Class IX, BIOLOGY, "Frog"

Frog

Frog is found in he water or near the water. It belongs to class Amphibia. it passes a specific part of life in water and remaining life on land. Its bilogical name is Rana Tigrina. It is a cold blooded animal i.e. blood temperature changes with that of the environment. At the start of winter, water is decreased and temperature is lowered, the frog lives buried in the mud to over come winter.
(Figure)

Coelom

The body cavity of frog is called Coelom. It contains many organs which form different systems.

Digestive System of Frog

This system consists of alimentary canal and accessory glands like liver and Pancreas.

Alimentary Canal

It is a coiled tube through which food passes. It consists of buccal cavity, pharynx, oesophagus, stomach and intestines.

Buccal Cavity and Pharynx
Mouth is present between upper and lower jaws. Upper jaw has a row of pointed maxillary teeth. Lower jaw lacks teeth. There is a pair of set of vomerine teeth on the roof of the buccal cavity. Frog does not chew the food with teeth. But teeth are used only to grasp the food. The old teeth are continuously replaced by the new teeth through its life. Near the vomerine teeth, internal nostrils are present on the roof of the buccal cavity. These open outwards into external nostrils. Behind them, two large bulges indicate the position of eyes. The tongue of frog is sticky. The tongue is attached at anterior end of buccal cavity. The posterior end of tongue is free and bifid. the frog feeds on different insects. To capture its prey, it suddenly throws its tongue on to the prey, which sticks to the tongue and is brought to the buccal cavity, when the tongue is drawn back.
Near the maxillary joints, pair of opening of eustachian tubes are present. In male frog, there is also, present a pair of opening of vocal, sacs on the lateral side of floor of the buccal cavity which help the croaking. Buccal cavity narrows, posteriorly to form pharynx. The digestive system, respiratory system and ears are linked to pharynx. In posterior part of pharynx, there is another opening called Glottis. This leads to lungs through tachea. It closes at the time of digestion of food but remains open when animal is respiring.
(Diagram)

Oesophagus, Stomach and Intestine
Pharynx leads into a small but wider tube called oesophagus or gullet. The oesophagus opens into the 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. 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 are called Small intestine. 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. Digested food is absorbed by illium and surplus water is absorbed by recturm. Remaining undigested food is expelled though cloacal aperture. A membrane keeps the intestine intact at a place and prevents strangulation of small intestine. This membrane is called mesentery.

Respiratory System of Frog

There are three methods of respiration in frog.

* Pulmonary Respiration
* Cutaneous respiration
* Buccal Respiration

(Diagram)

Pulmonary Respiration

The exchange of gases through lungs is called pulmonary respiration. In this process, frog keeps its mouth closed. Air reaches buccal cavity through nostrils. Nostrils are closed floor of buccal cavity is raised; glottis opens, and air is pushed into the lungs. The intake of air is called inspiration. In frog, there is a pair of balloon shaped lungs. Each lung consists of small thin walled chambers called alveoli which greatly increase the surface area of the lungs. On each alveolus, there are many blood capillaries. When lungs are filled with air, then exchange of gases occurs between blood and air in lungs at the site of alveoli. During this, the exchange of gases occurs between blood and air present in buccal cavity. After this air is removed from the lungs. Frog uses its nostrils and floor of buccal cavity for inspiration and expiration.
Oxygen present in the air is dissolved in moisture present on lining of lungs. Then oxygen is diffused into the blood where it combines with hemoglobin to form oxyhaemoglobin. This oxygenated blood goes to all parts of the body by means of capillaries. Where oxygen separates from oxyhaemoglobin molecules and is absorbed by the cells. Carbon dioxide from cells comes out into the blood, which carries it to the lungs, and from here carbon dioxide is expelled.

Cutaneous Respiration

In frog, exchange of gases occurs through skin during hibernation and swimming. This is called cutaneous respiration. Skin is richly supplied with capillaries. Skin is moist. Oxygen diffuses through skin to capillaries and is carried by blood and CO2 diffuses back to blood from cells and is discharged out.

Circulatory System of Frog

It consists of blood vascular and lymphatic systems.

Blood Vascular System

The blood vascular system of frog consists of following parts:

* Heart
* Arteries
* Veins
* Capillaries

Structure of Heart

Heart is conical organ. It is muscular. It has three chambers. It is present in the body cavity between the oesophagus and sternum. Like a pump, it contracts and is relaxes. As a result of this, blood continuously circulates in the body. The heart is surrounded by a membrane which is called pericardial which protects the heart.
(Figure)
The three chambers of heart are as follows:
1. Right Atrium
2. Left Atrium
3. Ventricle
the two atria form the broader interior part of the heart. The right atrium is larger than left atrium. Both atria are thin walled. The posterior conical thick walled part of the heart is called ventricle. A broad vessel, which is called truncus artenosus, arises from dorsal side of the ventricle and then divides into two branches near the atria. A thin walled triangular sinus venosus opens into the right atrium. Some biologists consider truncus arteriosus and sinus venosus as chambers of the heart.

Function of Heart

* The chambers of the heart beat in a rhythmic way.
* First of all sinus venosus contracts. Then, the two atria contract. After this ventricle and finally truncus arteriosus is contracted.
* The deoxygenated blood from the whole body except lungs is carried to sinus venosus by two precavals and one post caval.
* Sinus, venosus opens into the right atrium through an opening.
* Oxygenated blood from the lungs is brought into the the left atrium by two pulmonary veins.
* Both the atria open into the ventricle and push their blood collectively into the ventricle by a common aperture, which is guarded by a valve.
* This valve maintains the unidirectional flow of blood in the heart and prevents the back flow of blood.
* In the middle of ventricle some mixing of oxygenated and deoxygenated blood takes place. On the two sides the blood remains unmixed due to rapid flow of blood.
* When ventricle contracts the blood goes to the truncus arteriosus through an aperture. This aperture controls the speed and direction of the blood by a spiral valve present at the start of truncus arteriosus.

Arterial System of Frog

The blood vessels which carry the blood from heart to different parts of the body are called arteries. The system consisting of arteries is called arterial system.
(Diagram)
It starts from truncus arteriosus. It is divided into two main branches each of which further divides to form three small branches.

Carotid Arch

It supplies blood to lower jaw, tongue, eye and brain.

Pulmocutaneous Arch

It carries blood to lungs and skin.

Systemic Arch

Right and left systemic arches joint posteriorly to form dorsal aorta. But prior to their union, each systemic arch gives out arteries supplying blood to vertebral column, oesophagus and fore limbs.

Dorsal Aorta

It runs along the vertebral column towards hind limbs. It gives off following branches.

Coeliacomesenteric Artery
It supplies blood to digestive system.

Renal Arteries
These supply blood to kidneys and general organs.

Posterior Mesenteric Artery
It supplies blood to rectum.

Illiac Arteries
These supply blood to hind limbs of their sides.
After reaching their specific organs, all the arteries divide and redivide to form capillaries. The walls of capillaries are very thin. Due to this reason, the exchange of materials take place between blood and tissues. The capillaries join to form venules. These venules join to form veins. Then these veins carry blood back to heart.

Venous System

The blood vessels which bring the blood from different body parts, back into the heart are called veins. The system containing of veins is called Venous system.
Following are the major veins in frog.

Pulmonary Veins

Blood from right and left lungs goes to left atrium through pair of pulmonary veins. These have oxygenated blood.

Right and Left Precavals

Each precaval is formed by union of three veins which bring blood from tongue, lower jaw, head, shoulders, forelimbs and skin. Both veins open in sinus venosus. From here blood goes to right atrium.

Postcaval

It is formed by union of five or six pairs of renal veins from the kidney and the genital veins. While passing through the liver, it receives two hepatic veins. Then it enters the sinus venosus. Therefore, venous blood from different body parts enters the heart.

Renal Portal Vein

The veins which bring blood from the hind limbs and pelvic region combine to form Renal portal vein. The renal portal vein enters the kidney of its side and form capillaries. Blood from kidney goes to the post caval through renal veins. Post caval caries the blood to the heart.

Abdominal Vein

The Pelvic veins of two sides combine to form abdominal vein. Before entering the liver, it divides into branches. In liver, it is further divided to form capillaries. The blood from the liver is drained into post caval by hapatic veins of both sides.

Hepatic Portal Vein

The blood vessels (veins) bringing blood form various organs of digestive system (stomach, duodenum, illiums, rectum, pancreas and spleen etc) combine to form a large vein. This is known as hepatic portal vein. Near the liver a branch of abdominal vein combines with it. Then it enters the liver and divides and redivides to form capillaries. The, blood entering the liver through hepatic portal veins goes to the post caval by means of hepatic veins. The blood from post caval goes to heart through sinus venosus.
The blood coming back into the heart is of two types.

* Oxygenated blood which comes from lungs by pulmonary veins.
* Deoxygenated blood from all parts of the body enters sinus venosus through precavals and post caval and then enters the right atrium.

(Diagram)

Lymphatic System of Frog

In circulatory system, due to blood pressure, many components of blood plasma come out of the capillaries and fill the inter cellular spaces. These components are also in the form of fluid and called tissue fluid or interstitial fluid. Much of it reenters the capillaries and some of it enters the lymph vessels where it is known as lymph. The flow of lymph is unindirectional. Through lymph “vessels” lymph goes to big veins. Thus, lymph again enters the blood.
1. The lymph keeps the tissues wet.
2. The lymph helps in transport of various substances from blood to tissues and vice versa.

Excretory System of Frog

In frog, waste materials are removed in different ways e.g. through skin, lungs, liver digestive system etc. But for removal of nitrogenous wastes, there are two kidneys. Kidneys are attached to dorsal wall of body cavity. These are present close to vertebral column in posterior part of body cavity. These are elongated and made up of urinary tubules. Urinary tubules combine to form collecting ducts which open into Ureter. The urine from kidneys comes into ureters after illustration. Both ureters which start from edges of kidneys open into the cloaca. From here, urine is excreted directly or stored in the urinary bladder, which on opening of cloacal aperture is expelled. The carbon dioxide and water are excreted through lungs and skin while through liver and digestive system; undigested food and some wastes are excreted.
(Diagram)

Reproductive System of Male Frog

The reproductive system of male frog consists of a pair of testes and reproductive ducts. Each testis is attached to kidney by means of a membrane. At anterior end of testis, there is present fat body. Each testis is composed of small ducts called seminiferous tubules in which sperms are produced. Sperms enter the kidney via vesa efferentia. Sperms reach the cloaca through ureter. From here, these are dischaged in the water through cloacal aperture in this way, ureter in male frog does two jobs, one is removal of urine and other is removal of sexual material, so it is called urinogenital duct and the urinary system and genital system are collectively call urinogenital system.

Reproductive System of Female Frog

The reproductive system of female frog consists of a pair of ovaries and reproductive ducts. Ovaries are present close to the kidneys. At their anterior ends, there are present fat bodies. Each ovary contains many follicles in which eggs (ova) are produced. During breeding season, ovaries are enlarged. Ova are released into the body cavity through the coelomic fluid, these enter the oviduct. The anterior part of oviduct is funnel like called oviducal funnel and reach the uterus. The uterus opens into the cloaca. At last, ova are discharge in the water through cloacal aperture. In water, union of sperm with egg results in formation of zygote. From zygotes, offsprings are formed and in this way continuity of race is ensured.

Nervous System of Frog

It consists of three parts:
1. Central Nervous System
2. Peripheral Nervous System
3. Sympathetic Nervous System

Central Nervous System

It consists of brain and spinal cord.
(Diagram)

Brain
Brain is enclosed in protective layers and is located in cranium or brain case, which is major part of skull.
(Diagram)
Brain is divided into three parts:
(a) Fore Brain
(b) Mid Brain
(C) Hind Brain

(a) Fore Brain
This is anterior part of brain. This is associated with sense of smell. It controls the secretion of many hormones. It also receives messages from internal and external environment of the body.

(b) Mid Brain
This is central part of brain. This is associated with eyes and vision.

(c) Hind Brain
This is the posterior part of brain. It controls and coordinates body movements and maintains balance of the body. It also controls respiration, circulation, taste and digestion.

Spinal Cord
The posterior part of the brain is continuous with spinal cord. It runs through the vertebral column. The spinal cord controls the movements of trunk region.

Peripheral Nervous System

It consists of nerves. These nerves connect the central nervous system (CNS) with various parts of the body. Some nerves originate from brain. These are called cranial nerves other nerves originate from spinal cord. These are called spinal nerves. In frog, there are 10 pairs of cranial nerves and 9 or 10 pairs of spinal nerves.

Cranial
Basically, nerves are of three types:

Sensory Nerves
These take messages from sensory organs to CNS.

Motor Nerves
These take messages from CNS to glands and muscles.

Mixed Nerves
These do both above mentioned functions.

Cranial Nerves
In these nerves, first, second and eight pairs are sensory nerves which are associated with senses of smell, sight and hearing. Third, fourth and sixth pairs are motor nerves which carry message from brain to the eye. Nine and ten pairs are mixed nerves, which are supplied to jaw, face, tongue and heart.

Spinal Nerves
These are all mixed nerves. These control functions of different organs.

Ear of Frog

The organ of hearing in frog is “Ear” like other vertebrates.

Structure of Ear of Frog

The ear of frog consists of following three parts.
1. External Ear
2. Middle Ear
3. Internal Ear

1. External Ear
External ear consists of a bone. The vibration is produced in external ear when sound waves strike with it.
(Figure)

2. Middle Ear
Middle Ear consists of a tympanic membrane. On the inner side of the membrane is a cavity known as tympanic cavity. The cavity contains small rod like bones called ossicles. The middle ear is connected to internal ear by a tube which is called Eustachian tube; it transfers the vibrations towards the internal ear.

3. Internal Ear
The internal ear is a very delicate organ. It consist of three semi circular canals. These canals are filled with a fluid and sensory cells are located at special places in these canals.

Function of Ear of Frog

When sound waves strike the tympanic membrane, it is set into vibration, this is in turn vibrates the internal ear and thus sound waves stimulate the hearing receptors in the inner ear. The internal ear, in addition to hearing also keeps the balance of the body.

Eye of Frog

The frog has two eyes one on each side of the head. If we make vertical section of the eye, we find that the innermost layer of the ball is the sensory retina. The retina contains photoreceptor cells. Outside the retina is the choroid, which is richly supplied with blood capillaries supplying nutrients to the retina. The sclerotic is the hard, outer most layer of the eye. It provides shape to the eye ball. The anterior transparent part of the eye is called cornea. Behind the cornea is Iris. The Iris has a window called the pupil. Behind the pupil is the lens of the eye. The cornea, pupil and lens focus light on the retina. A watery fluid is present in between the cornea and lens. Similarly a jelly like fluid is present between the lens and retina, through which light passes before it strikes retina. Optic nerve takes the sensory messages from the eye to the brain.
(Diagram)