FROG (Rana tigrina)

Phylum : Chordata

Class : Amphibia

Order : Anura

Genus : Rana

Species : tigrina

The most common frog found in India is the Indian bullfrog.

It is the largest frog and is named as bullfrog because of its large size and loud call.

Indian bullfrog is found in fresh water marshes, ditches, ponds, and shallow lakes.

They undergo aestivation (summer sleep) in summer and hibernation (winter sleep) in winter.

They are carnivorous (feeding upon other animals, insects, etc.), poikilothermic i.e. the body temperature changes with environment.

They develop protective coloration to camouflage, i.e., to hide in surroundings.

Frogs belong to the Phylum Chordata, Subphylum Vertebrata or Craniata, Superclass Gnathostomata, Class Amphibia and Genus Rana.

The most common species is known as Rana tigrina (Indian bull frog).

The scientific name of common toad is Bufo melanostictus. Frogs exhibit sexual dimorphism.

Male and female are distinguishable externally only during breeding season when the males develop nuptial pad in the first digit of forelimbs.

Vocal sacs are well developed in males so they produce louder sound as compared to the females which are devoid of vocal sacs.

The vocal sacs help to produce mating calls.

Total number of bones is 153.

External Morphology

Skin is made up epidermis and dermis. Mucous glands are present in the dermis and their ducts open at the surface.

Blood capillaries and pigment cells (chromatophores) are present in the dermis.

Skin is without scales or any other cover or exoskeleton.

Body is divisible into head and trunk, neck is absent. The trunk is provided with a pair of fore and hind-limbs. The hind-limbs are much larger and muscular than the fore-limbs. Fore-limbs end in four digits and the hind limbs end in five digits. The digital formula of fore-limbs is 02233. The digital formula of hind limbs is 22343.

Shank or crus is associated with hind limbs.

Sexual dimorphism :

The male () and female () frogs exhibit certain differences in their external features, which become more pronounced during breeding season.

Generally, male frogs are larger than females.

During breeding season, however, the females become bloated with large ovaries and numerous ova, and appear considerably larger.

Only the males possess a pair of ventro-Iateral, wrinkled pouch-like vocal sacs located a little behind the mouth.

These sacs become specially large and distensible, in breeding season.

By inflating these repeatedly with air from the lungs, the males produce a loud croaking sound meant to call the females for copulation (amplexus).

The sound is actually produced by a pair of vocal cords in the larynx; the sacs only increase its pitch, like resonators.

The females produce a low pitch sound by their vocal cords alone.

The forelimbs in both male and female frogs bear small articular pads dorsally at the joints of digits, but the males also possess a special nuptial, copulatory or amplexusary pad on ventral side of the first finger of each forelimb.

Normally, these pads appear merely as rough patches but during breeding season, these become thick and sticky.

In amplexus, the male strongly grips a female under her armpits by means of these pads.

Internal Morphology

Digestive System

Since the frogs are carnivorous, their alimentary canal is short in length.

Tadpole larva is herbivorous so alimentary canal is very long and coiled in the form of spring.

The mouth is present as a terminal, wide opening.

It opens into bucco-pharyngeal cavity, which contains numerous maxillary teeth arranged along the margin of the upper jaw and vomerine teeth are present in the roof of the buccopharyngeal cavity.

The lower jaw is toothless. Salivary glands are absent.

Opening of eustachian tube, vocal sacs (only in male) gullet and glottis can be seen clearly in the bucco-pharyngeal cavity.

The muscular tongue is bilobed at the tip and free from behind. It is used for capturing the prey.

The gullet opens into a narrow and short tube-like oesophagus, which continues in large and distended stomach.

It contains a thick muscular layer, which helps in converting food into chyme.

It secretes gastric juice containing HCl and proteolytic enzymes. Stomach is followed by a coiled small intestine.

Intestinal wall has numerous finger-like folds called villi and microvilli, projecting into its lumen to enhance surface area for absorption of digested food.

The first part of the small intestine lying parallel to stomach is called duodenum. Intestine continues into a wider rectum, opening into cloaca.

The urinary bladder opens into cloacal chamber through the ureter.

The gastric and intestinal glands occur in the walls of stomach and intestine respectively.

The other important digestive glands associated with the alimentary canal are liver and pancreas.

Liver secretes bile which is temporarily stored in gall bladder before being released into the duodenum.

Bile helps in digestion of food by changing its pH from acidic to alkaline and by emulsifying the fats.

Liver does not secrete any digestive enzymes. Pancreas is an irregular, elongated gland, situated in a thin mesentery and lies parallel to the stomach.

It produces pancreatic juice containing digestive enzymes like trypsin, amylopsin, etc.

Respiratory System

Three types of respiration: cutaneous, buccopharyngeal and pulmonary occur.

Cutaneous respiration on land is through the body surface. During hibernation and aestivation, frog respires only through this method.

Buccopharyngeal respiration occurs through the lining of buccal cavity. It occurs only when frog is out of water. The mucus membrane of the buccal cavity is moist which dissolves oxygen whose diffusion occurs into the blood capillaries.

Pulmonary respiration: Lungs in frog are not efficient respiratory organs because only mixed air enters into them and they mainly function as hydrostatic organs.

Lungs are a pair of thin walled, translucent sacs with inner surface divided into alveoli by septa. Pulmonary respiration has a maximum frequency of 20/minute.

It occurs when more energy is required. Mouth and gullet are kept closed during pulmonary respiration.

Respiratory movements in pulmonary respiration are because of buccopharyngeal cavity which acts as a force pump.

These movements are carried out by set of paired muscles -sternohyal and pterohyal muscles.

Sternohyal muscles are attached with hyoid and coracoid processes, clavicles of the pectoral girdle and on contraction, depress the buccal floor enlarging the buccopharyngeal cavity.

Pterohyals are attached in between hyoid and pro-otics of the skull and on contraction lift the floor of buccal cavity.

With the depression of buccal floor, air enters buccal cavity through the nares.

External nares are then closed by pushing tuberculum prelinguale and the movable premaxillae.

It is followed by raiSing of the buccal floor by pterohyal muscles which reduces the volume and the air is pushed into the lungs where exchange of gases takes place.

Buccal floor is again lowered enlarging its volume which draws air into the buccal cavity.

External nares are opened fol'lowed by raising the buccal floor, pushing the air out through external nares.

Sound producing organ of frog is laryngo-tracheal chamber.

It is supported by one cricoid, two arytenoids and two prearytenoid cartilages.

It has a pair of muscle strands (vocal cords) which actually prod uce sound. Male frog has vocal sacs which act as resonating chambers.

Circulatory System

Circulatory system is closed type.

The heart lies enclosed by a thin, transparent, two layered sac, pericardium.

Frog's heart is a three chambered structure made of two upper auricles and a single lower ventricle.

The two additional chambers connected to the heart of frog are sinus venosus and truncus arteriosus.

Frogs also possess two well developed portal systems: Renal portal system and Hepatic portal system. Frog also has two pairs of lymph hearts.

Nervous System

The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic chains ofganglia).

There are ten pairs ofcranial nerves.

Brain is enclosed in a bony structure or brain box (cranium) which has two occipital condyles for attachment with the first vertebra (atlas).

The brain is divided into fore-brain, mid-brain and hind-brain.

Forebrain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon.

The mid-brain is characterised by a pair of optic lobes.

Hind-brain consists of cerebellum and medulla oblongata.

The medulla oblongata passes out through the foramen magnum and continues into spinal cord which is contained in the vertebral column.

Jacobson's organ, also called Vomero-nasalorgan opens into nasal chamber and acts as additional olfactory organ.


Eye is guarded by immovable upper eyelid, movable lower eyelid and transparent nictitating membrane.

Vertical section of frog's eye (Diagrammatic)

Outersclerotic ring is cartilaginous and cornea is the part exposed out. Middle, highly vascular, pigmented layer is choroid.

Iris is yellow pigmented and perforated by a central aperture, the pupil.

Retina is the innermost coat of eyeball and consists of inner pigmented layer and an outer receptor layer.

Rods and cones are light sensitive structures found in retina.

Rods have rhodopsin or visual purple meant for night vision while cones have iodopsin responsible for colour vision in day light.

The anterior chamber present in front of lens is filled with aqueous humour while the chamber behind the lens is posterior chamber, filled with vitreous humour.

Eye ball is moved in the eye orbit by a set of six muscles -two are oblique and four are recti.

Besides, there are retractor bulbi and levator bulbi muscles for intrusion or protrusion of eye ball into the eye orbit, respectively.


Ear of frog has only middle and internal ear.

Tympanic membrane is present at the body surface.

Middle ear has single bone called columella auris.

Its outer end is attached to the ear drum while inner end to the stapedial plate.

Pressure of air in middle ear is controlled by eustachian tubes.

Membranous labyrinth or internal ear consists of utriculus, sacculus and semicircular canals. Endolymph fills the membranous labyrinth .

Excretory System

The main organ of excretion is a pair of kidneys.

These are compact, dark red and bean like structures situated little posteriorly in the body cavity on both sides of vertebral column.

The frog excretes urea, thus is a ureotelic animal.

Urea is carried by blood into the kidney where it is separated and excreted.

Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons.

Ureter emerges from the kidney as urinogenital ducts in the males.

A common ureter opens into the cloaca.

A thin-walled urinary bladder is present ventral to rectum which also opens in the cloaca.

Reproductive System

Male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum called mesorchium.

Vasa efferentia are 10-12 in number and after arising from testes, run through the mesorchium and enter the kidneys of their side.

In kidneys, these open into Bidder's canal which finally communicates with the urinogenital duct.

This duct emerges from the kidneys and finally opens into the cloaca.

The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior.

A pair of ovaries, situated near kidneys, comprises the female reproductive organs.

However, these have no functional connection with kidneys.

A pair of oviduct opens into the cloaca, separately.

The release of ovum in female is termed as spawning.

A mature female can lay 2,500 to 3,000 ova at a time.

Fertilisation is external and takes place in water.

Development involves a larval stage called tadpole.

Tadpole undergoes metamorphosis to form the adult.


Earthworm, Cockroach and Frog show characteristic features in body organisation.

In Pheretima posthuma (earthworm), the body is covered by cuticle.

All segments of its body are alike except the 14th, 15th & 16th segment, which are thick and dark and glandular, forming clitellum.

A ring of S-shaped chitinous setae is found in each segment.

These setae help in locomotion.

On the ventral side spermathecal openings are present in between the grooves of 5 & 6, 6 &7, 7 & 8 and 8 & 9 segments.

Female genital pores are present on 14th segment and male genital pores on 18th segment.

The alimentary canal is a narrow tube made of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus.

The blood vascular system is of closed type with heart and valves.

Nervous system is represented by ventral nerve cord.

Earthworm is hermaphrodite.

Two pairs of testes occur In the 10th and 11th segment, respectively.

A pair of ovaries are present on 12th and 13th intersegmental septum.

It is a protandrous animal with crossfertilisation.

Fertilisation and development takes place in cocoon secreted by the glands of clitellum.

The body of Cockroach (Periplaneta americana) is covered by chitinous exoskeleton.

It is divided into head, thorax and abdomen.

Segments bear jointed appendages.

There are three segments of thorax, each bearing a pair of walking legs.

Two pairs of wings are present, one pair each on 2nd and 3rd segment.

There are ten segments in abdomen.

Alimentary canal is well developed with a mouth surrounded by mouth parts, a pharynx, oesophagus, crop, gizzard, midgut, hindgut and anus.

Hepatic caecae are present at the junction of foregut and midgut.

Malpighian tubules are present at the junction of midgut and hindgut and help in excretion.

A pair of salivary gland is present near crop.

The blood vascular system is of open type.

Respiration takes place by network of tracheae.

Trachea opens outside with spiracles.

Nervous system is represented by segmentally arranged ganglia and ventral nerve cord.

A pair of testes is present in 4th and 5th segments and ovaries in 4th, 5th and 6th segments.

Fertilisation is internal.

Female produces 10-40 ootheca bearing developing embryos.

After rupturing of single ootheca sixteen young ones, called nymphs come out.

The Indian bullfrog, Rana tigrina, is the common frog found in India.

Body is covered by skin.

Mucous glands are present in the skin which is highly vascularised and helps in respiration in, water and on land.

Body is divisible into head and trunk.

A muscular tongue is present, which is bilobed at the tip and is used in capturing the prey.

The alimentary canal consists of oesophagous, stomach, intestine and rectum, which open into the cloaca.

The main digestive glands are liver and pancreas.

It can respire in water through skin and through lungs on land.

Circulatory system is closed with single circulation.

RBCs are nucleated.

Nervous system is organised into central, peripheral and autonomic.

The organs of urinogenital system are kidneys and urinogenital ducts, which open into the cloaca.

The male reproductive organ is a pair of testes.

The female reproductive organ is a pair of ovaries.

A female lays 2500-3000 ova at a time.

The fertilisation and development are external.

The eggs hatch into tadpoles, which metamorphose into frogs.