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, esophagus,
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.
Iliac 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)
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