Phylum: Chordata

Sub Phylum: Vertebrata

Group: Gnathostomata

Class: Amphibia

Order: Anura

Genus: Rana

Species: tigrina

Common Name: Frog

Habit and Habitat:

Frog is an amphibian animal, it can live both in water and land. Its tadpole larva is completely aquatic in habitat. Later metamorphos into adult, the characters of larva are completely disappearing in the adult.

On the land frog locomotes by jumping movement and in the water swims actively due to webbed foot. It is completely solitary and free living animal but during rainy season always live in colony. The body skin is always in moist condition. Therefore easily respire by cutaneous respiration. Nutritionally larva is herbivorous and adult is carnivorous or insectivorous.

During breeding season usually at night, frogs gather nearby the pond or shallow water. In the group, there are more males than the female, only the males produces high pitch croaking sound to attract female. Only those males who produce best croaking sound will be select by the females for copulation. The distribution of frog is almost cosmopolitan nearby watery areas.

Locomotion: The frog’s powerful hind legs are adapted for both swimming and leaping. The strong extensor muscles of the thigh contract, extending the limb and thrusting the foot against the ground or against the water. In the water the webbed hind feet provide a greater surface area for pushing backwards on the water. On moving from water to land or over rough ground the frog will crawl rather than leap.

Croaking: The sound made by the male frog during breeding season is called croaking. It is a call for mating.

Hibernation: Is also called winter sleep. It is the resting period. They burry themselves into the deep mug and take rest. When summer start they become active again.

Body structure:

Body is short and broad possesses head and trunk only due to absence of neck. Head is almost triangular shape possess;

# 1 pair of buldging eye ball.

# 1 pair of extends hare or nostrils or nasal opening.# 1 pair of tympanum or eardrum and

# a terminal mouth.

Head:  Head immediately follows broad trunk due to absent of neck. In the ventral position of the trunk possess shorter 4 limbs with 4 tree digits and longer hind limbs with 5 webbed digits. Body skin is always kept in moist due to secretion of mucin or mucus from skin mucous glands. Skin may or may not possess pigments and do not possess effective poison glands.

Skin: Skin is moist, smoothThe outer most layers is epidermis.Below epidermis,two glands are present. Poison gland and mucous gland. Mucous gland keeps skin moist.Skin is protecting covering of the body. Poison glands are also present in the skin, which protects from enemy.

Mouth: A wide opening called mouth is situated at the anterior end of the snout.

Eyes: Its eyes protrude in such a way that they are above water when the rest of the body is immersed. The eyes have movable lids but, in addition, the whole eyeball can be withdrawn farther into the head by muscles. This can be seen to happen sometimes when the frog is swallowing.

Nostrils: Its nostrils are situated so that air can be breathed while the frog is swimming at the surface, they can also be closed. Behind the eyes are circular ear-drums. Sounds in the air or water set these thin membranes vibrating, the vibration being transmitted by a small bone to a sensory region which sends nervous impulses to the brain.

Limbs: 2 pairs of limb are in lateral position bears from the trunk. The fore is slightly shorter than the hind limb. Hind limb is stronger than the forelimb and mainly used for the purpose of Jump.

Digestive System

Digestive is a process by which the complex insoluble food particles are converted into soluble from by action of enzymes. The digestive system consists of the alimentary canal and digestive gland.

The alimentary canal is a long, coiled path starts from mouth and ends in cloaca.It consists of organs such like mouth or buccal cavity, pharynx, esophagus,stomach, small intestine, large intestine and cloaca.

Mouth:The digestive system of a frog starts with the mouth. Mouth is a wide opening lies in between upper and lower jaw. Mouth helps in consumption of food. This process is known as ingestion.

Buccal cavity: Mouth leads into the buccal cavity and it is a wide space between upper and lower halves of the head.

A) Structure present on the upper jaw: The upper jaws of frog are fixed or immovable. The following structure found on the upper jaws:

a) Maxillary teeth: maxillary teeth are the situated on the either side of upper jaw. These are sharply pointed and not fixed in the sockets. These are directly attached with the maxilla. Such type of teeth is known as acrodont teeth. All teeth are of the same size and are called homodont type. These teeth breaks occasionally and is replace by new ones.this is known as polyphyodont condition. At the tip of the mouth, the maxillary teeth are more pointed called the premaxillary teeth. It helps in closing the tip of mouth.

b) Vomarine teeth: these are the two small patches of teeth bending backwards and are found one or either side of the median line of the roof of the buccal cavity. They are called the vomarine teeth as they are born in vomer.

Structure of teeth: the teeth of frog acrodont, homodont and polyphyodont.

The tooth is conical in its shape and consists of two regions: a base and crown. The wide lower part of teeth is called root or base. It is formed of a bone like material, whereas the upper part is called the crown. It is composed of hard substance called dentine. The crown is covered with hard and shining white substance called enamel.

The inner side of teeth is hollow and is called pulp cavity.it nourishing the tooth during the growth.

Function of teeth:

The teeth of frogs not help in cutting, grinding and mastication of food. They are simply serving to preserve the captured smooth prey from the slipping out.

c) Fossae: there are three pits are situated at tip of upper jaw just near the premaxillary teeth called fossae. The middle one is larger is called median sub rostral fossa while other two are smaller and is called lateral sub rostral fossae.

d) Internal nares: internal nares are two in no.lying closed outside the patches of vomarine teeth.

e) Bulging of eyeballs: there are two large and oval area on the roof of upper jaws and called the bulging of eye ball. These occupy the maximum portion of upper jaws and form a socket for eyes.

f) Eustachian tube: there are two small spindle shaped opening one on either side situated laterally near the jaw angles called Eustachian tube. It connects the cavity of the pharynx and middle ear to open into internal ear.

B) Structure resent on the lower jaw:

The lower jaw of frog is moveable and it can up and down in vertical plane. It bears no teeth. The following structures are found on lower jaw:

a) Tongue: the tongue of frog is long, large fleshy thick, fleshy, bifid and producible. The tongue is attached in front inside the tip of lower jaw and free behind. The tongue is bilobed at its free end.The upper surface of tongue bears the taste buds in the form of small papillae and mucous glands. The secretion of which keeps tongue moist. Neither taste buds nor mucous glands produce any digestive enzymes.

The tongue can be thrownout and retracted suddenly to capture and engulf prey. This action is accompanied by protractor and retracter muscle.

The throwing out of tongue and its retraction due to the muscular action is assisted by the sudden flow of lymph from the sublingual lymph sinus situated in tongue.

b) Glottis: Glottis is a longitudinal median slit like aperture in the pharynx behind the tongue that guards the entrance to the lungs. It leads into laryngo-trachel chamber. It is always opened during the breathing but become closed during the swallowing of food.

c) Vocal sac: in male frog, on the either side of the tongue on the angle of lower jaw there are two pores called vocal sacs. These acts as resonators at the time of cracking.

d) Gullet: Just above the glottis, there is horizontal opening called gullet. The food enters from the gullet to esophagus to stomach.

e) Prelingual elevations: there are three small prelingual elevations at the front end of lower jaw. They fit in the fossae of the upper jaw and helps in the closing of mouth.

Pharynx:The buccal cavity leads into the narrow pharynx. The buccal cavity and pharynx is collectively known as bucco-pheryngeeal cavity.

Oesophagus: Pharynx leads into a small but wider tube called oesophagus or gullet. The oesophagus opens into the stomach.

Stomach: The oesophagus opens into a large and thick sac like structure known as stomach.The anterior end of stomach is called Cardiac end while the posterior end is called pyloric end. The walls of stomach are muscular and glandular. The muscles of walls of stomach contract and relax, by which food is broken down into tiny pieces. The secretions of stomach have different enzymes which help in chemical digestion. In stomach, digestion of protein of food is started.

Small intestine (Duodenum and Ileum): After stomach, first part of intestine begins which is called duodenum. The ducts from liver and pancreas open into the duodenum. These ducts bring juices from these glands.The second part of intestine is Ilium. The secretion of pancreas is called Pancreatic Juice. Pancreatic juice enters the bile duct by small duct. This juice digests the food and brings in such form which can be absorbed by the blood through intestine. In illum the internal lining forms many longitudinal folds. Digested food is absorbed by illium.

Large intestine (rectum): It is wide tube measures about 4cm. its inner lining forms numerous folds. In rectum undigested food material is stored which is ready to expel from cloaca.

Cloaca: It is a small sac like structure used to remove undigested food material.

Digestive gland:

Liver: It is the largest gland of the body. It is reddish-brown in colour and situated in the anterior region of body cavity. Liver consist of two lobes, left lobe is larger than the right one. As far as digestion is concerned, it secretes fluid called bile.

Bile juice produced by the liver is stored in the gall bladder. Bile is yellowish greenish, alkaline fluid. Bile emulsifies fats which help in breaking them into small globules. The acidic food coming from the stomach becomes alkaline, when it is mixed with the bile. It is an extremely important step which ensures further digestion of the food. The digestive enzymes that are brought in the duodenum and ileum can catalyze the breakdown of food only in alkaline medium.

Function of liver:

· The liver secrets bile, which is used in small intestine for digestion of food.

. It store excess of sugar as glycogen.

· It regulates the amount of sugar in the blood.

· It maintains the protein concentration in blood.

· It stores copper and iron and forms vitamin A.

. It destroys the injured RBC.

. It helps in the removal of some excretory wastes.

Gastric glands: They are present on the stomach wall. They secret HCL and enzyme pepsinogen.

Pancreas:  It is long and irregularly lobed gland and yellowish in colour. It lies in between stomach and duodenum. It consists of many small lobes. Its secretion is known as pancreatic juice which contains several enzymes which are poured into duodenum through a common duct (Hepatopancreatic duct). The enzymes help digestion of ingested food.

Functions:

. It functions as both endocrine and exocrine glands.

· The Enzymes secreted by pancreas helps in digestion of ingested food.

. It secrets a hormone called insulin it help in to convert glucose into glycogen.

· Hormones secreted by pancreas helps in metabolism of carbohydrates and regulate the storage glycogen in liver and muscles.

Physiology of digestion: Frog is insectivorous (insects eating).They catch prey by the action of tongue and swallowed without mastication. The teeth only prevent the prey to escape out. The food becomes lubricated by mucous secreted by mucin gland.

Digestion:The process of conversion of complex organic food into simple soluble material is called as digestion. When the food reach to stomach the chemical digestion starts.

Gastric Digestion: the digestion starts 2-3 hours later. The gastric glands present on stomach wall secrete Hydrochloric acid (HCl) and the Pepsinogen enzyme. The food is mixed with HCl which kills the bacteria, and softens the hard food. The pepsinogen is inactive enzyme. But by presence of HCl, it becomes active and then it is called Pepsin which digests protein into proteases and peptones.

Pepsinogen + HCl                                         Active Pepsin

Pepsin + Protein                                            Proteoses and peptones

The food is gradually digested and changed into a semi-liquid state called Chyme. From the pyloric constriction, the chyme slowly moves towards duodenum.

Intestinal Digestion: In the duodenum the chyme reacts with bile, pancreatic and intestinal juice. Bile juice is a kind of alkaline juice secreted by liver. It has mainly two functions like It neutralizes the acidic food and It emulsifies fat i. e. the fat droplets are broken into fine droplets and mixes with the food.Pancreatic Juice it is also a kind of juice secreted from the pancrease. It contains three important enzymes amylopsin, trypsinogin and lipase.

Trypsinogen – In presence of enterokinase it is converted into trypsin and it coverts protein into peptones and polypeptides.

Trypsinogen +enterokinase                                           Trypsin

Protein                                 Peptones and polypeptide

Amylase – It digests the carbohydrate into maltose.

The food is mixed with intestinal juice in ileum, which contains following enzymes.

Proteoses and peptones                                      Amino acid

Maltose + Maltase                                       Glucose

Fats + Lipase                                      Fatty acids + glycerol

Sucrose + Sucrase                                      Simple sugar

Absorption of Digested food.

It is the process by which digested food is taken into blood.The completely digested food material contains glucose, amino acids, fatty acids, glycerols etc. All these simple compounds are now absorbed through the villi of intestine. There are two methods to absorb food materials. The final products of fats pass into lymph vessels called lacteals. The food materials are absorbed by diffusion or osmosis through the villi and are mixed into the blood around the intestine. It is slow process and food molecules pass into blood from the high concentration in lumen of intestine. This method is called passive method. There is another method of absorption i.e. active method. It is fast and the food molecules are absorbed by using energy against concentration gradient from the lumen of intestine into the blood. The remaining undigested and unabsorbed materials are stored in rectum for short time and passed out through anus.

Assimilation:It isthe utilization of absorbed food material by living tissue.

Circulatory System:

Circulatory system is the system of blood, heart, and blood vessels. The distribution of the digested food and oxygen to various parts of the body through blood is called circulation. The circulatory system of frog consists of: i) The blood vascular system and ii) The lymphatic system.

The blood vascular system consists of heart, blood vessels and blood.

Blood: It is a liquid connective tissue containing fluid plasma. The plasma contains different kinds of cells floating in it. These cells are called blood corpuscles, i.e. RBC, WBC and platelets.

Plasma: It forms about 60% of blood. It is composed of 90% water and 10% of organic and inorganic substances. The organic substance contains fibrinogen, albumins, gloobulins etc. and inorganic substance contains Na, K, Ca, Fe etc. It also contains hormones and antigens and antibodies.

Heart:

External structure:

The heart is a muscular and somewhat conical. Heart is triangular muscular pumping organs. It is reddish in color. Heart of frog is situated ventrally to the liver in the pericardial cavity. Heart remains enclosed within in a double walled sac, the pericardium and it contain pericardial fluid, which protects the heart from mechanical shocks, allows it to function without any friction. The frog consists of three chambered heart. Upper two chambers are called auricle and lower one chamber is called ventricle. Its anterior end is broader than posterior end. The ventricle is thick walled than auricles and the right auricle is larger than left auricle.

The hearts of frog have two additional chambers- truncus arteriosus and sinus venosus. The tubular structure is present on right side of anterior part of ventricle, which is called truncus arterious gives two branches called aortic trunks.

Sinus venosus: It is a thin walled triangular chamber. It opens into right auricle. The right precaval, left precaval and post caval veins open into sinus venosus.

Internal structure of heart:

The internal structure of heart of frog is studied with the help of vertical section. It is a conical hollow, muscular structure with different chambers. The different chambers remain separated by valves which help to keep the one way flow of blood during rhythmic contraction and relaxation of heart.
The two auricles are thin walled and remain separated from each other by a thin vertical inter-auricular septum. Right auricle is larger than left one as it receives blood from different parts of the body. The sinus venosus opens into dorsal wall of right auricle by a large oval aperture, sinu-auricular aperture. It lies at middle close to inter-auricular septum and remains guarded by a pair of flap like values. The common pulmonary veins open into left auricle, near inter-auricular septum, by a small opening without valves. Both auricles open into ventricle by a common large auriculo-ventricular aperture guarded by two pairs of flap like auriculo-ventricular values.
The ventricle is thick walled, muscular and spongy in nature. Its inner surface has irregular ridges or folds called the columnae carneae with depressions which is also known as fissures. These folds reduce cavity of ventricle. The flaps of auriculo-ventricular values remain connected with wall of ventricle by thread like chordae tendineae.
The opening of ventricle into truncus anteriosus is guarded by three semilunar values, which prevent the back flow of blood from truncus arterious into ventricle. The spirally twisted cavity of truncus anteriosus remains divided by three semilunar valves into a long thick walled proximal conus arteriosus or pylangium and a short distal, thin walled bulbous arteriosus or synangium. A longitudinal spiral valve is present on pylangium which remain attached dorsally while free ventrally. It divides cavity of pylangium into left dorsal cavum pulmoutaneum and right ventral cavum aorticum. The common opening of two pulmocutaneous arches lies in cavum pulmocutaneum while separate openings of carotid and systemic arches lie in synangium.

Working of heart: Heart constantly beats during life time under nervous control to pump blood into blood vessels. Contraction of heart is called systole, while relaxation is called as diastole. When sinus venosus contracts, its non-oxygenated blood passes into right auricle through sinu-auricular aperture the blood from lungs come into left auricle through pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into ventricle through auriculo-ventricular aperture.

A) OLD VIEW: This view is purposed by Brucke (1858). According to the old view, two kinds of blood do not mix to any great extent on being forced into single ventricle. The two lines of blood could not mix to any great extent because of their viscous nature and also because of the network of columnae carneae.
When ventricle starts contraction, first the deoxygenated blood from the right side being nearer flows into the truncus arteriosus and directed by the spiral valve into the common opening of pulmo-cutaneous arches and carried to lungs and skin for oxygenation. Spiral valve now closes the opening of pulmo-cutaneous arches. Next follows the mixed blood which is pushed through cavum aorticum into the systemic arches and is sent to the body and limbs. Finally enters the oxygenated blood of left side and directed through carotid arches to the head. Thus, according to old view the spiral value in truncus arteriosus plays an important role in directing blood into different arches.

B) MODERN VIEW: This view was put-forward by Vandervale (1933) and Foxon (1953), show that it is actually completely mixed blood in ventricle and truncus which flows simultaneously through the three pairs of arches to all parts of the body. The blood received from skin and buccal cavity into sinus venosus and right auricle is equally oxygenated to the blood received from lungs to left auricle.

Arterial system:

The blood vessels, which carry blood away from heart to different part of the body, are called arteries. And the system called as arterial system. It starts from truncus arteriosus. The truncus arteriosus devides into two, right and left aortic trunks and each trunk again divides into 3 arches.

a) Carotid arch:      it arises from anterior side and gives off two arteries.

Lingual artery–      it supplies blood to tongue and lower jaw.

Internal carotid– It divides into three branches:

Palatine:  it supplies blood to buccal cavity.

Opthalmic: Supply blood to the eyes.

Cerebral: Supply blood to the brain.

b) Systemic arch: It arises from the middle part of each aortic trunk. It is the largest of all the three arches. The two systemic archer curve dorsally around the oesophagus and join with each other behind the heart to form the dorsal aorta. In its course each systemic arch gives off 3 arteries:

a) Oesophageal artery: It is small artery and supplies blood to oesophagus.

b) Occipito-vertebral artery: It immediately gives two branches i.e. occipital artery that supplies blood to head, eye, orbit and jaws, and a vertebral artery that supplied blood to vertebral column and spinal cord.

c. Sub-calvian artery: It is a large artery supplying blood to shoulder region and arm of forelimb.

Branches of Dorsal Aorta: It is formed by the union of both the systemic arches. It lies mid-dorsally just beneath the vertebral column. It gives off the following arteries:

a) Coeliac-mesentric artery: It is a single large artery arising from the junction of the two systemic arches. It has two main branches i.e. coeliac artery that supplies blood to stomach, and liver; and the anterior mesenteric artery which gives four branches that supplies blood to the duodenum and pancreas, spleen, small intestine.

b) Gonadial artery: A  dorsal aorta gives a pair of short arteries to gonads called spermatic artery in male frog and ovarian artery in female frog supplies blood to testis and ovary respectively.

c) Renal artery: Dorsal aorta gives out 5-6 pairs of small renal arteries in a series into both the kidneys.

d) Posterior mesenteric: It arises from the anterior mesenteric artery. It supplies blood to large intestine or rectum.

e) Commo iliac artery: The dorsal aorta finally bifurcates posteriorly into two common liliacs, each supplying an epigastric artery to ventral body wall,hypogastric artery to urinary bladder, femoral artery to hip and upper thigh, and sciatic artery to lower portion of hind limbs.

C. Pulmo-cutaneous arch: It divides into main two arteries i.e.

Pulmonary artery: Gives blood to the lung.

Cutaneous artery: Gives blood to the skin of dorsal and lateral sides.

Fig: Arterial system

Venous System

The blood vessel which carries de-oxygenated blood from different parts of body towards Heart. The venous system includes the network of veins or those blood vessels in which blood of body returns to the heart. In frog the venous system can be studied in three headings i.e.pulmonary venous system, Systemic venous system and Postcaval venous system.

Fig: Venous System

1. Pulmonary venous system:Oxygenated blood from two lungs is collected by right and left pulmonary veins which unite to form a common pulmonary vein opening directly into the left auricle on the dorsal side.

2. Systemic venous system: Deoxygenated blood from rest of the body travels towards heart by three large vessels i.e. two anterior venacava and single posterior venacava, all the three unite to form a triangular shaped structure called sinus venosus, which poens into right auricle through sinu-auricular aperture.

a) Anterior venacava: There is two anterior veins which collect blood from anterior parts of body.

i. External jugular vein: It is formed by the union of lingual vein form tongue and mandibular vein from outer margin of lower jaw. It collect blood from mouth, tongue and from lower jaw.

ii. Innominate vein: It is formed by the union of internal jugular vein and sub-scapular vein. It collects blood from skull, brain and from shoulder and arm.
iii. Subclavian vein: It is formed by the union of brachial vein and the musculo-cutaneous vein. It collects blood from fore limbs and from skin, muscle and trunk and superficial parts of the head.

b) Posterior venacava or post-caval: The single postcaval is a large, dark coloured vein lying ventral to dorsal aorta. Its posterior end is formed between the two kidneys form which it drains blood by 5-6 pairs of renal veins. It also receives a pair of gonadial veins (spermatic vein in male and ovarian vein in female) from gonads. The post-caval then runs forwards, dorsally to the liver receiving blood from a pair of short hepatic veins.

3. Portal venous system: A vein which collects blood form one organ of body and supplies alone to another organ instead of going to heart directly is called portal vein. It is a double capillary system.Frog consists of two postal systems i.e. renal portal system and Hepatic portal system.

a) Renal portal system: The veins which carry blood to a capillary system in kidneys constitute the renal postal system. Blood of each hind leg is collected by two veins, an outer femoral vein and an inner sciatic vein. On entering the abdominal cavity the femoral divides into a dorsal renal portal and ventral pelvic vein.
The renal portal unites with the sciatic vein of its side and while running along there outer border of kidney of its side it receives blood from lumbar region by a dorso-lumbar vein. Renal postal veins enter the kidney by several branches which break up into capillaries.

b) Hepatic portal system: The vains which carry blood to a capillary system in liver constitute the hepatic portal system. It consists of hepatic portal vein and anterior abdominal vein.
Hepatic portal vein is large and formed by the union of several branches form stomach, intestine, spleen and pancreas and carrier blood form alimentary canal to the liver.

Anterior abdominal vein: it is formed by union of the pelvic veins of both sides. It receives blood form urinary bladder and ventral abdominal wall and runs forwards to enter liver into which it breaks up into capillaries.

Lymphatic System:

The lymphatic system is a part of complex immune system consists lymph nodes is a network of thin lymphatic channels (similar to thin blood vessels), and organs such as the spleen and thymus.

Lymph: Lymph is the blood without erythrocytes so they are colourless. It contains large number of WBC. It carries food and oxygen to the cells and takes away water and other waste from the cells. Thus, it acts as a medium of exchange between the blood and tissue cells.

Lymph Vessels: These are thin walled, delicate and branched vessels. They are formed by the union of lymph capillaries. They are made up of unstraited muscle and connective tissue. They have no direct connection between blood capillaries.

Lymph Simnuses: The important lymph sinuses are:

1.Subcutaneous lymph sinuses: Thesebare the large space between skin and muscle.

2. Sub-vetebral lymph sinus: It is found around the dorsal aorta.

3. Pericardial Sinus: It lymph is called Pericardial fluid. And enclose heart.

4. Coelom: It is a large lymph space. And its lymph is ceolomis fluid.

5. The peritoneal cavity.

6. The synovial cavity of the joints.

Lymph Heart: these are two pairs of thin, transparent muscular sac like structures. One pair of anterior heart lies on sides of vertebral column and posterior part lies on either sides of urostyle.

Spleen: It is small dark red and round structure lies near the anterior end of the rectum. Spleen is a solid mass of reticiloendothelial tissue covered in a fibrous capsule. Spleen is a blood bank of the body.it is a store house of RBC and WBC and also help in the production of Antibodies.

The Respiratory System

The process of gaseous exchange (Oand CO2), i.e. intake of oxygen and liberation of carbon-dioxide and utilization of oxygen to breakdown food to release energy is called respiration. The energy is released during the process of respiration.

C6H12O6 + 6O2                                       6CO2 + 6H2O + energy

There are three types of respiration in frog:

1. Pulmonary respiration: Frog respires by pulmonary respiration when the need for oxygen is great, i.e. during the time of swimming and jumping.The respiration through the lungs is called pulmonary respiration. Many organs function during the respiration:

i) External nares:A pair of opening lies between the eyes and snout.

ii) Internal nares: A pair of small opening is found in between the maxillary and vomer teeth on buccal cavity.

iii) Nasal passage: space between external and internal nares.

iv) Buccopharynheal cavity: the space between buccal cavity and pharynx.

v) Glottis: A slit like opening.

vi) Bronchi: It is a pair of small tubes.

vii) Lungs: There is a pair of lungs which are thin walled elastic sacs. They are present within thoracic cavity on either side of heart.Numerous small air sacs are present in the lungs called alveoli.The alveoli are very thin walled and supplied by blood vessels. The air enters into the alveoli of the lungs through the external nares, internal nares, buccopharyngel cavity, glottis, laryngotrachial camber, and bronchi. The alveoli increase the respiratory surface of the lungs.

Mechanism of pulmonary respiration

During pulmonary respiration the mouth of the frog remains closed. The inspiration and expiration of the air is done by the action of the buccal cavity.

Inspiration: The process of intake of air is called inspiration.The mouth remains closed. The sternohyalas contract and the floor of buccal cavity is lowered. Due to which the space in cavity is increased and air pressure is decreased. Therefore, air is taken into cavity through external nares.Then, the nares remain closed which decrease the space in cavity and pressure is increased. By this mechanism the air passed into lungs.In lungs, alveoli are filled with air and gaseous exchange takes place between blood and alveoli by diffusion.

Expiration: The process of exhaling of CO2 is called expiration. It is the reverse process of inspiration. The external nares remains closed. The floor of the cavity is lowered and the air is drawn into the cavity from the lungs. Finally the glottis is closed and premaxillae comeback into their original position due to which external nares open and the cavity raises and then the air is passed out through the external nares.

Buccopharyngeal respiration: While the respiration done through the buccopharyngeal cavity then it is called buccopharyngeal respiration. The buccal cavity consists of moist mucous membrane and richly supplied with blood capillaries. The lungs do not take part in this respiration.The air enters into the cavity through external nares and gaseous exchange takes place through the lining of buccal cavity between blood and oxygen (air) present in the cavity by diffusion process.

Coetaneous respiration: The skin consists of chief respiratory organ. The respiration through skin is called coetaneous respiration. During hibernation and aestivation the frog totally depends on this type of respiration.The skin of frog is thin and has fine blood vessels. Due the presence of mucous gland the skin of frog always remains moist.During gaseous exchange the oxygen first dissolve moisture of body and then diffuse into the blood capillaries. And the carbon dioxide diffuses out from the blood into the environment.

Reproductive System:

Frogs shows the sexual dimorphism, i.e. male and female frog can easily distinguish. The male frog is slightly smaller than female frog. During breeding season male frog produce special sound known as croaking by the help of vocal sac. The croaking is invitation for copulation.

Male reproductive system:

It consists of a pair of testes, vasa efferentia, urinogenital ducts, and seminal vesicles.

Testes: Testes are yellowish, elongated or oval, is found attached to the anterior ventral side of each kidney. They are suspended by a double fold of peritoneum called Mesorchium. Each testes consists coiled structures called seminiferous tubules or crypts. The epithelial lining of seminiferous tubules consists of germinal cells, which produce spermatozoa.

Vasa efferentia: Many seminiferous tubules unite to form vasa efferentia, which is narrow tube like structure. They enter into the kidney. In kidney it opens into the Bidder’s canal, it is longitudinal tube, which is then connected to the ureter. The vasa efferentia conduct mature sperms from testis to the ureter.

Urinogenital Duct: it is a tubular duct where the urine and sperms pass through ureter so that it is also called as urinogenital duct.

Seminal Vesicle: Each urinogenital duct expands to form seminal vesicle where the sperms are stored until they are ejected out during copulation.

Female reproductive system

It consists of ovaries, oviducts, ovisacs.

Ovary: They are paired much folded sac. They are lies on ventral to the kidneys and hang in loops of peritoneum called mesovarium. It is sac like structures composed of ovarian follicles consists of countless ova. Groups of germ remaining cells form ovarian follicles and one of them undergoes oogenesis to form ovum. The mature ova are shed into the abdominal cavity and reach to the oestium by pressure of fore arms of clasping of male.

Oviduct: They areLong coiled tubes one on either side of abdominal cavity. Anterior of oviducts oviducal funnel called astium is present. At maturity the eggs are shed and ultimately find their way into the oviduct.

Ovisacs: The coiled part of the oviduct posteriorly dilates to form a thin-walled ciliated ovisacs. They opens by a narrow aperture in the cloaca.

Copulation:

Copulation takes place in water during breeding season. They produce croaking sound with the help of vocal sacs. The vocal sacs act as amplifiers. The croaking is a mating call for the female frogs. The female frogs getting attracted approach the male frogs. The male frog rides over the female frog and embraces it. The male frog holds the female frog firmly by its forelimbs and nuptial pads. The couple remains in this condition for 2-3 days. The Frog takes a long time to become sexually excited, as they are cold-blooded animals and devoid of copulatory organs. The male holds the female more tightly at the state of orgasm. At this stage the female discharges a large number of eggs in water from its ovisac through the cloacal aperture. The male frog right at the same moment discharges its sperms over the eggs falling in water. The two animals separate from each other on completion of this process.