Frog- Rana tigrina (Amphibian)

A frog is an amphibian, living both on land and water. Amphibians are the first group of chordates that lived outside water.  These are coldblooded vertebrates tetrapods.

Classification of Rana tigrina (Frog):

Habits & Habitats in frogs

The frogs are cosmopolitan in distribution, they are usually found in freshwater ponds, rivers, ditches, under-stones, and in damp places except in arid parts of the country. They are found active during the spring and rainy seasons and become inactive during summer.

Indian frog is also known as the “Indian bullfrog” as it is of large size and loud voice.

Characteristic features of Rana tigrina

Frogs characteristically have smooth skin, strong hind legs for leaping, and webbed feet. Most reproduce in water, laying eggs that develop into larvae (tadpoles). It shows the following habits:

  1. Resting: Most frogs rest on land in a squatting position, in this, they keep their short forelimbs upright and the long hind limb folds inside the body.
    • On any disturbance, they suddenly jump in water by extending their hind limb to escape from enemies.
  2. Feeding habit: The frog is a carnivorous animal, it feeds on earthworms, insects, spiders, snails, and tadpoles. The tadpoles feed on the aquatic plants as they are herbivorous.
  3. Aestivation: (Summer sleep): as the frogs are cold-blooded animals, their body temperature varies according to the environment, so, they cannot survive in adverse conditions. The frogs go on aestivation (summer sleep) to tide over unfavorable conditions. They bury themselves in the soft mud and become sluggish and inactive.
  4. Hibernation (Winter sleep): During the winter season the frogs bury themselves in deep mud and take rest, this is termed as winter sleep or hibernation.
    • In winter the metabolism gets slow and its temperature falls down.
    • Due to a fall in temperature and slow body activity, the frogs become sluggish and inactive.
    • Frogs do not respire with the lungs; they respire through the skin.
    • General vital activities get slow to save energy to maintain life.
    • During this time period, frogs do not take food but consume energy stored in the glycogen in the body.
Frog- Rana tigrina

External features of Frog- (Rana tigrina)

Locomotion in frogs:

Locomotion in frogs takes place by two methods: (i) leaping on land and (ii) swimming in water.

Leaping: While resting on land, the frog keeps its short forelimbs upright and the long hind limbs folded like “Z”. The animal jumps by sudden extension of hind limbs that acts like springs throwing the body into the air.

  • The forelimbs are used to hold up the anterior part of the body and to manipulate the direction.
  • A frog may leap a distance of 1.5 to 2 meters in a single jump.

Swimming: The frog swims in water by alternately flexing and extending its hind limbs, as it starts to swim the legs are extended quickly.

  • During their backward strokes, the toes are spread apart and the broad webs push against the water, moving the body forward.
  • The forelimbs help propel the frog to the extent and also guide the direction of the movement.

Sexual dimorphism in frogs:

In frogs, the male and female can be distinguished by their following morphological characteristics:

  • The males are usually smaller and darker in color than the female
  • Males are slimmer while the females are stouter
  • The male frog has vocal sacs which help in croaking loudly. The female has no vocal sac.
  • The forelimbs in male frogs possess a swollen copulatory pad on the first inner finger, it is absent in females.

Breeding: The animal breeds during the rainy season (June to September). During the rainy season, male frogs croak to attract females for copulation. It is a mating call. Copulation takes place in shallow water, and they lay down eggs for fertilization.

  • Fertilization is external
  • after fertilization development of the tadpole larva
  • Tadpole goes through a metamorphosis to become an adult.
  • There is no parental care for frogs.

Croaking sound: It is the characteristic sound made by frogs during the breeding season. It is a mating call, produced by forcing air from lungs over vocal cords into the mouth cavity and back again. It is louder in males than in females due to the presence of vocal sacs.

Body of frog:

The body of the frog is bilaterally symmetrical. It is somewhat dorsoventrally flattened and streamlined which offers the least resistance during swimming. A light yellow line runs mid-dorsally from the tip of the snout to the cloacal aperture, called the mid-dorsal line. It measures about 12 to 18 cm in length and 5 to 8 cm in width.

Endoskeleton of frog: The frog consists well-developed endoskeleton

Exoskeleton: There is the absence of an exoskeleton in frogs.

Structure of body:

The body of a frog is divided into two regions: Head and trunk. (Tail present only in the larval stage)

Head: It is triangular in shape and jointed directly to the trunk. The head bears Mouth, nostrils, eyes, and tympanum.

  • Mouth: It is bounded by upper and lower jaws, cheeks and lips are absent.
  • Nostrils: A pair of small openings on the anterior tip of the snout.
  • Eyes: There are two large prominent eyes on each side of the head. The eyes are round, protuberant, and dorsolaterally placed on the top of the head.
  • Tympanum: Behind and below the eye, on either side, a circular obliquely placed membrane, the eardrum or the tympanum. (Pinna is absent)

Trunk: It is the posterior portion of the body. The trunk contains the cloacal aperture between two legs. The cloacal aperture is common for feces, urine, sperm, or ova discharge.

The digestive system in Rana tigrina:

The digestive system consists of the Alimentary canal and Digestive glands.

Alimentary Canal of Frog:

It is a long coiled tube situated between the mouth to anus (cloacal aperture). The following organs are associated with an alimentary canal in the digestive system:

Buccopharyngeal Cavity: The buccopharyngeal cavity consists buccal cavity and pharynx together. It lines between the upper jaw and lower jaw.

  • Teeth: two types of teeth, maxillary teeth are found in the upper jaw. They are polyphyodont (replaceable) teeth and homodont (all are similar in size). Vomerine teeth are present on either side of the buccopharyngeal cavity, these help to capture prey.
  • Tongue: A thick, fleshy, muscular, and bifurcated (bifid), protrusible organ, i.e. the tongue can be thrown out and retracted. It arises from the lower jaw.
    • The tongue secrets a sticky substance by which they stick the prey in the tongue.

Vocal Sac: In a male frog on either side of the tongue on the lower jaw there are two pores called vocal sacs, which produce a croaking sound.

Pharynx: The posterior part of the buccopharyngeal cavity is called the pharynx, which opens into the esophagus

Esophagus: It is a short muscular, broad tube that opens into the stomach.

digestive system in frog

The digestive system in the frog

The stomach: It is a large, thick-walled muscular bag. The anterior part of the stomach is called the cardiac part and the posterior part is called the pyloric part.

  • The gastric glands present on the stomach wall secrete HCL and pepsinogen enzymes.
  • Food mixed with HCL softens the food.
  • The pepsinogen is an inactive enzyme, in the presence of HCl, it becomes active, and then it is called pepsin which digests protein into proteases and peptones.
  • The stomach is internally folded, and stores ingested food. The food becomes half solid, called chime, which passes slowly through pyloric constriction.

Intestine: It is a long, coiled part that starts from pyloric constriction, it is divided into two parts:

Duodenum: it is c shaped structure, 3-5 cm long where the hepatopancreatic duct opens. Chyme moves towards the duodenum. The food is mixed with bile and pancreatic juice.

  • Bile: It is a kind of alkaline juice secreted by the liver, it neutralizes acidic food and It emulsifies fat
  • Pancreatic Juice: It is also a kind of juice secreted from the pancreas. Pancreatic juice contains Trypsinogen – In presence of enterokinase it is converted into trypsin and the trypsin digests the protein into peptones and polypeptides, and Amylase – It digests the carbohydrate into maltose.

Ileum: it is a coiled part. It is about 20-25 cm long. The ileum is internally highly folded. Folding s is called villi, which increase the absorptive surface

The food mixed with intestinal juice in the Ileum contains enzymes-

  • Encryption: It digests peptones and proteoses into amino acids.
  • Peptidase: It digests peptides into amino acids
  • Sucrase: It digests sucrose into glucose.
  • Maltase: It digests maltose into glucose
  • Lactase: It digests lactose into glucose.
  • Nucleotidase: It digests nucleic acid into nucleotides

Rectum: It is 4-5 cm long tube. It opens outside through the cloaca and that opening is called the cloacal aperture. It stores undigested food for a short time.

Absorption of Food :

The completely digested food material contains glucose, amino acids, fatty acids, glycerol, etc. Water, minerals, and other simple molecules are not required to digest. The food is now absorbed through the villi of the intestine. The energy is used as ATP hence all food materials are absorbed completely into the blood.

The remaining undigested and unabsorbed material is stored in the rectum for a short time period and passes through the anus.

Respiratory system in Frog:

The process of gaseous exchange (O2 and CO2) and utilization of oxygen to break down food to release energy is called respiration.

Three types of respiration in frogs:

  1. Cutaneous respiration
  2. Buccopharyngeal respiration
  3. Pulmonary respiration
Cutaneous respiration:

The process of respiration through the skin is termed cutaneous respiration.

  • It occurs during hibernation and aestivation in frogs. The skin always becomes moist by mucous secreted from the mucous glands.
  • In this respiration, due to moist skin, the oxygen from the environment diffuses into the blood through the skin and the carbon dioxide diffuses out from the blood into the environment.
Buccopharyngeal respiration:

In this type of respiration, the exchange of O2 and CO2 is through the buccopharyngeal cavity. The air enters into the cavity through nares and gaseous exchange takes place through the lining of a buccal cavity between blood and air present in the cavity.

Pulmonary respiration:

The respiration through the lungs is called pulmonary respiration. This type of respiration is only in the condition of more need for oxygen during jumping or swimming.

  • A pair of lungs present in the thoracic cavity on either side of the heart in frogs, are thin-walled elastic sacs.
  • Numerous small air sac called alveoli is present in the lungs. These are thin-walled and supplied by blood vessels.
  • The air enters the alveoli of the lungs through organs in sequence – the external nares – internal nares – buccopharyngeal cavity – glottis – laryngotracheal camber and then the bronchi.
The Process of Expiration in Frogs:

The process of exhalation of CO2 is called expiration. Expiration follows sequences as such-

  • The lungs get contracted.
  • The external nare remains closed.
  • The floor of the cavity is lowered and the air is drawn into the cavity from the lungs.
  • Then the nares open and the cavity rises and air passes out through the nares.

The circulatory system in frogs: (Blood vascular system)

The system consists of the Heart, Blood, and blood vessels.

Heart of Frog:

The shape of the heart in a frog is a triangular, muscular pumping organ. it is situated ventrally to the liver in the pericardial cavity.

  • The heart of a frog is three-chambered.
  • The upper two chambers are called auricles, these are anterior and broader parts, and the lower chamber is called the ventricle (posterior)
  • The ventricle is thicker than the auricle.
  • The right auricle is larger than the left auricle. The opening of the right auricle, sinus venosus called sinu- the auricular aperture is guarded by sinu-auricular valves. It allows a flow of blood from the sinus venous to the right auricle and prevents the backflow of blood.
  • Two auricles are separated by a septum called the internal auricular septum.
  • The left auricle has an opening of a pulmonary vein, the valve is absent.
  • A tubular structure, Truncus anteriocus is present on the right side of the anterior part of the ventricle. Two branches named aortic trunks rise from it.
  • Auricles open into ventricles by auriculo-ventricular aperture, which is guarded by four auriculo-ventricular valves.
dorsal view of heart of frog

Dorsal view of the heart of the frog

Arterial System of Frog:

Arteries carry oxygenated blood from the heart to all parts of the body. Truncus arterious has two branches right aortic trunk and left aortic trunk, each divided into three branches:

  1. Carotid Arch
  2. Systemic arch
  3. Pulmocutaneous arch

Carotid Arch: The Carotid arch is divided into the Lingual artery and the common carotid.

  • The lingual artery supplies blood to the tongue and hyoid.
  • Common carotids supply blood to the buccal cavity and brain.

Systemic Arch: Two systemic arches move upwards; it bears-

  • Oesophageal artery- supplies blood to the esophagus
  • Occipital vertebral artery- supplies blood to the head and vertebral column
  • Subclavian – it supplied blood to the shoulder and forelimb. From the junction of two systemic
  • The Coeliac artery- The hepatic artery – supplies blood to the liver, and the Duodenal artery – supplies blood to the duodenum.
  • Posterior mesenteric artery- It is a Long Branch and supplies blood to the rectum

Dorsal Aorta: it supplies blood through the following-

  • Gonadal artery: Supplies blood to testes and ovary
  • Renal artery: Supplies blood to kidneys.

The dorsal aorta at the end, runs posterior and bifurcates into the right and left common iliac arteries. From each iliac artery the rise of the following:

  • The femoral artery – supplies blood to the hip and thigh
  • The sciatic artery – supplies blood to the lower region of the hind legs
  • Epigastric artery– it supplies blood to the urinary bladder

Pulmocutaneous Arch: It is divided into the pulmonary artery and the Cutaneous artery.

  • Pulmonary artery: it receives deoxygenated blood from different parts of the body and opens into the lungs.
  • Cutaneous artery: It supplies oxygenated blood to the skin.
The venous system of the frog:

The vessels that carry deoxygenated blood from different parts of the body are called veins, consisting venous system.


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