Morphology And Anatomy Of Frog

Rana tigrina is the most widely distributed species in Northern India. Generally frogs are found in ponds, tanks, pools, ditches, etc. However, they may leave their aquatic habitat to come on land to hunt for their prey, which are mostly insects. The frog is called an amphibious animal because it lives both in water and on land.

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The frog is very well adapted both for jumping and leaping on the land and swimming or floating in water. Normally frog can jump up to three feet. When a frog jumps to a shorter distance, it is known as leaping. The alternate kicks of the hind-limbs push the webbed toes against water, thereby providing the force for swimming. The boat-shaped body of the frog, mainly due to the absence of neck, is helpful in swimming.

The adult frog is carnivorous and mostly feeds on insects and their larvae, spiders, earthworms and in some cases evens their own larvae. It will capture only a moving prey and swallow it as a whole.

When the prey is within a catching distance, the free posterior end of the tongue is shot out suddenly with much speed and is taken back into the buccal cavity with the prey sticking to it. The tadpole (larva) of frog is herbivorous.

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During the rainy season, which is also the breeding season of the frog, the frogs make peculiar sounds with the help of their vocal cords, known as croaking. The male frogs croak louder than the females, because in them the pitch of croaking is increased due to the presence of two vocal sacs situated one on each side of the floor of the throat. The vocal sacs act as resonators. The croaking is a mating call to attract the females.

It takes place through lungs, buccopharyngeal cavity and skin. However, in early tadpole, gills are the organs for exchange of gases. Being a cold-blooded (poikilothermal) animal, the frog cannot withstand very cold weather and therefore, buries itself deep in the mud.

During this period it lives in a dormant life of very slow vital body activities. Whatever little energy required during this period is obtained from the fat-bodies and from the stored glycogen of the liver, which had been stored in sufficient quantity earlier.

The oxygen need of the animal is met with by respiration through skin. This winter-sleep is known as hibernation. With the coming of spring, the hibernating frogs come out and start their normal life.

In the mid-summer, when the temperature is very high, the frogs escape to the places of concealment (mud, etc.) temporarily, but often coming out to feed themselves. This period of little activities is usually referred to as summer sleep or aestivation.

However, it is believed that during this period frog takes rest and recuperates its energy. The frog is capable of changing its body colour, though gradually, with the change in its surroundings and climatic conditions. This capability of changing colour is known as metachrosis.

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The breeding season normally takes place during the rainy months. The male frog jumps on the back of the female frog and holds her tightly with the help of his fore-limbs behind her fore-limbs with the aid of nuptial pads or amplexusary pads. This sexual embrace is called the amplexus.

The female discharges its ova followed by the shedding of the sperms by the male to ensure fertilization, which takes place in water. Eggs of frog are mesolecithal and telolecithal. Mesolecithal eggs contain moderate amount of yolk.

In telolecithal eggs the yolk is concentrated in the vegetal half. The fertilized ovum (egg) undergoes development to produce a fish-like tailed larva, the tadpole, which respires with the help of gills and feeds upon vegetable matter (herbivorous).

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Morphology of Frog

The body of frog is divisible into two parts only: the head and the trunk. The neck and tail are not present. The ante­riorly placed mouth is a wide transverse opening. A pair of external nares (nos­trils) is present at a short distance above the mouth. There is present a pair of dorsolaterally placed prominently bulg­ing out eyes on the head. The eyes are protected by upper and lower eye-lids.

The upper eye-lid is prominent, thick and slightly movable, while the lower eye­lid is vestigial and immovable. A little above the lower eye­lid, a transparent nictitating membrane, mistakenly referred to as the third eye-lid, is present.

This membrane is with­drawn and unfolds itself with the help of certain eye muscles to protect the eye from water and any other damaging matter by covering the eye ball. Just behind the eyes on each side a pigmented tympanum (ear covering) is present.

Between the two eyes on the mid-dorsal line is present a small circular and lightly coloured, but a conspicuous patch, known as brow spot which represents vestigial pineal eye. Only in the male frogs, there is situated on the posterior part of the throat a pair of somewhat bluish and wrinkled vocal sacs. The vocal sacs help in increasing the croaking which is referred to by some people as the “mating call”.

The neck being absent in frog, the head is directly continued into the trunk. The trunk comprises an anterior portion known as thorax and a posterior larger portion called abdomen. A pair of fore-limb and hind-limb is appended to the trunk.

The fore-limb consists of the proximal upper arm (brachium), the middle fore-arm (an- tebrachium), wrist and a hand (manus) bearing four fingers. The finger corre­sponding to our thumb or pollex is rudi­mentary and invisible externally. During the breeding season, there develops a dark swelling on the inner finger of the male frog, the nuptial pad which helps the male frog in mating.

The hind-limbs are much longer than the fore-limbs and more powerful. The hind-limb consists of proximal thigh, the middle shank, a long ankle and a foot (pes) bearing five toes. The first toe is called hallux. The toes are connected together by a thin membranous web, which helps the animal in swimming. A medially placed cloacal aperture is present for the outlet of faecal matter, urine and reproductive product.

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Sexual Dimorphism

This phenomenon means that the male and female animals can be distinguished by their external features.

In case of frog the male and female can be differ­entiated from each other due to the following facts:

(i) The male frog possesses vocal sacs, which are most developed during the breeding season,

(ii) During the breeding season an amplexusory or nuptial pad is developed on inner finger of each hand of the male frog.

Anatomy of Frog

Skin (Integument)

The skin consists of the following two layers


It is composed of stratified epithelium. The epidermis is non-vascular layer. The innermost layer of the epidermis consists of columnar cells, which is called stratum germinativum or Malpighian layer, which undergoes active mitosis to produce new cells towards outside.

Later on the cells shift to the outer surface and become extremely thin, inactive and ultimately dead to form the outermost layer of epidermis, which is termed as stratum corneum.

The layers between stratum corneum and stratum germinativum are referred to by a common name of stratum transitionale. The stratum corneum is protec­tive in function and is periodically cast off (moulting).

This phenomenon, known as ecdysis, is affected by the secretions (hormones) of two ductless glands, the pituitary and thyroid. On the surface of stratum corneum, are present the openings of the cutaneous glands. The stratum germinativum rests on basement membrane.


The thick dermis consists of an upper bigger area, the stratum spongiosum and the lower smaller portion, the stratum compactum. The stratum spongiosum is com­posed of highly blood vascular network of loose fibrous connective tissues, therefore, spongy in nature and richly supplied by nerve fibres and contains many lymph spaces.

The sensitivity of the skin is due to the presence of touch corpuscles and nerve endings. Just beneath the stratum germinativum there are present many scattered pigment cells (chromatophores) in the stratum spongiosum. The pigment cells are responsible for the colour of the animal.

The chromatophores are of three types

(i) Lipophores are yellow fatty cells which have red and yellow pigment,

(ii) Guanophores are specialized cells which contain white pigment called guanine,

(iii) Melanophores are black pigment cells which contain dark brown or black pigment, called melanin. The olive greenish colour is due to the combined effect of these three pigments.

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There are two kinds of glands in the stratum spongiosum

(i) Many mucous glands secrete the mucus which makes the skin moist and slippery in nature,

(ii) Many poison glands, only infrequently present in frog, but mostly present in toads.

They secrete a poisonous fluid, which protects these animals from their enemies. Both mucous and poison glands are formed by the stratum germinativum and invaginated below with their necks opening on the surface of the skin.

The inner region of dermis consists of compact dense connective tissue, known as stratum compactum. There are present many horizontal and some bundles of vertical connective tissue fibres. The stratum compactum is also supplied with nerves and blood vessels.

Body Cavity (Coelom)

The space or cavity lined by parietal peritoneum externally and visceral peritoneum internally is known as coelom or cavity.

The coelom of frog is divisible into

(i) An anterior small pericardial coelom or cavity enclosing the heart and

(ii) Posterior spacious abdominal (pleuroperitoneal) coelom in which the rest of the internal organs lying in the trunk are lodged.

Digestive System of Frog

It consists of the alimentary canal and the associated digestive glands.

Alimentary canal

The alimentary canal consists of the following parts:


It is bounded by immovable upper jaw and movable lower jaw.

Buccopharyngeal cavity

It is so-called because the buccal cavity and pharynx have been merged into one cavity. It comprises the following structures.

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The teeth are absent on the lower jaw while the upper jaw bears similar small, backwardly directed conical teeth known as maxillary teeth. In addition there are present two patches of vomerine teeth one on each vomer bone, near the internal naris. All teeth are similar and are, therefore, known as homodont.

If the teeth are broken or worn out, they are easily replaced many times. Such teeth are called polyphyodont. Unlike man, the teeth of frog are not lodged in sockets but fixed by an adhe­sive secretion, the cement. Such teeth are known as acrodont. The teeth of frog are not masticatory. They only help in injuring the prey and prevent its escape from the buccopharyngeal cavity,

Sub-rostral fossae and pre-lingual elevations

Three depressions, the sub-rostral fossae can be seen behind the upper jaw. Three small elevated structures, called the pre-lingual elevations are placed at the tip of the lower jaw opposite to the sub-rostral fossae. These elevations get fixed into the sub-rostral fossae and help in closing the mouth firmly during exchange of gases,

Internal nares

A pair of internal nares, in the form of two small openings is present on the roof of the buccal cavity lying outer to vomerine teeth. They help in exchange of gases,

Bulging’s of eye balls

Behind the internal nares are present two prominent bulging’s of eye balls which indicate the position of the eye-orbits.

Openings of Eustachian tubes

On each lateral side of the buccopharyngeal cavity, an opening of the Eustachian tube can be observed which connects the cavity of the middle ear with the buccopharyngeal cavity,

Openings of vocal sacs

In male frog only, there are present two openings of the vocal sacs near the angles of lower jaw. They help in croaking,


The tongue is fixed in front but its hinder end is free and bilobed. The free bilobed posterior end can be thrown out and retracted backward with a great speed after catching the prey. This sudden action of the tongue is brought about by a protractor and a retractor hypoglossus muscles,

Glottis and opening of oesophagus (gullet)

A slit like opening, the glottis opens into the laryngotracheal chamber. Larynx and trachea are fused to form laryngotracheal chamber in frog. Medianly placed is a wide opening of oesophagus that leads into the oesophagus.


Because of the absence of neck in frog, the oesophagus is only a short tube. The oesophagus leads to the stomach.


It is divisible into two parts

(i) Cardiac stomach, the anterior larger part is present near the heart. The opening of the oesophagus into the cardiac stomach is guarded by a cardiac sphincter, a powerful ring of circular muscles, which prevents the backward flow of food,

(ii) Pyloric stomach is a posterior narrow tapering part, which is separated from the duodenum by a muscular constriction, the pyloric constriction externally, which indicates the position of pyloric sphincter and controls the entry of food into duodenum.

Small Intestine

It is the long­est part of the alimentary canal suspended by mesentery, and is divisible into duode­num and ileum. They have large number of finger like branched projections known as villi which increase the absorptive surface area.

Rectum (Large Intestine)

The ileum leads into the rectum. Posteri­orly, it opens into the cloaca through an aperture known as anus which is guarded by an anal-sphincter. The rectum stores the faecal matter and water is absorbed by its wall.

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It receives the rectum in both the sexes, but in female frog, the cloaca also receives the ureters and oviducts, while in male the urinogenital ducts are received in addition to the rec­tum. The urinary bladder also opens into the cloaca. The cloaca opens out through a cloacal aperture, situated at the posterior-most end of the abdomen.

Digestive Glands of Frog

In frog, the digestive glands are as follows


The liver is the largest gland of the body. It is composed of two lobes, the left and the right joined with each other. The left lobe is larger and divided into two smaller lobes. In between the two main lobes of the liver, a muscular, rounded, dark bluish green bag like structure, the gall bladder, is situated.

The liver secretes a liquid, which is called bile. The bile is transferred to the gall bladder for storage through many hepatic ducts. The bill contains organic and inorganic salts. The sodium carbonate salt of bile makes it alkaline. The bile as such does not contain any digestive enzyme.

The gall bladder gives rise to a duct, the cystic duct, which also receives a few hepatic ducts from the liver directly. The cystic duct proceeds towards the duodenum as the bile duct. When the bile duct passes through pancreas, it receives some pancreatic ducts from the pancreas and opens into the duodenum as a common hepatopancreatic duct.


It is a branched flat elongated pale yellow gland lying in mesentery extended in the loop between stomach and duodenum. The pancreas is made up of lobules, connective tissue and inner core of islets of Langerhans. The lobules produce pancreatic juice which is poured into the bile duct through many pancreatic ducts and finally reaches the duodenum via common hepatopancreatic duct.

The islets of Langerhans secrete a hor­mone known as insulin which converts glucose into glycogen in the liver. The enzymes present in the pancreatic juice are trypsinogen (pro-enzyme), chymotrypsinogen (pro-enzyme), peptidase, amylase, maltase and lipase.

Gastric glands

The glands are present in the stomach.

The gastric glands secrete gastric juice and are made up of two types of cells

(a) Peptic cells secreting pro-enzyme, pepsinogen,

(b) Oxyntic cells producing hydrochloric acid. In addition there are also present many mucus secreting cells.

Intestinal glands

These glands are present in the small intestine which secretes the intestinal juice. Intestinal juice contains several peptidases such as tri and dipeptidases, maltase, lipase and activator enterokinese enzyme.


The adult frog is carnivorous, eating insects, spiders, earthworms etc. However, its larva is strictly herbivorous feeding on water plants.


The act of taking the food material into the buccopharyngeal cavity is called ingestion. During ingestion the mouth is opened and the muscular sticky tongue is shot out with much speed to catch the prey. The prey gets adhered to the sticky tongue. The tongue along with the prey is withdrawn with great rapidity into the buccopharyngeal cavity and the mouth is firmly closed.

The prey is held, injured or killed by the maxillary and vomerine teeth. The bulging of eye balls and raising up the throat help in pushing down the food towards the oesophagus. The movement of the epithelial cilia of the buccopharyngeal cavity and peristalsis further aid in the downward movement of the food into the oesophagus.

The peristalsis is due to the alternate constrictions and relaxations of longitudinal and circular muscles present in tubular muscular organs like alimentary canal, bile duct, ureter, reproduc­tive ducts, etc., passing from one end of the tubular organ to the other end. During this, the contents of the tube are pushed onwards.


Digestion is a process by which the non-diffusible food material is converted into diffusible form by the action of various digestive enzymes. The buccopharyngeal cavity and the oesophagus do not secrete any enzymes.

Digestion that takes place in the stomach is known as gastric digestion. The walls of the stomach secrete a hormone, the gastrin, which stimulates the gastric glands to release the gastric juice. The gastric juice contains hydrochloric acid and a pro-enzyme pepsinogen and mucus.

Hydrochloric acid converts inactive pepsinogen into active pepsin. The acid also provides the acidic medium, which is essential for the action of pepsin. The pepsin converts proteins into proteoses and peptones. The carbohydrates and fats remain unaffected by the proteolytic enzymes.

In the stomach the food is stored for quite a long time. Due to the massive peristalsis of the stomach wall, the food is broken into smaller pieces and thoroughly mixed with the gastric secretions including the mucus. The food is now in a semi-liquid state known as chyme. The latter passes into the duodenum through pyloric sphincter at regular intervals.

Digestion that takes place in the intestine is called intestinal digestion. In the duodenum, the acidity of the chyme is neutralized by the salts of bile and now the chyme becomes alkaline in nature. With the entry of bile, pancreatic juice, the intestinal juice and more of mucus, the food is now in a more liquid form and is called chyle.

A hormone, the cholecystokinin, produced by the duodenum reaches the gall bladder through blood circulation where it stimulates the gall bladder to release the bile. The bill contains no enzymes, but its salts provide the alkaline medium necessary for the action of the pancreatic and intestinal enzymes.

The bile salts lower down the surface tension of fat material, which is then broken into fine fat droplets/globules and this process is called emulsification. The duodenum also secretes secretin hormone which reaches the pancreas through blood circulation, where it stimulates the pancreas to release the pancreatic juice.

The pancreatic juice contains pro-enzymes trypsinogen, chymotrypsinogen and enzymes peptidase, amylase, maltase and lipase. The inactive trypsinogen is converted into trypsin by the action of an activator enzyme, the enterokinase of the intestinal juice. The trypsin converts inactive chymotrypsinogen into chymotrypsin.

All these three enzymes— trypsin, chymotrypsin and peptidase change proteoses and peptones into peptides. Amylase converts starch into maltose. Maltase converts some maltose into glucose. Lipase changes fat into fatty acid and glycerol.

Intestinal juice contains peptidases, maltase and lipase enzymes. The enzyme enterokinase present in this juice converts inactive trypsinogen of pancreatic juice into trypsin. Many peptidases (tri and dipeptidases), change peptides into amino acids. Maltase changes the rest of the maltose into glucose and the lipase converts the remaining fat in fatty acids and glycerol.


It is a process by which the digested food diffuses into the circulatory system (blood and lymph) through the wall of the alimentary canal. The absorption starts from the stomach. Most of the absorption is done by small intestine.

The wall of the alimentary canal is richly traversed by blood and lymph capillaries. The lymph capillaries of the intestine are known as lacteals. Amino acids, glucose, vitamins and water are absorbed into blood capillaries, while fatty acids and glycerol enter the lymph-capillaries (lacteals).


The indigestible food comes to rectum by peristaltic action, where water is mainly absorbed. After that, the faecal matters are transferred to the cloaca and from there they are egested via cloacal aperture.

Respiratory Organs of Frogs

The adult frog respires in three different manners

(i) Cutaneous or skin respiration;

(ii) Buccopharyngeal respiration;

(iii) Pulmonary or lung respiration.

Cutaneous Respiration

The skin of frog provides an extensive surface for the exchange of gases. It is thin, richly supplied with blood and kept moist by the mucus and water. The cutaneous respiration is always carried out. During hibernation (winter sleep) and aestivation (summer sleep), it is the only method of respiration in frog.

Buccopharyngeal Respiration

It occurs when the animal is on the land or partially immersed in water. There are present two external nares situated at the tip of the snout. Each external naris (nostril) leads into a nasal chamber which opens into the buccopharyngeal cavity by an internal naris.

During this mode of respiration, the mouth and glottis are tightly closed while the nares are kept open. There are two sets of muscles which operate the mechanism of buccopharyngeal respiration. These muscles are stemohyal muscles and petrohyal muscles.

(i) Stemohyal muscles are attached by one end to the sternum and at the other end to the hyoid apparatus,

(ii) Petrohyal muscles are attached at one end to the hyoid apparatus and at the other end to the auditory capsule (prootic bone).

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Pulmonary Respiration

It is less frequent than the cutaneous and buccopharyngeal respiration. It occurs when need for oxygen is increased. Respiratory path includes external nares, nasal chambers, internal nares, buccopharyngeal cavity and glottis. The glot­tis opens into the laryngotracheal chamber.

This chamber leads posteriorly to the lungs through a pair of apertures. The lungs are two delicate, elastic, pinkish, ovoid, thin walled hollow sacs lying on either side of the oesophagus. Each lung looks like a honey-comb.

The inner surface of the lungs is divided by a series of partitions, the septa into many small cavities known as alveoli. The wall of the alveoli is richly supplied with blood cap­illaries. Exchange of gases takes place through alveoli. During pulmonary respiration, the mouth is tightly closed. The sternohyal and petrohyal muscles play very important role in the mechanism.

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Sound Producing Organ

A thin walled laryngotracheal chamber is the sound producing organ in the frog. It corresponds to the larynx and trachea of higher animals. There is also present a pair of elastic bands, the vocal cords which extend longitudinally across the laryngotracheal chamber. These cords can be stretched and relaxed by the laryn­geal muscles attached to the cartilages of this chamber. The vocal cords are the true sound producing organs.

A narrow slit like gap, the rima-glottis is present between the free inner edges of the vocal cords, which helps in the production of sound. The inner edges of the vocal cords, which lie freely, get vibrated when air from the lungs is expelled out forcibly to produce a characteristic sound (croak).

The male frogs produce louder and prolonged sound than the females because males have a pair of vocal sacs. These sacs are the pouches of the skin on the throat, opening into the buccopharyngeal cavity and acting as resonators. When the air is expelled out from the lungs the vocal sacs are inflated with air to raise the pitch of the sound in the male frog.

Circulatory System of Frog

The circulatory system of frog consists of blood vascular system and lymphatic system. Blood Vascular System. It is of closed type as the blood flows in the blood vessels. It represents single circulation. It means both the oxygenated and the deoxygenated blood enters the heart and get mixed in the ventricle. Blood vascular system comprises blood, heart and blood vessels.


The blood is a mobile connective tissue, composed of a fluid, the plasma and the cells, the blood corpuscles.


It is the non-cellular alkaline, almost colourless fluid.

Blood Corpuscles

There are present three types of cells in the blood plasma

Erythrocytes (Red blood corpuscles)

These are large, oval and biconvex nucleated cells. The cytoplasm contains a respiratory red pigment, known as haemoglobin, which is made up of a blood protein, the globin and haematin.

Leucocytes (White blood corpuscles)

Their number is also numerous but less than erythrocytes.

The leucocytes are of two types

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They are of three types

(a) Neutrophils,

(b) Basophils,

(c) Eosinophil’s.


The agranulocytes are of two types

(a) Lymphocytes,

(b) Monocytes.

Thrombocytes (Spindle-shaped cells)

They are small, numerous and spindle shaped nucleated cells. They are produced by the bone marrow. They help in the clotting of blood, by producing the thrombin enzyme.

Heart of Frog

The heart is a modified blood vessel to receive and distribute the blood.


The heart is situated mid-ventrally in the thorax in between the two lungs, just anterior to the liver. The heart is protected by the pectoral girdle ventrolaterally and by vertebral column dorsally.


The heart is enclosed in a delicate transparent two layered sac, the peri­cardium. The outer layer of the sac is called parietal pericardium and inner layer is known as visceral pericardium. In between the two layers, a space, the pericardial cavity is present, which is filled with a pericardial fluid. The pericardium protects the heart from the shocks, and mechanical injuries and also allows its free movements.

Structure of Heart

After removing the pericardium, the external characters of the heart are clearly seen. The heart of frog is three chambered, viz., two auricles and a single ventricle. In addition to the auricles and ventricle, there are also two other associated structures, known as sinus venosus and truncus arteriosus (= conus arteriosus).

The two thin walled auricles are separated from each other by the inter auricular septum. The sinus venosus opens into right larger auricle through sinuauricular aperture which is guarded by a pair of sinuauricular valves. It only allows the blood to flow towards the right auricle. The common opening of pulmonary veins is present in the smaller left auricle. The opening is not guarded by any valve.

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Both the auricles open into a single ventricle by a common wide auriculo-ventricular aperture, which is guarded by a valve, the auriculoventricular valve bearing four flaps— one dorsal, one ventral and two lateral in position. The free edges of these flaps project into the ventricle and are connected to the wall of the ventricle with fine contractile but tough thread-like structures known as chordae tendineae.

The chordae tendineae regulate the movement of the auriculo-ventricular valve so as to allow the blood to pass from the auricles into the ventricle and further prevent the backward flow of the blood into the auricles.

The ventricle is a muscular and thick walled chamber, the inner surface of which is projected into ridges, the columnae carneae. The latter divide the cavity of the ventricle into smaller spaces, known as fissures.

The right side of the ventricle opens into truncus arteriosus on the ventral side. At the place from where the truncus arteriosus arises from the ventricle, a row of three pocket shaped semilunar valves is present.

These valves allow blood flow from the ventricle to the truncus arteriosus. The latter consists of a proximal muscular part, the pylangium and the distal part, the synangium. A row of the three semilunar valves separates the pylangium from the synangium.

The pylangium part is further divided vertically into two incomplete chambers by a spiral twisted valve. The right chamber is called cavum aorticum and the left cavum pulmocutaneum. The spiral valve is a flap of tissue running vertically in the pylangium and is attached with the middle valve of the anterior row of the three semilunar valves. The spiral valve is free ventrally and posteriorly.

The cavity of the synangium is further divided into a dorsal and ventral chamber by a horizontal partition called primary septum or septum principle. The dorsal chamber com­municates with cavum pulmocutaneum while ventral chamber with the cavum aorticum.

The synangium bifurcates into two branches as aortic trunks. Each aortic trunk di­vides into three arches, the pulmocutaneous, systemic and carotid arches. A small opening of the pulmocutaneous arch is present in the cavum pulmocutaneum just behind the anterior row of semilunar valves. This opening is guarded by a small valve.

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Thus, the cavum pulmocutaneum leads into the dorsal chamber of synangium and then dorsal chamber opens into pulmocutaneous arch. While the cavum aorticum leads into the ventral chamber of synangium and then the ventral chamber opens into both systemic and carotid arches.

In addition to the above mentioned structures, there is present one sinus venosus. Histologically, the wall of the heart consists of three layers: outer smooth visceral pericardium, middle muscular myocardium and inner endothelium comprising epithelial cells.

Working of Heart

Heart of frog is myogenic (heart beat originates from a muscle, however, it is regulated by the nerves). Wave of contraction originates from the sinus venosus and spreads over the wall of the sinus venosus and both the auricles. It compels the heart to beat. The contraction of the heart is termed as systole while its expansion is called diastole and both constitute one heart beat.

The sinus venosus contracts, whose contraction drives the mixed blood from the sinus venosus to the right auricle. At the same time, the left auricle receives oxygenated blood through the opening of the pulmonary veins from the lungs. When the two auricles are full of blood, they contract simultaneously. The backward flow of the blood into the sinus venosus is prevented by the sinuauricular valves.

The blood cannot go back into pulmonary veins as the openings of the pulmonary veins are small and oblique which are closed auto­matically by the contraction of the left auricle.

Thus, the blood from both the auricles is forced into the single ventricle through auriculoventricular aperture. The auriculoventricular node is stimulated by the auricular contraction and passes a fresh wave of the contraction over the wall of the ventricle.

Thus both oxygenated and deoxygenated bloods are mixed in the ventricle. Now the ventricle contracts and the backward flow of blood into auricles are prevented by the auricu­loventricular valve. The mixed blood of the ventricle enters the truncus arteriosus. From the truncus arteriosus mixed blood goes to three arches (carotid, systemic and pulmocutaneous) and ultimately reaches all the parts of the body.

Control of Heart Beat

The contraction of heart (systole) and the relaxation of heart (diastole) constitute the heart beat. The rate of heart beat is controlled by the medulla oblongata of the brain and by certain hormones such as adrenalin and thyroxine.

Increase in carbon dioxide and temperature also affects the sinus venosus and thereby it increases the rate of heart beat. In frog cardiac cycle is completed in about 0.9 seconds. This means that frog’s heart beats about 64 times per minute.

Blood circulation

It can be represented by the following graphical sketch.

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Blood Vessels of Frog

Arteries, arterioles, veins, venules and blood capillaries are called blood vessels. Arteries carry blood from the heart to different body parts. Veins bring blood from different body parts to the heart. Arteries divide to form arterioles.

The arterioles divide to form blood capillaries which unite to form venules. The venules join to form veins. The veins have valves to prevent backward flow of blood. Exchange of nutrients, hormones, gases, etc. takes place between blood capillaries and body cells.

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Significance of Renal Portal System

The renal portal system collects the blood from the hind parts on the body from which the urea and uric acid present in the blood are first got filtered in the kidneys before the blood goes into postcaval and then to the heart. Thus, the blood going to the heart contains comparatively less impurities after passing through the renal portal system.

Significance of hepatic portal system

The hepatic portal system has following sig­nificance

(i) The blood which comes from the alimentary canal contains digested food like glucose and amino acids. The excess of glucose is converted into glycogen which is stored in the liver for later use. When an individual feels deficiency of food, the glycogen is converted into glucose and is transferred to the blood stream via hepatic veins,

(ii) Harmful nitrogenous waste like ammonia is converted into urea which is later removed by kidneys. Thus the blood is detoxified (purified) of harmful nitrogenous waste;

(iii) Liver produces blood proteins which are put into blood circulation.

Lymphatic System

It is the second part of the circulatory system, through which a mobile connective tissue, the lymph, is circulated. Lymphatic system comprises (a) Lymph, (b) Lymph capillaries, (c) Lymph sinuses and (d) Lymph hearts.


Lymph is a mobile connective tissue like blood which is composed of two parts


It is similar to blood plasma but has got less number of proteins. Mainly globulin proteins are present. Other components of the lymph plasma are very much like that of blood plasma, i.e., organic, inorganic substances.


Red blood corpuscles (Erythrocytes) are completely absent. Leucocytes are numerous which are mainly lympho­cytes. Thrombocytes (spindle shaped cells) are few in number due to which the clotting of the lymph is very slow.

Lymph Capillaries

Lymph capillaries lie very close to the blood capillaries but differ from them, as they end blindly in contact with the body cells or the tissue spaces. The lymph always flows from the tissues towards the lymph hearts. Their walls are extremely thin and permeable to colloids, crystalloids and water.

They do not have a constant shape. The lymph capillaries of the intestine transport the digested fat, which does not pass through blood stream directly. Because these capillaries contain fat, they appear milky white, hence are termed as lacteals (lactos = milk).

Lymph Sinuses

In the frogs, lymph vessels are not found. Instead of these vessels, there are present thin walled spaces or lymph sinuses around the tissues and between the organs. The important lymph sinuses are subcutaneous sinuses and sub-vertebral sinus.

Lymph Hearts

There are present two pairs of thin walled, muscular lymph hearts, one pair is located behind the transverse processes of third vertebra and the other is situated in the region of urostyle. The flow of lymph is from the lymph capillaries → lymph sinus → lymph hearts → veins.


It is a dark, red oval structure lying near the anterior end of the rectum. It produces RBCs and WBCs in adult too. It stores blood hence it is called “blood bank”.

Nervous System of Frog

The nervous system of frog is divisible into three parts

(i) Central nervous system

It comprises the brain and spinal cord.

(ii) Peripheral nervous system

The nerves arising from the central nervous system constitute the peripheral nervous system.

(iii) Autonomic nervous system

It includes the nerves and ganglia that control and coordinate such organs which are not under voluntary control. It comprises sympathetic nervous system and parasympathetic nervous system.


It is lodged in the cranial cavity of the skull. The brain is covered by two membranes or meanings (sing meninx). The outer tough, thick membrane is duramater, and the inner thin, more delicate and vascular membrane is pia arachnoid membrane. The brain is divisible into three parts: Fore brain, mid brain and Hind brain.

Fore Brain

It comprises two olfactory lobes, two cerebral hemispheres and a diencephalon. Dorsal surface of the diencephalon has anterior choroid plexus to supply nourishment to the anterior parts of the brain.

From the dorsal side of the diencephalon also arises a short cylindrical pineal stalk, which carries knob like pineal body. The pineal body is an endocrine gland. The position of pineal body is indicated externally by the brow spot.

A pair of thick optic nerves crosses and forms optic chiasma on the ventral side of the diencephalon. Just behind the optic chiasma, a small outgrowth is formed by the diencepha­lon, which is termed as infundibulum.

Further, behind the infundibulum and attached to it a flattened ovoid sac, the hypophysis (pituitary gland) is present. It is an important endo­crine gland (ductless gland). It produces very important hormones.

Morphology And Anatomy Of Frog 18

Mid Brain

It consists of optic lobes and crura cerebri. Behind the diencephalon two ovals, large slightly pressed to the outside, optic lobes are present. These are well seen in dorsal view. On the ventral side, the brain is thickened to form two fibrous strands of nervous tissue, the crura cerebri (sing, crus cerebrum), which carry the stimuli between the cerebral hemispheres and medulla oblongata.

Hind Brain

It comprises cerebellum and medulla oblongata. Immediately behind the optic lobes, there is present a sort of transverse ridge on the dorsal surface known as cerebellum, which is poorly developed in frog. It maintains balance of the body.

The last part of the brain is medulla oblongata, which is stout and somewhat triangular in shape and continues as spinal cord posteriorly. The medulla oblongata controls involuntary activi­ties of the body. The dorsal side of the medulla oblongata is non-nervous but highly vascular and forms posterior choroid plexus. The cranial nerves from 5th to 10th are connected with the medulla oblongata.


The brain is hollow containing cavities, which are filled with the cerebro spinal fluid secreted by anterior and posterior-choroid plexuses. The cavities of the brain are known as ventricles.

Each olfactory lobe encloses a ventricle termed as rhinocoel which leads into the ventricles of the cerebral hemisphere, the lateral ventricles or paracoels or first and second ventricles. Two lateral ventricles open into the ventricles of the diencephalon, the diocoel or third ventricle through an aperture, the foramen of Monro.

Each optic lobe con­tains a cavity which is called optocoel. The two optocoels open into a narrow tubular cavity, the iter which connects the diocoel with the myelocoel or fourth venticle (a triangular ven­tricle of the medulla oblongata). The ventricle of the cerebellum is called the metacoel. The myelocoel communicates with the central canal of spinal cord.

Morphology And Anatomy Of Frog 19

Functions of Brain

(1) The olfactory lobes control the sense of smell.

(2) The cerebral hemispheres are the seat of intelligence, memory, will and emotions, but these qualities are poor in frogs, because cerebral hemispheres are not very well formed. They also control the voluntary activities of the animals.

(3) The diencephalon acts as a relay centre, because it conveys the impulses to cerebral hemispheres. It also regulates the autonomic nervous system. Diencephalon also controls the metabolism of fat, water and carbohydrates. The secretion of pituitary body influences a number of activities of the animal.

(4) Since optic lobes receive impulses from the eyes, they control the sense of sight.

(5) The crura cerebri serve to communicate impulses from the fore-brain to the hind-brain or vice versa.

(6) Cerebellum controls and coordinates muscular movements, thus it mainly controls equilib­rium and locomotion of the animal. It is a reduced structure in frog.

(7) Medulla oblongata controls and regulates the various involuntary activities, such as heart beat, respiration, nutrition, sound production and metabolism in general.

Spinal Cord (Myelon)

It lies in the neural canal of vertebral column and extends from the medulla oblongata of the brain to almost the end of back-bone. The spinal cord is surrounded by the same two protective membranes; the meanings, as in the brain viz., a thick outer diameter and a thin inner vascular pia-arachnoid membrane.

In frog, it is short, somewhat flattened structure which widens anteriorly and tapers towards the posterior end into the urostyle, where it is called filum terminate. The spinal cord shows two swollen parts, one near the arms and the other near the hind limbs known as brachial and sciatic enlargements respectively. These enlargements supply large nerves to the arms and legs.

All along its length, spinal cord has two median longitudinal grooves, the dorsal fissure on the dorsal side and ventral fissure on the ventral side while the latter is more grooved.

Spinal cord serves two main functions

(i) The stimuli are passed from and to the brain through spinal cord.

(ii) It is the centre of spinal reflex action.

Cranial Nerves

The nerves which connect the brain and leave the brain box (cranium) are known as cranial nerves. The number of cranial nerves is definite in a particular group of animals. In frog, the number is ten pairs. The serial number of a nerve is also definite.

Morphology And Anatomy Of Frog 20

Olfactory Nerve

It carries nerve impulses (smell) from the nasal chamber to the brain, hence, the nature of the nerve is sensory. Optic Nerve. It carries impulse of sight from the eye to the brain, and its nature is sensory.

Optic nerve

It carries nerve impulses of sight from the eye to the brain, and its nature is sensory.

Oculomotor Nerve

It innervates four eye muscles viz., inferior oblique, internal rectus, superior rectus and inferior rectus. This nerve caries the impulse from brain to these muscles for controlling the movements of the eye ball and is motor in nature.

Trochlear (Pathetic) Nerve

It is distributed to one eye muscle, the superior oblique. This nerve helps in controlling the movement of eye ball and its nature is motor.

Trigeminal Nerve

It divides into three branches

Ophthalmic Nerve

It innervates the skin receptors of the snout. Thus, it conveys impulses from the skin of this area to the brain and, hence sensory in nature,

Maxillary Nerve

It supplies the receptors of the skin, upper jaw, upper lip, lower eye-lid and carries the impulses from these areas to brain and hence is sensory in nature,

Mandibular Nerve

It innervates the skin and muscles of the lower jaw and tongue. It is mixed branch. It carries sense of touch from lower jaw to the brain. It also controls the movement of lower jaw.

The total nature of trigeminal nerve is mixed.

Abducens Nerve

It supplies the remaining eye muscle, “the external rectus and controls the movement of the eye ball and is motor in nature.

Facial Nerve

It divides into two branches

Palatine Nerve

It supplies the roof of the buccal cavity (palate) and carries the sense of taste to the brain. So, it is sensory in nature,

Hyomandibular Nerve

It innervates the muscles of hyoid apparatus, muscles and skin of lower jaw. It controls the activities of these organs and also conveys the sense of touch from skin of lower jaw. Its nature is mixed. Thus, total nature of the facial nerve is mixed.

Auditory Nerve

It supplies the ear and carries impulses from the ear to the brain, thus helps in hearing and balancing the organs. The nature of this nerve is sensory.

Glossopharyngeal Nerve

It supplies the taste buds of tongue and muscles of the pharynx. Some fibres carry impulses from tongue, while the other fibres are responsible for pharynx movement as they are concerned with swallowing reflex. Thus, both sensory and motor nerve fibres are present in this nerve. The nature of this nerve is mixed.

Vagus Nerve

It is to be noted that it is the only cranial nerve, which enters the body cavity after emerging from the brain box (cranium).

It divides into the following branches

Laryngeal Nerve

It supplies the laryngeotrachial chamber,

Cardiac Nerve

It inner­vates the heart,

Gastric Nerve

It supplies the stomach,

Pulmonary Nerve

It innervates the lungs. Vagus is mixed in nature because it carries both motor and sensory fibres.

Spinal Nerves

These nerves arise from the spinal cord and are normally nine pairs, sometimes 10th unpaired nerve is also seen. Rarely 10 pairs of spinal nerves are found.

As soon as the spinal nerve comes out of the vertebral column, the origin of each nerve is cov­ered with soft white calcarious masses, which are called glands of Swammerdam or periganglionic glands in which cal­cium is reserved. As soon as two roots come out of the vertebral column, they unite to form a very small branch, which immediately divides into three branches.

Morphology And Anatomy Of Frog 21

Dorsal branch or ramus dorsals

It is a short branch supplying the skin and muscles of the dorsal side of the trunk.

Ventral branch or ramus ventrails

It is thicker, larger and supplies the skin and muscles of the ventral and lateral side of the trunk mostly, but in some cases it is also connected with the limbs.

Ramus communicans

It is a short fine branch, which arises near the origin of each ventral branch and joins sympathetic cord of its side.

Distribution, Functions and Nature of Spinal Nerves

Except the first pair of spinal nerve, all are distributed to the skin and muscles of the respective parts of the body. They carry impulses from the skin receptors to the spinal cord and from the spinal cord to the muscles. Thus, their nature is mixed.


The first spinal nerve is also known as hypoglossal Nerve. It supplies muscles of the tongue, floor of the buccal cavity and some muscles of the shoulder and back. It also takes part in the formation of brachial plexus. It is a motor branch.

2nd and 3rd

The hypoglossal nerve, the second and the third spinal nerves form the brachial plexus. From the brachial plexus, the second nerve continues as a brachial nerve to the skin and muscles of fore-limb.

4th, 5th and 6th

The 4th, 5th and 6th spinal nerves supply the skin and muscles of the body wall of the belly region.

7th, 8th and 9th

The 7th, 8th and 9th spinal nerves form the sciatic plexus, which gives off branches to the skin and muscles of the abdomen and hind-limbs. This plexus also supplies some branches to the large intestine, genital ducts and urinary bladder.


The tenth nerve is also known as coccygeal nerve, which is not commonly found in Rana. Whenever this nerve is present, it emerges from the urostyle through an aperture and joins the sciatic plexus. The fine branches of 10th nerve supply the urinary bladder and cloaca.

There is a noteworthy point that the roots of seventh to tenth nerves first run inside the neural canal of the vertebral column for some distance to form a horse-tail shaped structure, the cauda equina. Later on these nerves come out of the neural canal.

Autonomic Nervous System

It is a system of nerve fibres and ganglia which control and coordinate the involuntary activities of the visceral organs, such as secretion of digestive fluid, action of heart, etc.

This system is autonomic in the sense that it regulates such activities of the body in which the will power of the animal is not involved, for example, the secretion of the digestive fluid is always under the control of autonomic nervous system, but the animal is not aware of it.

The autonomic nervous system is divisible into

(i) Sympathetic nervous system,

(ii) Para-sympathetic nervous system.

Sympathetic Nervous System

It consists of two longitudinal sympathetic cords, one on either side of the vertebral column ventral to the dorsal aorta, which run forward along the outer sides of the systemic arches. Each cord bears at intervals small black nine to ten sympathetic ganglia connected with the corresponding spinal nerves by the ramus communicans (a branch of spinal nerve).

Anteriorly, each sympathetic cord enters the skull along the vagus cranial nerve (10th) and joins with the vagus ganglion and ends in the gassenan ganglion of the trigeminal cranial nerve (5th). Posteriorly each sympathetic cord joins the 9th spinal nerve and later on ends there.

The nerves arising from the sympathetic ganglia supply the respective visceral organs, such as heart, liver, stomach, intestine, kidneys, gonads, blood vessels, urinary bladder, etc. On stimulation the sympathetic nerve fibres secrete a chemical sympathin, which stimulates the organs to function. Example: It increases heart beat.

Parasympathetic Nervous System

There is no such cord or ganglia as found in the sympathetic nervous system. It consists of very small parasympathetic ganglia situated in the walls of visceral organs (viscera) and nerve fibres. These are connected with the central nervous system after travelling in some cranial and spinal nerves.

On stimulation, the parasympathetic nerve fibres secrete a chemical called acetylecholine, whose function is just opposite to that of sympathin. Example: It slows heart beat. The function of sympathetic and parasympathetic nervous system is antagonistic to each other. Autonomic nervous system is ultimately controlled by diencephalon (hypothala­mus) of the brain.

Sense Organs of Frog

Frog has five types of sense organs

Tango receptors (organs of touch)

These are nerve endings and touch corpuscles which are found in the skin. Nerve endings are also present in the viscera (soft internal organs),

Olfactoreceptors (organs of smell)

They are present in the nasal epithelium of the nasal chambers,

Gustatoreceptors (Organs of taste)

These are found as taste buds which are present in the epithelium of the tongue and the buccopharyngeal cavity,

(iv) Photoreceptors (organs of sight) — eyes,

(v) Statoacoustic receptors (Hearing and balancing organs)-Ears.

Out of these eyes and ears are well organised. They are described below.

Structure of Eye of Frog

Eye lids and gland

Each eye is provided with upper and lower eye lids which are actually simple folds of skin. The upper eye lid is thick and very slightly movable. The lower eye lid is vestigial and immovable.

The nictitating membrane is movable, semitrans­parent and can be drawn over the eye when animal is inside water where it prevents aquatic infection and also enables the frog to see under water. This membrane is retracted when the frog is on land. A harderian gland is present below the lower eye lid whose secretion lubricates the eye ball and nictitating membrane.

Morphology And Anatomy Of Frog 22

Muscles attached to the eye ball

Six other muscles are attached to the eyes which bring about the movements of the eye ball. The four muscles are recti, namely, superior rectus, inferior rectus, internal rectus, external rectus and two oblique muscles: superior oblique and inferior oblique.

The superior oblique is supplied by fourth cranial nerve, inferior oblique, internal rectus, superior rectus and inferior rectus are innervated by 3rd cranial nerve while external rectus gets sixth cranial nerve.

Layers of eye ball

The wall of the eye ball mainly consists of three layers: Outer fibrous coat, middle vascular coat and inner nervous coat.

Outer Fibrous Coat

It is the outermost layer of eye ball which is fibrous in nature. Two third part of this layer is opaque and lies inside the eye orbit and is known as sclerotic. The muscles controlling the movement of eye ball are attached on it.

The remaining one-third of the outer layer is transparent and bulges out to form the cornea. The cornea is lined externally by a thin transparent membrane, the conjunctiva, which is continuous with the lining of the eye lids.

Middle Vascular Coat

Choroid lies next to outer layer which is richly supplied with blood capillaries and black pigmented cells. It forms a circular pigmented yellow membrane, the iris towards the anterior side to enclose an oval aperture, the pupil.

The iris contains both circular and radial muscles. Due to the contraction of radial muscles the diameter of the pupil is increased and by the contraction of circular muscles the diameter of the pupil is decreased. A vascular fold of the choroid, the ciliary body lies behind the iris. The ciliary processes arise from the ciliary body. The ciliary body is poorly developed in frog.

Just behind the iris, a transparent, crystalline and almost spherical lens is situated, which is enclosed in the delicate transparent lens capsule. It is held and kept in position by the fibres of the suspensory ligament, which extend from the lens capsule to the ciliary body.

The lens divides the cavity of the eye ball into aqueous chamber and vitreous chamber. The aqueous chamber contains a transparent watery fluid, the aqueous humour, while the vitreous chamber contains a jelly like substance, the vitreous humour.

Both these humours help in refracting the light and in maintaining the shape of the eye ball. There are two protractor lentis muscles, one dorsal and one ventral, which extend between the cornea and the inner part of the ciliary body. They help in the slight adjustment of the object.

Inner Nervous Coat (Retina)

It is the innermost layer of the eye on which the image is formed, that is why it is nervous and perceptive layer. It is also called retina. It is made up of four layers. The outer layer is non-sensory pigmented cuboidal epithelium, which lies very close with the choroid.

Next to the epithelial layer, a layer of cones and rods is present. Rods and cones contain visual pigments. The rods mainly enable the animal to see in the darkness, that is why these are present in large numbers, in nocturnal animals.

The cones are chiefly concerned with distinguishing colour and day light vision. The rods and cones are followed by a layer of bipolar neurons. Just inner to the bipolar neurons a layer of ganglion-cells is present, whose cells give rise to optic nerve fibres to form optic nerve in each eye.

The optic nerve is connected with the brain. Where the optic nerve arises, that point is called blind spot; no image is formed at this spot because sensitive cells of retina are absent at this point. A point on the retina where sharpest image is maintained, lies more or less opposite the centre of the pupil, is called yellow spot or area centralis. This spot contains numerous cones.

Since the eyes of frog are situated one on each side of the head, each eye focuses its own object and both the eyes cannot focus on one object, this type of vision is known as monocular vision.

So each eye of this animal forms a separate image of the object. It is just reverse to the binocular vision present in human beings in which both the eyes can be focused simultaneously on a common object. Monocular vision in frog is due to the absence of neck.

Structure of Ear of Frog

The ears are hearing and balancing organs, which are situated just behind the eyes.

Each ear consists of two parts

(1) Middle ear, and

(2) Internal ear.

Middle ear

Exter­nally, it bears a rounded patch, the tympanum or tympanic membrane, which is tightly mounted on a cartilaginous ring, the tympanic ring. Inner to the tympanic membrane an air filled chamber is present which is known as tym­panic cavity. This cavity communicates with the buccopharyngeal cavity through a narrow Eusta­chian tube.

A club shaped rod like structure, partly formed of bone and partly of cartilage, the columella auris extends from the cen­tre of the tympanic membrane and attaches with a plug of cartilage, the stapes. The stapes lies over a small oval aperture, the fenestra ovalis. The fenestra ovalis leads into auditory capsule.

Morphology And Anatomy Of Frog 23

Internal ear

It is lodged in the bony auditory capsule, which is mainly formed by prootic bone, whose cavity contains a watery fluid, the perilymph. This capsule lodges the actual statoacoustic organ known as membranous labyrinth.

It consists of dorsal main body, the utriculus, and the ventral, projecting downwards smaller part, the sacculus. Three semicircular canals (anterior vertical semicircular canal, posterior vertical semicircular canal and horizontal semicircular canal) arise from the utriculus.

The anterior and posterior semicircular canals are united at their origin. Each semicircular canal is enlarged at one end to give rise to small rounded ampulla. The anterior, vertical and horizontal canals bear the ampullae at their anterior ends while the posterior vertical canal contains an ampulla each at its posterior end.

Thin narrow tube, the ductus endolymphaticus, arises from the sacculus and enters the cranium (brain box) to form a thin walled sac, the endolymphaticus sac over the hind-brain.

This sac contains a milky fluid along with the crystals of calcium carbonate, whose function is not known. Posterior portion of sacculus forms two small rounded outgrowths, a relatively large lagena and smaller pars basilaris. Lagena is the forerunner of the cochlear duct of higher vertebrates.

Pars basilaris seems to be a part of lagena. A small similar outgrowth, the pars neglecta arises from utriculus. The pars neglecta is located close to lagena and pars basilaris. Ampullae contain sensory spots called cristae. Utriculus, sacculus and legena also have sensory spots known as maculae. The ampullae, utriculus, sacculus, lagena and three canals are supplied by the branches of auditory nerve.

Morphology And Anatomy Of Frog 24

The functions of ear are hearing, equilibrium and movement of the body.

Skeletal System of Frog

There is no exoskeleton in frog. Endoskeleton consists of axial skeleton (skull, vertebral column and sternum) and appendicular skeleton (pectoral and pelvic girdles and bones of fore and hind limbs). Ribs are absent in frog. The skull of frog consists of cranium, sense capsules and jaws.

There is a foramen magnum through which brain joins the spinal cord. Frog’s skull is dicondylic (with two occipital condyles). Hyoid apparatus lies in the floor of the buccal cavity and provides attachment and support to the tongue. Vertebral column (back bone) of frog consists of ten vertebrae including urostyle.

First vertebra is called atlas. 2nd to 7th vertebrae have same structure hence called typical vertebrae which are procoelous (centrum is concave in-front and convex behind). 8th vertebra is amphicoelous (centrum is concave in both the sides). 9th vertebra is acoelous (centrum convex on both the sides). 9th vertebra is also called sacral, Urostyle is 10th vertebra.

It is sword like. Spinal cord in frog extends up to the anterior part of the urostyle. Sternum of frog has four parts: episternum, omosternum, mesosternum and xiphisternum. Pectoral girdle (shoulder girdle) is made up of similar right and left halves. Each half has a glenoid cavity.

The head of humerus fits into the glenoid cavity. Pelvic girdle (hip girdle) is ‘V’ shaped. Each half of pelvic girdle is composed of ilium, ischium and pubis bones. Each half has a cavity called, the acetabulum. The head of femur fits into the acetabulum.

A fore limb consists of humerus, radio-ulna, carpals, metacarpals and four digits. The digital formula for the fore limb of frog is 02233. A hind limb comprises femur, tibio- fibula, tarsals, metatarsals and five digits. The digital formula for the hind limb of frog is 22343. Characteristic feature of humerus is deltoid ridge and of femur is trochanter.

Morphology And Anatomy Of Frog 25

Urinogenital System of Frog

Since the urinogenital system comprises two closely associated urinary (excretory) and the genital (reproductive) system, it is collectively called as the urinogenital system.

Morphology And Anatomy Of Frog 26

Excretory System of Frog

The excretory system in both male and female frogs is similar except a few minor differences.

It comprises the following organs


There are two kidneys, situ­ated one on either side of the vertebral column in the posterior part of the body cavity. The kidneys are the chief excretory organs which are oval, dorsoventrally flattened, dark, red bod­ies. Peritoneum covers each kidney only on its ventral surface.

An elongated irregular yellow structure, the adrenal gland is present on the ventral surface of each kidney, which produces hormones. The fat bodies are attached anteri­orly. Renal arteries and veins are associated with the kidneys.

Internally, each kidney is made up of a large number of minute (microscopic) tubes (about 2,000), known as uriniferous tubules or nephrons bound together by connective tissue. Each uriniferous tubule is the structural and functional unit of the kidney.

Thin walled expanded cup shaped blind end of each tubule, is called ‘Bowman’s capsule. The Bowman’s capsule leads into a long coiled ciliated urinary tubule. The urinary tubule opens into trans­verse collecting tubules which ultimately communicate with the ureterurinogenital duct (to be described ahead).

The kidneys receive renal arteries and each artery divides into arterioles, one arteriole goes to each Bowman’s capsule known as afferent arteriole. This breaks up into smaller branches (not capillaries) to form a network, termed as glomerulus.

From the glomerulus, an efferent arteriole emerges out of the capsule and proceeds towards the urinary tubule, where efferent arteriole breaks up into peritubular capillaries network. The latter unite to form venules which join to form renal vein.

The renal portal vein after entering the kidney also breaks up into capillaries, which unites with the capillaries and finally connect the renal veins. A few ciliated funnels, the nephrostomes, are situated on the ventral surface of the kidney to collect the excretory matter from the coelom and pour it into the renal vein.

Thus, Bowman’s capsule receives only the arterial blood supply, while the renal tubules get arterial as well as the venous blood supply.

Morphology And Anatomy Of Frog 27

Each kidney contains a longitudinal Bidder’s canal (Fig. 7A.69) to which the transverse collecting tubules are connected. All the transverse collecting tubules lead into the ureter urinogenital duct.

Morphology And Anatomy Of Frog 28

Ureters Urinogenital ducts

From the outer border of each kidney a fine trans­parent duct arises, which is known as ureter in the female and urinogenital duct in the male. They run backwards from the kidneys and open into the cloaca, and carry urine from the kidneys to cloacal chamber.

In female ureters carry urine alone, while in male both sperms and urine are carried from kidney to cloacal chamber, hence called urinogenital ducts. In some species of frog the posterior part of each urinogenital duct dilates to form a seminal vesicle. The latter stores the sperms temporarily. Seminal vesicles are not well-developed in Rana tigrina.


The terminal portion of the alimentary canal is known as cloaca which receives faecal matter, genital products and urine. It opens outside through an opening, the cloacal aperture provided with a sphincter.

Urinary bladder

It is a transparent, highly distensible bilobed sac, attached to the ventral side of the cloaca, below the opening of the ureters urinogenital ducts. When it is completely empty, it shrinks.

It can undergo periodic contractions due to the presence of un-striped muscles. It does not have direct connection with ureters urinogenital ducts. The opening of the urinary bladder into the cloaca has a sphincter to regulate its passage. It can store the urine for some time.

Reproductive System of Frog

Reproductive system is meant for the production of sex cells (sperms or eggs) and transporting the same through certain ducts to outside. The sexes are separate. The male and female can be distinguished by their external features. This phenomenon is known as sexual dimorphism.

The vocal sacs are present only in male, and during the breeding season, a nuptial pad is developed at the base of the first finger of the male frog. The vocal sacs increase the pitch of the sound, while the nuptial pads help in grasping the female during amplexus. As already mentioned in the excretory system, the two systems (excretory and repro­ductive) are closely associated with each other and form urinogenital system.

Male Reproductive System


These are two, oval yellow coloured structures. Each is lying attached on the outer side of the anterior most part of each kidney on the ventral side. With the help of a thin peritoneum, the mesorchium, each testis remains attached with the dorsal wall of the abdomen and kidney. Testis is composed of a large number of small tubes, the seminiferous tubules, which look rounded in section.

The wall of the seminiferous tubules is made up of germinal epithelium, which forms sperms by spermatogenesis. The space between the seminiferous tubules is filled with the connec­tive tissue, blood capillaries, nerve fibres and interstitial cells. The interstitial cells secrete a male sex hormone, which produces the secondary sexual characters in male viz., nuptial pads during breeding season, etc.

There are present some finger like outgrowths, yellowish in colour attached to the anterior end of each testis which are collectively known as fat-body. The fat bodies store the food material specially for hibernation period. They increase in size during summer.

Vasa efferentia

There are 10-12 very fine tubes connecting the testes to kidneys on each side. The vasa efferentia run transversely through the mesorchium. They open into the anterior part of Bidder’s canal.

The sperms produced by the germinal epithelium of seminiferous tubules are passed into Bidder’s canal via vasa efferentia and thence carried to the transverse collecting tubules and then to the urinogenital duct via longitudinal collecting tubule.

Urinogenital ducts

Each kidney gives rise to a muscular urinogenital duct at its outer border, which carries urine as well as sperms. Each urinogenital duct runs posteriorly, and before opening into cloaca, it dilates to form a seminal vesicle to store the sperms temporarily. In some frogs the seminal vesicles are not found.


It is a common chamber for receiving faecal matter, urine and sperms. All these products are passed out through cloacal aperture. Copulatory organ present in higher animals, is absent in frog. During amplexus, the male frog sheds his sperms on the female’s ova. It happens in the surrounding water where external fertilization takes place.

Germinal epitheliums of seminiferous tubules produce sperms which are transferred to the Bidder’s canal via vasa efferentia. From the Bidder’s canal, the sperms are carried to the transverse collecting tubules, longitudinal collecting tubule and then to the urinogenital duct.

The latter carries the sperms to the seminal vesicle where they are stored temporarily. From the seminal vesicles, the sperms are transferred to the cloaca and then they are shed into water through cloacal aperture. Male frog sheds its sperms on the female’s ova during amplexus.

Female Reproductive System

The female reproductive system comprises the following parts


These irregular shaped, lobulated structures, occupying the same position as the testes in male frog. The ovaries are attached to the dorsal wall of abdomen, and kidneys with the help of thin membrane, known as mesovarium. The ovaries have no internal connections with the kidneys. During the breeding season, the wall of the ovary ruptures to release the ova into the coelom.


These are paired, white, glandular, long, coiled tubes lying one on either side of the body cavity. Each oviduct consists of ovarian funnel, ovarian tubule and ovisac.

Morphology And Anatomy Of Frog 29

By the rupturing of the ovary wall, the ova are released into body cavity and reach the ovarian fun­nels, through ostia, where they find their way to the ovarian tubule, from where they pass to the ovisac, cloaca and then outside.

This all happens due to the pressure exerted by the fore-limbs of male on the body of the female frog during amplexus (the clasping of the female frog by the male frog before discharging sexual products in water is called amplexus, and by the beating of cilia present in the internal epithelial lining of the oviduct.

A mature female frog can lay 2500 to 3000 ova at a time fertilization is external. Development is indirect involving a larval stage called tadpole.

Endocrine Glands of Frog:

Frog has the following endocrine glands (ductless glands) which secrete hormones:


They are paired in frog. Each thyroid gland is situated behind the hyoid apparatus. They secrete thyroxine. Thyroxine regulates metabolism. It also brings about metamorphosis (change from larva to adult).


These are two pairs of small bodies located on either side of the posterior portion of the hyoid plate. They secrete parathormone (PTH) which regulates the calcium phosphorous balance in the blood.

Adrenals (Adrenal glands)

There is a pair of adrenal glands. Each adrenal gland is a yellow or orange struc­ture present on the ventral surface of each kidney. In higher vertebrates like mammals, the adrenal gland consists of distinct outer cortex and inner medulla. But in frog the adrenal glands consist of two types of tissue closely intermixed.

Infact, the two tissues gather in a com­paratively small area and come to lie on the ventral side of the kidneys as irregular yellowish streaks. The harmones of the adrenal cortex are called cortin or adrenocorticoides (e.g, cortisol, aldosterone).

Morphology And Anatomy Of Frog 30

Cortisol regulates the metabolism of carbohydrates, proteins and fats. Aldosterone in­creases reabsorption of sodium in the uriniferous tubules (nephrons) of the kidneys. Adrenaline is the hormone of the medulla which increases heartbeat, blood pressure, blood sugar, respiratory rate, etc.

Pituitary gland (Hypophysis)

It is present on the ventral side of the diencephalon of the brain. The pituitary gland is also called ‘master endocrine gland’ because some of its hormones influence other endocrine glands.

It consists of anterior lobe, intermediate lobe and posterior lobe (a) Anterior lobe secretes the following hormones, (i) Growth hormone (GH) influences growth of the body (ii) Gonadotrophic hormones (GTHs) affect growth and functioning of the gonads (testes and ovaries) (iii) Thyroid stimulating hormone (TSH) stimulates growth of thyroids and production of thyroxine, (iv) Adrenocorticotrophic hormone (ACTH) stimulates cortex of adrenal glands to grow and secrete its hormone, (b) Intermediate lobe secretes melanocyte stimulating hormone (MSH) which influences the colouration of the skin, (c) Posterior lobe secretes antidiuretic hormones (ADH) or vasopressin which increases the reabsorption of water in the uriniferous tubules (nephrons) of the kidneys.

Pineal gland (Epiphysis cerebri)

It lies on the dorsal surface of the diencephalon of the brain. It secretes melatonin hormone which affects the colour of the skin and inhibits gonad function.


The thymus in the adult frog is a small compact lymphoid body located behind each tympanum under the depressor mandibulae muscle. It is much smaller in the adult than in the tadpole. It secretes thymosin hormone. Thymosin stimulates the production of lymphocytes (a type of WBCs), increasing resistance to infection. It also hastens attain­ment of sexual maturity.


Endocrine part of the pancreas is composed of islets of Langerhans. Some cells of islets of langerhans secrete insulin hormone which converts glucose into glycogen in the liver.


Certain cells of the mucous membrane (=mucosa) of the stomach secretes a hormone called the gastrin which stimulates the gastric glands to produce the gastric juice.


Certain cells of the mucous membrane (=mucosa) of the duodenum secretes some hormones such as secretin and cholecystokinin. Secretin causes the release of sodium bicarbonate solution from the pancreas for pancreatic juice and from the liver for bile cholecystokinin stimulates the gall bladder to release the bile into the duodenum.


These are attached to the antero-lateral part of the kidneys, whose primary function is to produce sperms but there are also present endocrine cells known as intersti­tial cells to secrete male sex hormone, which brings about secondary sexual characters (the characters which appear only during sexual maturity) in male, viz., development of nuptial pads. Sex hormone also influences the male bahaviour during mating.


They have the same position as those of testes. The ovaries not only produce ova, but also female sex hormone to cause secondary sexual characters in female viz., she receives the male at a certain period or periods of the year. If the ovaries are removed the female sex hormone is not produced, the secondary sexual characters do not appear and the sex instinct is never present.

Interaction of Frog with Mankind

Frog is useful animal for human being because it eats up insects which are harmful for the crops. Thus the frog saves expenditure on insecticides. Frog also eats mosquitoes which act as vectors for parasites (e.g., malarial parasite) of man. Frog has been used as an experimental material for teaching and researches. It was ideal animal for dissections.

The muscles of the legs are used as food by man in some parts of India and many other countries. Frogs were exported in large quantities but it is now legally banned. Frog-lets (very small frogs) are used as fish bait.

Due to habitat loss and large scale use of this animal, it has become a threatened species. Since frog is important animal in the food chain, it helps to maintain our ecosystem. So it should be protected. Use of frogs for dissection purpose should also be discouraged as far as possible.

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