior surface of the scapula and coracoid, there are four or five small brachial ossicles or pterygiophores which support the dermotrichia or fin-rays of the pectoral Jin. The pelvic girdle is represented on each side by a thin plate—the basipterygiom, which bears the dermotrichia of the pelvic Jin.
3. Muscular System of Bhetki Fish:
The muscles of the fin, gill region and head are small. The trunk and tail, however, bear powerful muscles arranged in the form of V-shaped segments called myotomes. Successive myotomes are separated by delicate partitions of connective tissue. These muscles are utilised for swimming.
4. Body Cavity of Bhetki Fish:
Bhetki has a large body cavity to accommodate the visceral organs. This is divided into two parts—an anterior pericardial cavity and a posterior perivisceral cavity. The small pericardial cavity encloses the heart and is lined by the pericardial membrane.
The large perivisceral cavity contains the digestive organs, swim- bladder and the urinogenital organs. It is lined by the peritoneum which encloses most of the organs between its two folds, thereby fixing them in their proper place within the body cavity (see Plate).
5. Alimentary System of Bhetki Fish:
The alimentary system consists of:
(1) A digestive canal and
(2) The associated glands.
The alimentary canal is complete with a mouth in front and an anus behind. The mouth is a wide opening bounded by upper and lower jaws which bear sharp conical teeth. The mouth leads into the buccal cavity, which in turn is continued backwards into the pharynx. The tongue is small and non-muscular; it is firmly attached to the floor of the buccal cavity and supported by the basihyals.
Numerous mucous glands pour their secretion into the mouth but salivary glands are absent. There are no internal nares on the roof of the buccal cavity. The pharynx is supported on each side by bony gill-arches. There are five gill-slits on either side in between the gill-arches by which the pharynx communicates with the gill-chambers.
Inner margin of each gill-arch is provided with horny gill-rakers which act as strainers and prevent escape of food into the gill-chambers. The pharynx leads through a short oesophagus into the stomach. There is a strong oesophageal sphincter which prevents entry of water into the stomach.
The stomach of the fish is a recurved U-shaped sac. The portion nearer to the oesophagus is the cardiac stomach and the opposite portion is the pyloric stomach. The cardiac stomach is directed backwards and ends blindly, whereas the pyloric stomach is directed forwards and is continued into the intestine. There is a pyloric constriction separating the pyloric stomach from the intestine.
The intestine is long and narrow. Five blind tubular pouches called pyloric caeca originate from the intestine close to its junction with the pyloric stomach. These caeca are either secretory structures producing digestive juice or they may help in the absorption of the digested products. The intestine extends backward and then bends forward forming a loop.
Finally, the tube again curves backward to open at the anus. The anus is situated in the anterior part of the vent in front of the anal fin. A short posterior part of the intestine is known as the rectum but this cannot be sharply demarcated from the intestine. The mesentery supporting the gut is darkly pigmented and loaded with fat.
The large liver lies in the anterior part of the body cavity. It has two lateral lobes connected by a flat median portion. The elongated lateral lobes extend backwards into the posterior part of the body cavity. Bile formed within the liver passes into the gall-bladder and thence by the bile-duct into the first part of the intestine. The gall-bladder consists of two thin-walled sacs connected by a narrow tube.
One of the sacs lies attached to the posterior extremity of the right lobe of liver and the other one is placed on the median portion. There is no separate existence of pancreas. Pancreatic cells are present in a diffused state amongst the tissues of the liver. Thus the pancreatic secretion is mixed with bile and is conveyed into the intestine by way of the bile-duct.
Its principal food is the small fry of fishes. The wide mouth, and the jaws with sharp teeth act as a trap for capturing the prey, and not for chewing food. The swallowing is helped by the mucous secretions, which do not contain any enzyme. The swallowed food is conducted through the short oesophagus into the stomach. The stomach and the first part of the intestine are the digestive regions.
By the movement of the wall of the stomach the food substances are triturated and then partly converted into simpler forms by the action of the gastric secretions. In this condition the food gradually passes into the first part of the intestine through the pyloric constriction. Now, it is digested by the different juice secreted from the liver, pancreatic cells, wall of the intestine and probably from the pyloric caeca.
As a result of the enzymatic action the protein food substances are converted into amino acids, fat into fatty acids and glycerol, and carbohydrate into simple sugars such as glucose. Now the completely digested food is ready for absorption. The absorption takes place through the numerous intestinal folds into the blood, and the residue is egested through the anus via rectum.
There is a large swim-bladder or air-bladder in the dorsal portion of the body cavity under the vertebral column. This is a thin-walled elongated sac filled with gases, such as oxygen, nitrogen and carbon dioxide.
Originally the swim-bladder arises as an outgrowth from the anterior part of the alimentary canal, and its connection with the latter is often maintained by means of a narrow tube, the pneumatic duct; but a pneumatic duct is not found in Bhetki. A pneumatic duct is found in the common carp, Rohu.
The air-bladder consists of two chambers of which the anterior one is smaller than the posterior. The inner wall of the bladder is vascularised by bunches of capillaries derived from branches of the mesenteric artery. These capillaries are recognised as the rete mirabele.
The epithelial covering of the rete mirabele can isolate molecules of oxygen, nitrogen and carbon dioxide from the circulating blood and secrete the gases into the anterior chamber. The rete mirabele of the posterior chamber can reabsorb the gases.
The volume of the gas in the bladder may thus be controlled by alternate secretion and reabsorption. A full air-bladder means decreased specific gravity and consequent floating of the fish.
Reabsorption of the gases increases the specific gravity and the fish sinks to a greater depth. The fish makes this adjustment slowly when it moves up and down in a vertical plain. Thus the air-bladder serves as the hydrostatic organ of the fish.
6. Vascular System of Bhetki Fish:
The blood of the fish is pale and scanty in comparison with that of the toad. The blood plasma contains nucleated, elliptical erythrocytes and amoeboid leucocytes. The heart is enclosed within a thin pericardium and is situated in the anterior part of the body cavity ventral to the oesophagus.
It consists of a thin-walled sinus venosus, and auricle and a muscular ventricle. The sinus venosus is situated dorsally. It receives deoxygenated blood through right and left caval veins or ductus Cuvier. The auricle is situated ventral to the sinus venosus.
Ventral to the auricle and partly surrounded by its indented outer margin, lies the thick-walled conical ventricle. The deoxygenated blood passes from the sinus to the auricle through a sinuauricular opening and thence to the ventricle through the auriculo-ventricular opening. These openings are guarded by valves to prevent back-flow. Thus all the chambers of the heart contain deoxygenated blood.
A stout median artery, the ventral aorta, originates from the base of the conical ventricle. The base of the aorta is dilated to form a whitish, non-contractile bulbus aorta. The ventral aorta runs forward in the mid-ventral line along the floor of the pharynx.
It supplies paired afferent branchial arteries to the gills, thereby distributing the deoxygenated blood to the capillaries in the gill- filaments. The ventral aorta forks anteriorly at its terminal end to form the first pair of afferent branchial arteries which run into the first pair of gills.
The second pair of afferent branchial arteries originate from the aorta a little behind the first. They enter the ventral ends of the second pair of gills. Proximal to the second afferents, two stout branches arise from the ventral aorta, one on each side.
Each of these vessels divides to form the third and fourth afferent branchial arteries of the side which enter the corresponding gills. Evidently the ventral aorta and its branches are concerned with sending deoxygenated blood to the respiratory organs for the purpose of oxygenation; hence they constitute the afferent branchial system (Fig. 125).
The oxygenated blood is not returned into the heart but is conveyed directly from the gills to different parts of the body. The set of arteries carrying blood away from the dorsal ends of the gills constitutes the efferent branchial system (Fig. 126).
Corresponding to the four pairs of afferent arteries, there are four pairs of efferent arteries. The first and second efferent branchials unite with one another to produce an anterior epibranchial artery on each side. The anterior epibranchials join in the mid-dorsal line to form a median artery, the dorsal aorta.
The third and fourth efferent branchials unite with one another to form a posterior epibranchial artery on each side. These drain into the commencement of the dorsal aorta. A cephalic artery is given out anteriorly from each first efferent branchial.
This soon divides into an internal carotid for supplying the brain, and an external carotid to the tongue, hyoid arch, and orbital region. The internal carotids run side by side anteriorly. Ultimately, they enter the cranium to supply the brain. All the vessels described above are situated on the roof and lateral walls of the pharynx.
The dorsal aorta lies ventral to the vertebral column and pursues a backward course along the mid-dorsal line. It extends through the trunk to the tail, where it is known as the caudal artery. A stout coeliacomesenteric artery arises from the dorsa aorta the region where the posterior epibranchials open into it.
The coeliacomesenteric breaks up into branches to supply the swim- bladder, stomach, intestine, spleen, liver, etc. Distal to the coeliacomesenteric, arises a pair of subclavian arteries which supply the pectoral and pelvic fins.
During its backward course through the trunk, dorsal aorta gives off” paired renal arteries to the kidneys, genital arteries to the gonads and segmental arteries to the body muscles. The continuation of the dorsal aorta enters the tail through the haemal canal and is known as the caudal artery. It extends as far as the root of the caudal fin and supplies the muscles of the tail.
Veins of Bhetki (Fig. 125) are of two kinds:
(i) Systemic veins which carry deoxygenated blood directly into the Cuvierian ducts, and
(ii) Portal veins which break down into capillaries is some intermediate organ (either liver or kidney).
The principal systemic veins are the Cuvierian ducts which open into the sinus venosus.
Each ductus Cuvieri is mainly formed by the union of:
(i) An anterior cardinal sinus,
(ii) A jugular sinus, and,
(iii) A posterior cardinal sinus.
Anterior cardinal and jugular sinuses of each side bring blood from the dorsal and ventral regions of the anterior part of the body, whereas the posterior cardinals bring blood from the posterior part. Pectoral and pelvic veins from the paired fins and hepatics from the liver are slender veins draining into the ductus Cuvieri of their own side.
Blood from the tail is returned by a caudal vein which on entering the trunk, divides into two parts. The right division enters the right kidney. Passing through the kidney substance, it emerges at the anterior end as the right posterior cardinal sinus and joins the right Cuvierian duct.
The left division of the caudal vein enters the left kidney and breaks up into capillaries, thus forming a one sided renal-portal system. The capillaries of the renal-portal vein reunite to produce the left posterior cardinal sinus at the anterior end of the left kidney which drains into the left Cuvierian duct.
Both the posterior cardinal sinuses receive renal veins from the kidneys, genital veins from the gonads and segmental veins from the body muscles.
Blood from the alimentary canal and other associated structures is returned by small veins which unite with one another .to produce a large hepatic-portal vein. This enters the posterior surface of the liver, and breaks up into capillaries which reunite to produce the hepatic veins.
The blood in the fish travels only in one direction. The circulation, therefore, occurs in a single circuit. The heart maintains the circulation by rhythmic contraction (systole) and expansion (diastole). During a diastolic phase, deoxygenated blood comes back to the sinus venosus along the Cuvierian ducts. It is then pumped through the single auricle and ventricle by the rhythmic contraction of the heart.
The sinuauricular and the auriculo-ventricular valves prevent leakage and direct the blood stream to flow forwards. Ultimately the deoxygenated blood is, pumped into the gills through efferent branchial arteries. The latter break up into capillaries in the gills and the blood is aerated while circulating through the branchial capillaries.
This is branchial circulation. The oxygenated blood comes out of the gills via the efferent branchial arteries and circulates through the body of the fish supplying oxygen and nourishment. In course of circulation through the tissues the blood is deoxygenated. It is now returned by the veins to the sinus venosus.
While returning to the heart, the blood from the posterior part of the body is partially diverted into the left kidney, thereby forming a one-sided renal-portal system. Similarly, the blood from the alimentary tract is led into the liver forming a hepatic-portal system. Thus the heart of the fish has to pump only deoxygenated blood. The oxygenated blood never enters the heart.
The fish breathes through four pairs of gills. The gills are situated in two gill-chambers, one on either side of the pharynx. The gill-chamber is covered externally by a bony plate, the operculum. A branchiostegal membrane is attached to the posterior margin of the operculum.
The pharyngeal wall is perforated on each side by five gill-slits, which are separated from one another by four thickened strips. These are known as interbranchial septa or gill-arches. A gill-arch bears two rows of fine comb-like gill-filaments on its convex outer border, and teeth- like gill-rakers on the concave inner border.
The gill-rakers are pointed towards the pharynx and they prevent escape of food into the gill-chambers. Such a typical gill, bearing two rows of filaments hanging from the convex outer border of the gill-arch, is known as a holobranch.
In Bhetki all the four gills are holobranches. In other fishes a gill-arch may bear only one row of gill- filaments; such a gill is known as a demi-branch. Deoxygenated blood is carried to the gill by the afferent branchial artery, which lies superficially on the outer side of the gill-arch.
This blood is aerated while flowing through the capillaries in the gill-filament O2 being taken in and CO2 expelled through the delicate epithelium by diffusion. The aerated blood is conveyed through the efferent branchial artery which lies deeply on the inner side of the gill-arch. The fish requires a constant supply of oxygen charged water to flow through its gill-chambers, and cannot live long when it is taken out of water.
Mechanism of Respiration:
During inspiration, the opercula bulge out slightly but the branchiostegal membranes remain tightly opposed to the body wall, thereby sealing off the outer openings of the gill-chambers. The gill-arches are pushed out by the dilatation of the pharynx and buccal cavity. As a result of this increased space, water rushes in through the mouth and fill up the gill- chambers through the pharyngeal gill-slits.
The gills are thus washed in fresh-water current, and O2—CO2 exchange takes place. In the meantime the buccal and pharyngeal cavities contract and press upon the contained water. The oral valves are pushed out and the mouth-opening is sealed, thereby preventing escape of water through the mouth. The opercula are now lifted and the branchiostegal membranes give way. The water in the gill-chambers is thus forced out through the openings under the opercula.
This is expiration. Expansion and contraction of the buccal and pharyngeal wall occur alternately in quick succession due to the retraction and protraction of hyoid arch.
In a fish, inspiration is always associated with ingestion of food into the mouth; expiration, on the other hand, is associated with swallowing of the food. The food cannot escape into the gill-chambers due to combined action of the gill-rakers. There is a strong oesophageal sphincter which prevents entry of water into the stomach.
8. Nervous System of Bhetki Fish:
The nervous system is built on the same general plan as in the toad and consists of central, peripheral and sympathetic nervous systems. The central nervous system is composed of the brain and the spinal cord.
The brain of Bhetki consists of:
(ii) Midbrain, and
The forebrain is divided into an anterior telencephalon and a posterior diencephalon (thalamen-cephalon). The telencephalon is large and undivided. This enlargement is due to development of its ventral region, the corpus striatum.
Anteriorly, the telencephalon bears two well-developed olfactory lobes. This can be correlated with the fact that the fish depends more on the sense of smell than on sight.
Posterior to the olfactory lobes are the cerebral hemispheres with thin non-nervous roof. Thus the fish has no pallium and therefore the large telencephalon of the fish is merely a smell-brain and not a centre for higher acquisitions such as intelligence, etc., as in higher vertebrates.
The diencephalon is poorly developed. The minute pineal body attached to a slender epiphysis projects forward from the roof of the diencephalon. Its floor projects downwards as the infundibulum (hypophysis) to which is attached the pituitary body and saccus vasculosus. The saccus vasculosus is a highly vascular structure attached to the tip of the infundibulum of the fish.
It secretes cerebrospinal fluid. There is a lobus inferior on each side of the pituitary body which extends backward and meets in the mid-ventral line. The saccus vasculosus and the lobi inferiores are not found in toad and guinea-pig.
The fish has no optic chiasma in front of the pituitary body. The midbrain or mesencephalon is composed of two optic lobes dorsally, and the crura cerebri ventrally. This is the largest part of the brain. The two opdc lobes from the corpora bigemina.
The hindbrain is composed anteriorly of cerebellum (metencephalon) and posteriorly of medulla oblongata (myelencephalon). The cerebellum is located dorsally and covers the major part of the medulla; it is large and somewhat oblong in shape. The enlargement of the cerebellum is explained by the great muscular activity exercised by the fish.
The medulla oblongata is continued backwards as the spinal cord which extends throughout the entire length of the neural canal formed within the vertebral column. The peripheral nervous system consists of the cranial and spinal nerves. They originate in the same manner and from the same places as in toad.
There are ten pairs of cranial nerves:
(I) The olfactory nerves are long. They are distributed to the olfactory capsules.
(II) The optic nerves have no chiasma. They cross one another much anteriorly, the right nerve going to the left eye and the left nerve to the right eye.
(IV) trochlear, and
(V) abducens innervate the extrinsic muscles of the eyeball.
(VI) The trigeminal has, as usual, three divisions of which the ophthalmic divides into the two sensory branches:
(i) superficial ophthalmic innervating the top of the head and snout;
(ii) deep ophthalmic innervating the skin covering the eyeball.
(VII) The facial has a cutaneous branch in addition to its usual branches.
(VIII) The auditory is distributed to the membranous labyrinth of the internal ear.
(IX) The glossopharyngeal sends out a branch to the first gill-slit of its own side.
(X) The vagus divides into two main portions:
(i) Lateralis, and
The lateralis runs along the lateral line of the side innervating the lateral line sense organs. The visceralis sends out three branchial branches which supply the second, third and fourth gill-slits of the side.
The cardiac and the gastric branches for supplying the heart and the alimentary canal respectively arise from the branch to the fourth gill-slit. Paired spinal nerves emerge between the neural arches of successive vertebrae to supply the body segments of the trunk and tail. The sympathetic nerve trunks are devoid of ganglia.
9. Receptor Organs in Bhetki Fish:
These consist of sense organs such as eyes, ears, olfactory organs, and receptor cells for touch and taste.
The eyes, are located on the two sides of the head. They are devoid of eyelids and eyelashes. The transparent cornea is flat, and the rounded lens lies in opposition to the cornea. The anterior chamber, containing aqueous humor, is greatly reduced.
Each eye is composed of three coats:
(1) The cartilaginous sclera on the cuter side,
(2) The vascular choroid in the middle, and
(3) The sensitive retina internally.
There is the usual iris with a central hole, the pupil. The choroid layer is pigmented and contains vascular choroid glands. A peculiar silvery layer called argentea is placed between the sclera and the choroid.
There is a falciform process from the choroid extending into the posterior chamber which is filled by the jelly-like vitreous humor. A retractor lentis muscle is attached to the falciform process posteriorly and to the back of the lens anteriorly. There is no ciliary muscle.
The eye is normally focused for near vision. Accommodation for distant vision is not due to change of shape of the lens chamber op the vitreous humor but due to change of position of the lens.
Contraction of the retractor lenis muscle pulls the lens closer to the retina, so that the image is brought to the proper focus. But how far this accommodation is perfected is not known. The vision is monocular. While the right eye visualizes one object, the left eye can visualize another object. The fish cannot close its eye even during sleep.
Ear of fishes is comparable with the internal ear of higher vertebrates. It is more concerned in maintaining equilibrium than in hearing. The middle ear and the external ear are not developed in fishes. The ear sac or membranous labyrinth consists of three semicircular canals and three other chambers: utriculus, sacculus, and lagena. In each chamber there is an ear-stone or otolith.
Experimentally it has been found that some fishes can respond to sounds of different wave-length. The seat of hearing is located in the sacculus and lagena. How these structures carry out their function is not known. The ear of the fish receives all its sound vibrations through the body surface, because there is no eardrum. The lateral line sense organs help in perceiving low-frequency vibrations of the watery medium.
These are represented by two nasal sacs which do not communicate with the buccal cavity. Each nasal sac has two openings to the exterior. Olfactory cells of the nasal sacs are chemoreceptors for the sense of smell.
Tactile cells are usually found on the lips around the mouth and on the surface of the body.
Taste cells ate present in the mucous membrane of the mouth and over the whole body surface. These can discriminate food of different taste.
10. Endocrine System of Bhetki Fish:
Of the ductless glands, the following require special mention.
It is a ductless gland producing thyroxin, a compound rich in iodine. In fish, it is a small reddish mass found on the floor of the buccal cavity along the ventral aorta. The thyroid gland is the metabolic regulator of the body.
It is a paired mass found, one on each side, near the last branchial arch.
It is the largest ductless gland found attached to the intestine. It is principally concerned with the destruction of worn- out blood corpuscles and to serve as a storehouse of blood.
11. Excretory System of Bhetki Fish:
The greatly elongated kidneys lie ventral to the vertebral column but dorsal to the air-bladder. They extend through the coelom, from the gill-arches in front, to a point just anterior to the vent. The two kidneys are partially fused posteriorly but their anterior parts are separate. Each kidney is composed of a large number of uriniferous tubules held together by connective tissue.
The ventral surface of the kidney bears numerous minute apertures, the nephrostomes, which communicate internally with the uriniferous tubules. A duct or ureter arises from the posterior part of each kidney. The two ureters unite behind to form a single tube communicating with a thin-walled sac—the urinary bladder.
The bladder discharges the urine through a narrow canal which communicates with the urinary opening at the posterior part of the vent, behind the anus. The urine is formed by selective filtration at the glomeruli of the uriniferous tubules.
12. Reproductive System in Bhetki Fish:
Sexes are separate. Gonads are of enormous size in sexually matured individuals, particularly during the breeding season. It is difficult to distinguish testes and ovaries in the young fish.
In a male Bhetki, the gonads consist of a pair of smooth elongated testes situated in the posterior part of the body cavity. Each testis communicates by a duct, the vas deferens, with the reproductive opening of its own side. There are two reproductive openings which lie laterally at the two sides of the vent.
The ovaries found in female Bhetki are much bigger than the testes of a male of the same age and contain numerous small eggs or ova. Matured eggs are carried by the oviducts and discharged outside through, the reproductive openings. There are two reproductive openings lying laterally at the two sides of the vent.
Spawning takes place in winter. Seminal fluid or milt containing spermatozoa are shed by the male fish in water. Similarly, the female lays her eggs. Fertilization, of the ova by sperms is a matter of chance and takes place in water. A large quantity of the spawn is destroyed—being devoured by aquatic animals or by other natural factors and only a few of the fertilised eggs develop into the tiny fry.
The young fry hatches out of the fertilized egg in about 20 hours. It has a small yolk-sac, containing reserve food, attached to its ventral surface. In the course of a few days the yolk is exhausted and the sac disappears.
The fry has, by this time, enlarged in size. It feeds on algae, insect larvae and small crustaceans. It becomes predatory and devours fry younger than itself. The fish attains sexual maturity in about a couple of years.