Class Amphibia includes frogs, toads, salamanders and newts. They are small vertebrates that need water or moist environments to survive. They have thin, moist skin that allows for cutaneous respiration. They also have lungs and can breathe through their skin and mouth. The document then discusses the characteristics of amphibians, including having both aquatic and terrestrial stages of life. It provides details on the Indian bullfrog (Rana tigrina) as a type study, including its distribution, habitats, locomotion, feeding, breeding and other behaviors. The rest of the document describes anatomical features of R. tigrina such as its streamlined body shape, smooth skin, limbs adapted for jumping and

1. Class -Amphibia
Dr. A.B. Gaware
Assistant Professor,
Department of Zoology,
Shri Shivaji College, Motala, Dist.
Buldhana

2. Class -Amphibia
•Amphibians are small vertebrates that need water,
or a moist environment, to survive.
•The species in this group include frogs, toads,
salamanders, and newts.
•All can breathe and absorb water through their very
thin skin.
•Amphibians also have special skin glands that
produce useful proteins.

3. Characteristics of Class Amphibia
•The characteristics of the organisms present in class amphibia are as follows:
•These can live both on land and in water.
•They are ectothermic animals, found in a warm environment.
•Their body is divided into head and trunk. The tail may or may not be present.
•The skin is smooth and rough without any scales, but with glands that make it
moist.
•They have no paired fins. Unpaired fins might be present.
•They have two pairs of limbs for locomotion.
•They respire through the lungs and skin. Gills might be present externally in
some adults.
•The heart is three chambered.
•The kidneys are mesonephric. The excretory material includes ammonia and
urea.
•They possess ten pairs of cranial nerves.
•The lateral line is present during their development.
•The sexes are separate and fertilization is usually external. However, in
salamanders, the fertilization is internal.
•Development is indirect with metamorphosis.
•Breeding occurs in water. The copulatory organs are absent in males.

4. Type study
Rana tigrina

5. Distribution and habitat
Many frogs in this genus breed in early spring, although subtropical and tropical species may
breed throughout the year. Males of most of the species are known to call, but a few species are
thought to be voiceless. Females lay eggs in rafts or large, globular clusters, and can produce up
to 20,000 at one time.
Habits
(1) Locomotion : (a) Jumping and leaping, (b) Swimming. Absence of neck is helpful in
swimming in water and jumping on land.
(2) Feeding : The adult frog is carnivorous. Tadpole (larva of frog) is herbivorous.
(3) Croaking : The male frog croaks louder than the females because of the presence of two
vocal sacs in male frog. The vocal sacs act as resonators. The croaking is mating call to attract the
female frog.
(4) Hibernation (Winter sleep) : During hibernation frog respires through skin (cutaneous
respiration) only.
(5) Aestivation (Summer sleep) : During this period frog takes rest and recuperates its energy.
(6) Protective Coloration : The frog is capable of changing its body color with the change in its
surroundings. It can not only avoid its enemies but can catch its prey unnoticed.
(7) Breeding : The male frog jumps on the female frog and holds her tightly with the help of his
fore-limbs. Gripping of the female by the male is also very much aided by the presence of nuptial
pads. This sexual embrace is called the amplexus. Fertilization is external. During development, a
fish like tailed tadpole is produced, which respires with the help of gills and feeds upon vegetable
matter.
(8) Molting : The frog sheds off almost once a month its skin during its active life in the form of
small casting. This phenomenon is known as moulting.

6. Shape and Size:
•It has streamlined body The two ends, the anterior and the posterior, of the body are pointed and
the triangular flattened head, with its blunt apex directed forward, is broadly united to the trunk.
Thus, there is no neck and no tail.
•The size of adult frog varies from 12 to 18 cm in length and 5 to 8 cm in width. The colour of
the body at the dorsal side is green with black spots and streaks but ventrally it is paler.
•Skin is smooth, thin, moist and slimy, and fits loosely on the body.
•Skin of back is folded or thickened longitudinally called dermal plicae.
Head:
•The head is almost triangular and somewhat
flattened. Its anteriority directed blunt apex is
known as snout which terminates into a large,
transverse mouth.
•At the tip of the snout are two laterally placed
nostrils or external nares communicating with the
buccal cavity through internal nares, serving in
respiration.
•The head dorsolateral bears two large prominent
bulging eyes.
•This position enables the frog to see in all the
directions and, thus, compensate the
disadvantage on land due to the absence of the
neck.

7. •The eyes are protected by two eyelids or nictitating membrane
•There are no external ears but behind and below each eye there is a nearly circular obliquely
placed a tough transparent membrane-the tympanic membrane or ear drum.
•In the male frog under the head on either side are placed two bluish wrinkled patches of skin-the
vocal sacs which are used to produce croaking sound to attract the females for copulation.
Trunk:
• The head is broadly joined with short somewhat flattened ovoid trunk.
• At its dorsal side in the middle region in the resting stage there is a characteristic sacral hump
which is due to the linking of the hip girdle to the vertebral column.
• At the posterior end of trunk, in between the hindlimbs is present the cloacal opening or vent
through which fecal matter, urine and reproductive bodies (sperms and ova) are discharged.
Attached to the trunk are two pairs of limbs. The forelimbs are shorter, while the hindlimbs are
larger.

8. •The forelimbs are meant to hold and support the front part
of the body at the time of jumping but the hindlimbs assist
in jumping and swimming as the webs are present in
between the toes.
•Each forelimb comprises an upper arm (brachium),
forearm (ante brachium), wrist and hand (manus) with four
fingers (digits) and a vestigial “thumb” or pollex.
•In male the base of the first (inner) finger is thickened
especially in the breeding season, forming the nuptial pad
for clasping the female at the time of amplexus.
•Each hindlimb comprises an upper thigh, shank or lower
leg, ankle (tarsus) and long foot. The latter has a narrow
sole and five slender toes connected by broad thin webs of
skin which help in swimming.

9. Respiratory System of Frog
In adult frog, due to its amphibian life, respiration occurs through skin (cutaneous
respiration), lining of the Bucco-pharyngeal cavity (buccal respiration) and the lungs
(pulmonary respiration). Ordinary respiratory requirements are met by the skin and
Bucco-pharyngeal cavity, lungs are used only when the need of oxygen is great.
Cutaneous Respiration:
The skin of frog is an important organ of respiration. The frog, due to amphibious mode
of life, passes most of the time of its life in water. During this period the skin only
serves as an organ of respiration for gaseous exchange. Similarly, when frog undergoes
summer sleep (aestivation) and winter sleep (hibernation), the skin is the only organ of
respiration.
The skin of frog is very much suited for the respiratory function as it is very thin and
richly supplied with blood capillaries and remains moist with the water and also mucus,
secreted by mucous glands.
During gaseous exchange the oxygen first dissolves in the moisture present over the
body and then diffuses into the blood circulating in the blood capillaries, while the
resultant carbon dioxide passes out from the blood into the surrounding medium (water)
by diffusion. In cutaneous respiration, no movements are needed because skin always
remains exposed to air or water.

10. Bucco-Pharyngeal Respiration or Buccal Respiration:
•The mucous lining of the buccal cavity is richly supplied with blood capillaries and
remains moist by the mucus.
•The buccal respiration occurs by lowering and raising of the floor of the buccal cavity,
during the course of which the air is constantly sucked into the buccal cavity and is
drawn out through the external and internal nares.
•In this type of respiration, the mouth and
glottis remain closed.
•Thus, no air enters or goes out from the
lungs. When the floor of the buccal cavity is
lowered, the air enters the buccal cavity
through the nostrils or the nares.
•The oxygen of air dissolves in the layer of
mucus and then goes into blood.
•At the same time carbon dioxide is given
out into the buccal cavity from the blood
which is expelled along with residual air
through the nostrils when the floor of the
buccal cavity is raised

11. Pulmonary Respiration:
Respiration on land in air with the help of lungs is the pulmonary respiration. In frog,
lungs are poorly developed. The intake of oxygen by lungs is not sufficient to the body.
Therefore, oxygen intake through moist skin and buccal cavity is needed.
Organs of Respiration:
The organs of aerial respiration are a pair of lungs. The lungs are not only the organs of
respiration but are also hydrostatic organs as they enable frog to float in water when
they are inflated.
(a) Respiratory Tract:
It includes the external nostrils, nasal
chambers, internal nostrils, bucco-pharyngeal
cavity, glottis, laryngo-tracheal chamber and a pair
of bronchi. The median slit-like glottis on the floor
of pharynx opens into larynx (laryngo-tracheal
chamber). Larynx is a small sac whose walls are
supported by two arytenoid and one cricoid
cartilages. Cricoid cartilage is a slender ring
surrounding the larynx.

12. The arytenoid cartilages are a pair of semilunar valves, which rest upon the cricoid
cartilage. Their upper edges form the lateral margins of the glottis. They afford
attachment to muscles by which glottis may be opened or closed. The true sound
producing organs are a pair of elastic bands, the vocal cords, extending longitudinally
across the larynx.
Their median edges are thickened and lie near each other in the middle line. Sound
is produced by the expulsion of air from the lungs which set the free edges of the vocal
cords in vibration. Vibrations in the sound are caused by altering the tension on the
cords through the action of laryngeal muscles.
The vocal apparatus of the male frog is much larger than that of the female. Vocal
sacs found only in male frog serve as resonators to increase the croaking sound
produced by the vocal cords. The larynx opens behind into a pair of very small tubes,
the bronchi, which lead to corresponding lung.

13. Blood vascular system
The blood vascular system of frog is closed. It includes the heart, blood vessels, blood
and lymphatic system. The prime function of this system is to distribute the digested
food and oxygen to different parts of the body, in order to release energy to carry out
life activities and also to bring the excretory and gaseous wastes to organs of
elimination, i.e., kidneys and lungs.
Heart:
The heart is a muscular pumping organ pushing the blood into the closed circulatory
system.
i) External Structure:
The heart of frog is a dark red coloured conical muscular organ situated ventrally to the
liver in the pericardial cavity along the mid-ventral line at the level of forelimbs.
a) Pericardium:
The heart is enclosed within a sac formed of two membranes, an outer pericardium and
an inner epicardium which closely invests the heart. Between these two membranes a
serous or pericardial fluid is found which prevents friction and also keeps the heart
moist. It also protects the heart from outer shocks.

14. (b) Chambers of Heart:
The heart looks like an angular structure with broader anterior part and narrow posterior
part. The heart is 3-chambered. The broader part of the heart contains two atrium or
auricles, whereas the posterior part has a single ventricle. The atrium or auricle lies
anterior to the ventricle. Both the auricles are externally demarcated by a faint
longitudinal inter-auricular groove.

15. (ii) Internal Structure:
Internally the heart is three-chambered with two auricles and one ventricle. The blood
flows only in one direction through various chambers. Their openings are guarded by
valves.
(a) Auricles:
•The two auricles, right and left, are
separated from each other by thin vertical
inter-auricular septum.
•Left auricle is smaller than the right. In
the right auricle close to the septum there
is a transverse oval opening called sinu-
auricular aperture through which blood
enters into the auricle from the sinus-
venous.
• It is guarded by two lip-like sinu-
auricular valves, one arising from the
dorsal edge and the other from the ventral.
•These valves allow the free flow of blood only into the right auricle but prevent the
backward flow of the blood.

16. •In the left auricle slightly anterior to the sinu-auricular aperture but close to the septum
there is a small opening of pulmonary vein which has no valve.
•The two auricles open into a single ventricle by an auriculo-ventricular aperture which
is bounded by two pairs of auriculo-ventricular valves, one arising from the dorsal edge
and the other from the ventral edge of this aperture.
(b) Ventricle:
•The ventricle is a most conspicuous triangular chamber of the heart with muscular
walls.
•Its inner surface has irregular ridges, the columnae carnae with deep pockets between
them, which to some extent prevent the mixing of the blood from the two auricles.
These ridges reduce the lumen of the ventricle.
•The flaps of auriculo-ventricular valves are attached to the wall of ventricle by thread-
like chordae tendinae.

17. (c) Truncus Arteriosus:
•From the upper right side of the ventricle arises a tubular truncus arteriosus. Its
opening is guarded by three semilunar valves, whose edges are directed towards the
truncus.
•On contraction of the ventricle these valves are pushed apart and make a free passage
for the blood from the ventricle into truncus but they prevent the backward flow of
blood into the ventricle.
•The truncus arteriosus is formed of a basal
thick-walled conus arteriosus and a distal
thin-walled ventral aorta.
•Its conus arteriosus part which is next to the
ventricle is known as pylangium and the
distal ventral part as synangium.
•Pylangium is a short tubular structure, while
synangium is simply formed by the union of
the basal parts of the arteries.
•The distal end of pylangium or the conus
arteriosus is also provided with a row of
semilunar valves which actually mark the
boundary line of the pylangium and
synangium.

18. (iii) Working of the Heart:
 In the wall of the sinus venosus there is a pacemaker called sinu-auricular node
which initiates the heart to contract. Because the muscles of the auricles are
continuous with those of the ventricle, therefore, the wave of excitation or
contraction starts from the sinus venosus and ends at the truncus, thus, these
chambers contract in sequence.
 As soon as sinus venosus contracts, its impure blood is pumped into the right auricle
through sinu- auricular aperture. At the same time the left auricle receives blood
from the lungs via pulmonary veins. Both auricles contract almost simultaneously
driving blood into the ventricle through the auriculo-ventricular aperture.
 According to new theory propounded by Vandervael (1933) and Foxon (1953), the
blood coming into auricles whether from lungs or sinus venosus is all oxygenated
because oxygenation of blood takes place not only in the lung but also in the skin
and the buccal cavity. It means the blood which comes into sinus venosus from skin
and buccal cavity is equally oxygenated, if not more.
 On contraction of the two auricles the blood comes into ventricle where the two get
completely mixed up. This mixed blood is distributed to all parts of the body
through the common carotid, systemic, and pulmocutaneous arches.

19. Arterial system:
In frog arterial system begins with the truncus arteriosus which divides into two large right and
left branches or trunks. Each branch divides into three aortic arches- an anterior carotid arch, a
middle systemic arch and a posterior pulmocutaneous arch.
Carotid Arch:
The carotid arch of each side also divides into two, an external carotid or lingual going
to the lower jaw and tongue and an internal carotid to the orbit and brain. Each internal
carotid at its commencement bears a swollen carotid body or labyrinth formed from the
second pair of gill-slit. Carotid bodies or labyrinths have a network of capillaries and
chromoffin cells. They detect the pressure of oxygen and CO2 in the blood.
Systemic Arch:
The two systemic arches as pass outward and curve around the oesophagus to join to
form a median dorsal aorta going backward beneath the vertebral column.
Each systemic arch gives out three arteries:
(i) Oesophageal artery to the oesophagus,
(ii) Occipitovertebral artery to the head, vertebral column and spinal cord and
(iii) A large subclavian to the forelimbs.
The oesophageal artery very often arises from the occipitovertebral artery.
Pulmocutaneous Arch:
Each pulmocutaneous arch divides into two arteries- a pulmonary artery going to a lung
and a cutaneous artery to the skin and buccal cavity.

20. Dorsal Aorta:
It is an unpaired artery formed by the union of
two systemic arches. It runs posteriorly mid-
dorsally just beneath the vertebral column.
It gives off a number of arteries:
(i) Coeliaco-Mesenteric Artery:
A large but unpaired coeliaco-mesenteric artery
arises at the point where the dorsal aorta is
formed by the union of the right and left
systemic arteries.
(ii) Gonadial:
A pair of small gonadial arteries to the gonads,
called spermatic in male and ovarian in female.
(iii) Renal:
Dorsal aorta runs backward between the
kidneys and gives off 5 to 6 pairs of renal
arteries to both the kidneys.
(iv) Common Iliacs:
Posteriorly dorsal aorta divides into two large
iliac arteries supplying the hindlimbs.

21. Venous System In Frog
The blood vessels which carry the blood
towards the heart constitute the venous
system.
The venous system in frog constitutes
the following main veins:
Pulmonary Veins:
The oxygenated blood from two lungs is
returned by two pulmonary veins, which
open dorsally into the left auricle after
uniting with each other.
Caval Veins:
The deoxygenated blood from the rest of
the body comes to the sinus venosus
through two precavals (anterior venae
cavae) and one postcaval vein (posterior
vena cavae).

22. (i) Precavals or Anterior Vena Cavae:
Each precaval or anterior vena cava is formed by the union of three veins:
(a) External jugular receiving branches from the tongue (lingual) and floor of the mouth
(mandibular),
(b) Innominate receiving branches from the brain and orbit (internal jugular), and from
shoulder and back of arm (subscapular),
(c) Subclavian vein receiving branches from arm (brachial) and from skin and muscles
of the abdomen and also mucous membrane of mouth and head muscles (musculo-
cutaneous).
(ii) Postcaval or Posterior Vena Cava:
The postcaval vein receives blood from the kidneys by 5 to 6 pairs of renal veins and a
pair of gonadial veins (spermatic in male and ovarian in female) from gonads.
Renal Portal Vein:
The veins which collect the blood from the posterior side of the body constitutes renal
portal system. Two large veins, the femoral and sciatic, return the blood from each leg.
Hepatic Portal System:
Hepatic portal system collects the blood from the alimentary canal through many
branches and carries it to the liver where the veins break up into capillaries and the
blood is collected by hepatic veins to pour into the postcaval.

23. Excretory System
Since the excretory and reproductive systems are closely associated, hence, it is
customary to call the two systems together as a urogenital or urinogenital system,
though both are unrelated functionally. In frog the sexes are separate.
The urinogenital organs can be studied under the following heads:
1. Excretory System:
The excretory system in both male and female frog is similar. The excretion is mainly
carried out with the help of a pair of kidneys, a pair of ureters, a urinary bladder and
cloaca.
(i) Kidneys:
Both the kidneys are elongated, compact, flattened and dark red in colour. These are
found in the lymph spaces (subvertebral lymph sinus) above the coelom attached on
either side of vertebral column. In tadpole the kidneys are pronephros, whereas in adult
these are mesonephros. These are covered ventrally by peritoneum.

24. (ii) Ureters:
From the outer smooth convex posterior side of each kidney arises a mesonephric or
Wolffian duct or ureter which passes backwards to open into dorsal side of the cloaca.
The openings of the ureters are placed over a separate papilla on the dorsal side of
cloaca. In male frog the ureters dilate just posterior to the kidney to form a vesicula
seminal is in which sperms are stored.
In male frog the ureters convey the sperms and urine, and, hence, are called urinogenital
ducts. The ventral surface of each kidney has a yellow colored adrenal or supra renal
gland of endocrine function. To the anterior of each kidney are attached numerous
finger-like fat bodies, a testis in male and ovary in female. Fat bodies are reserves for
nourishment.

25. (iii) Urinary Bladder:
It is large, thin-walled bilobed distensible structure. It also opens into the ventral wall of cloaca
by a sphinctered aperture. Its aperture lies below and opposite to the openings of ureters. The
inner surface of bladder is lined with a layer of epithelium about three cells thick. The middle
layer of the bladder consists of a network of smooth muscle fibres and outside this layer is a thin
sheet of connective tissue covered externally by the peritoneum.
(iv) Cloaca:
It is a small, medium sac receiving the anus, urinogenital apertures and the opening of
urinary bladder. Cloaca opens outside by a cloacal aperture placed at the posterior end
of the body between the two hindlimbs.

26. Male Reproductive System
Male reproductive system includes a pair of testes attached to kidneys, vasa efferentia
and a pair of urinogenital ducts. Copulatory organs are absent.
i. Testis:
• The testis are rounded or ovoid, light yellow bodies attached to the antero-ventral
surfaces of the kidneys by a double fold of peritoneum, the mesorchium.
• Actually each testis is surrounded by peritoneum, which is extended dorsally as a
double membrane, the mesorchium, to the dorsal side of the body cavity, where its
becomes continuous with the general coelomic lining.

27. ii. Vasa Efferentia:
•The vasa efferentia consist of a variable number of slender tubes arising from the inner
margin of testis and extend within the mesorchium and then enter the inner margin of
the kidney to open into the Bidders, canal.
•The Bidders’ canal communicates with the ureter through collecting tubules of kidney.
In this way sperms enter the ureter of kidney through vasa efferentia.
•Bidders canal and collecting tubules.
•The vasa efferentia are originally outgrowths of the walls of the Malpighian corpuscles
which become connected with the testis.
iii. Urinogenital Duct:
Ureter in male frog is a urinary duct as well as a vas deferens to convey the urine and
spermetozoa. Hence, it is called a urinogenital duct. Both the ureters open into the
dorsal wall of cloaca separately on urinogenital papillae.

28. Female Urinogenital System
The excretory organs are the same in female frog as found in male frog, but they do
not have any connection with the reproductive organs. The ureter does not dilate as
vesicula seminalis and no ducts from ovaries open into the kidneys. The cloaca
serves as a common passage for urinary and genital systems as in the male frog.
Female Reproductive System:
Female reproductive system includes a pair
of ovaries and a pair of oviducts.
i. Ovaries:
Both the ovaries are attached to the dorsal
abdominal wall, close to the kidneys, by a
fold of peritoneum called mesovarium. The
ovaries are large, lobulated hollow sac-like
structures. In breeding season the ovaries
become greatly enlarged. Histologically, the
wall of each ovary is composed of visceral
peritoneum which forms germinal
epithelium and internal to it is the fibrous
connective tissue having blood vessels,
muscle fibres and nerves.

29. ii. Oviducts:
•On each side is a long and much coiled glandular and ciliated oviduct or Mullerian
duct. It starts near the base of the lung by a thin-walled ciliated coelomic or oviducal
funnel.
•At the posterior end near cloaca, each oviduct dilates to form a thin-walled ovisac
called uterus which opens by a narrow aperture on a papilla in the cloaca.
•The cilia of the oviduct direct the eggs posteriorly and the glands secrete
albuminous coat around each egg during their descent.
•Oviducts become much enlarged and coiled just before the breeding season.
•The eggs escape from the surface of the ovary into the coelom and are directed by
cilia into the oviduct and are temporarily stored in the ovisacs.

30. Parental care in Amphibians
Parental care is the care of the eggs or the young's until they become able to protect
themselves from the predators.
These devices fail under two heads:
(1) Protection by the parents by means of nests, nurseries, or shelters and
2) Direct caring or nursing by parents.
The different modes of protection are given below in the three important orders of class
Amphibia.

31. 1. Protection by Means of Nests, Nurseries and Shelters:
A number of different species of frogs and toads construct nests or shelters of leaves or other
materials in which the eggs are deposited and the youngs are developed.
A. In Enclosures in the Water (Mud Nests):
B. In Holes Near Water (Foam Nests):
C. In Nests on Trees (Tree Nests):
D. In Transparent Gelatinuous Bags:
E. On Trees or in Moss away from Water:

32. 2. Direct Nursing by the Parent:
A. Tadpoles Transferring to water D. Eggs in Back Pouches:
B. Eggs Protected by Male: E. In the Mouth or Gular Pouch:
C. Eggs Carried by the Parents: F. Coiling Around Eggs:
G. Viviparous or Viviparity: