NCERT Textbook: Structural Organisation in Animals | Biology Class 11 - NEET PDF Download

 Page 1


STRUCTURAL ORGANISATION IN ANIMALS 79
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ORGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Organ and Organ
System
7.2 Frogs
2024-25
Page 2


STRUCTURAL ORGANISATION IN ANIMALS 79
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ORGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Organ and Organ
System
7.2 Frogs
2024-25
80 BIOLOGY
organ in our body is made of one or more type of tissues. For example,
our heart consists of all the four types of tissues, i.e., epithelial, connective,
muscular and neural. We also notice, after some careful study that the
complexity in organ and organ systems displays certain discernable trend.
This discernable trend is called evolutionary trend (You will study the
details in class XII). In this chapter, you are being introduced to
morphology and anatomy of frog. Morphology refers to study of form or
externally visible features. In the case of plants or microbes, the term
morphology precisely means only this. In case of animals this refers to
the external appearance of the organs or parts of the body. The word
anatomy conventionally is used for the study of morphology of internal
organs in the animals. You will learn the morphology and anatomy of
frog representing vertebrates.
7.2 FROGS
Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry.  You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.
7.2.1 Morphology
Have you ever touched the skin of frog? The skin is smooth and slippery
due to the presence of mucus. The skin is always maintained in a moist
condition. The colour of dorsal side of body is
generally olive green with dark irregular spots.  On
the ventral side the skin is uniformly pale yellow.
The frog never drinks water but absorb it through
the skin.
Body of a frog is divisible into head and trunk
(Figure 7.1). A neck and tail are absent. Above the
mouth, a pair of nostrils is present.  Eyes are bulged
and covered by a nictitating membrane that
protects them while in water. On either side of eyes
a membranous tympanum (ear) receives sound
signals. The forelimbs and hind limbs help in
Figure 7.1  External features of frog
Eye
Fore limb
Hind limb
Head
Trunk
2024-25
Page 3


STRUCTURAL ORGANISATION IN ANIMALS 79
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ORGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Organ and Organ
System
7.2 Frogs
2024-25
80 BIOLOGY
organ in our body is made of one or more type of tissues. For example,
our heart consists of all the four types of tissues, i.e., epithelial, connective,
muscular and neural. We also notice, after some careful study that the
complexity in organ and organ systems displays certain discernable trend.
This discernable trend is called evolutionary trend (You will study the
details in class XII). In this chapter, you are being introduced to
morphology and anatomy of frog. Morphology refers to study of form or
externally visible features. In the case of plants or microbes, the term
morphology precisely means only this. In case of animals this refers to
the external appearance of the organs or parts of the body. The word
anatomy conventionally is used for the study of morphology of internal
organs in the animals. You will learn the morphology and anatomy of
frog representing vertebrates.
7.2 FROGS
Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry.  You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.
7.2.1 Morphology
Have you ever touched the skin of frog? The skin is smooth and slippery
due to the presence of mucus. The skin is always maintained in a moist
condition. The colour of dorsal side of body is
generally olive green with dark irregular spots.  On
the ventral side the skin is uniformly pale yellow.
The frog never drinks water but absorb it through
the skin.
Body of a frog is divisible into head and trunk
(Figure 7.1). A neck and tail are absent. Above the
mouth, a pair of nostrils is present.  Eyes are bulged
and covered by a nictitating membrane that
protects them while in water. On either side of eyes
a membranous tympanum (ear) receives sound
signals. The forelimbs and hind limbs help in
Figure 7.1  External features of frog
Eye
Fore limb
Hind limb
Head
Trunk
2024-25
STRUCTURAL ORGANISATION IN ANIMALS 81
swimming, walking, leaping and burrowing. The hind limbs end in five
digits and they are larger and muscular than fore limbs that end in four
digits. Feet have webbed digits that help in swimming. Frogs exhibit sexual
dimorphism. Male frogs can be distinguished by the presence of sound
producing vocal sacs and also a copulatory pad on the first digit of the
fore limbs which are absent in female frogs.
7.2.2 Anatomy
The body cavity of frogs accommodate different organ systems such as
digestive, circulatory, respiratory, nervous, excretory and reproductive
systems with well developed structures and functions  (Figure 7.2).
The digestive system consists of alimentary canal and digestive glands.
The alimentary canal is short because frogs are carnivores and hence the
length of intestine is reduced. The mouth opens into the buccal cavity that
leads to the oesophagus through pharynx. Oesophagus is a short tube
that opens into the stomach which in turn continues as the intestine, rectum
and finally opens outside by the cloaca.  Liver secretes bile that is stored in
the gall bladder. Pancreas, a digestive gland produces pancreatic juice
Figure 7.2 Diagrammatic representation of internal organs of frog showing
complete digestive system
Intestine
Ureter
Rectum
Cloaca
Urinary
bladder
Oesophagus
Liver
Stomach
Kidney
Gall
bladder
Fat bodies
Lung
Heart
Cloacal Aperture
2024-25
Page 4


STRUCTURAL ORGANISATION IN ANIMALS 79
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ORGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Organ and Organ
System
7.2 Frogs
2024-25
80 BIOLOGY
organ in our body is made of one or more type of tissues. For example,
our heart consists of all the four types of tissues, i.e., epithelial, connective,
muscular and neural. We also notice, after some careful study that the
complexity in organ and organ systems displays certain discernable trend.
This discernable trend is called evolutionary trend (You will study the
details in class XII). In this chapter, you are being introduced to
morphology and anatomy of frog. Morphology refers to study of form or
externally visible features. In the case of plants or microbes, the term
morphology precisely means only this. In case of animals this refers to
the external appearance of the organs or parts of the body. The word
anatomy conventionally is used for the study of morphology of internal
organs in the animals. You will learn the morphology and anatomy of
frog representing vertebrates.
7.2 FROGS
Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry.  You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.
7.2.1 Morphology
Have you ever touched the skin of frog? The skin is smooth and slippery
due to the presence of mucus. The skin is always maintained in a moist
condition. The colour of dorsal side of body is
generally olive green with dark irregular spots.  On
the ventral side the skin is uniformly pale yellow.
The frog never drinks water but absorb it through
the skin.
Body of a frog is divisible into head and trunk
(Figure 7.1). A neck and tail are absent. Above the
mouth, a pair of nostrils is present.  Eyes are bulged
and covered by a nictitating membrane that
protects them while in water. On either side of eyes
a membranous tympanum (ear) receives sound
signals. The forelimbs and hind limbs help in
Figure 7.1  External features of frog
Eye
Fore limb
Hind limb
Head
Trunk
2024-25
STRUCTURAL ORGANISATION IN ANIMALS 81
swimming, walking, leaping and burrowing. The hind limbs end in five
digits and they are larger and muscular than fore limbs that end in four
digits. Feet have webbed digits that help in swimming. Frogs exhibit sexual
dimorphism. Male frogs can be distinguished by the presence of sound
producing vocal sacs and also a copulatory pad on the first digit of the
fore limbs which are absent in female frogs.
7.2.2 Anatomy
The body cavity of frogs accommodate different organ systems such as
digestive, circulatory, respiratory, nervous, excretory and reproductive
systems with well developed structures and functions  (Figure 7.2).
The digestive system consists of alimentary canal and digestive glands.
The alimentary canal is short because frogs are carnivores and hence the
length of intestine is reduced. The mouth opens into the buccal cavity that
leads to the oesophagus through pharynx. Oesophagus is a short tube
that opens into the stomach which in turn continues as the intestine, rectum
and finally opens outside by the cloaca.  Liver secretes bile that is stored in
the gall bladder. Pancreas, a digestive gland produces pancreatic juice
Figure 7.2 Diagrammatic representation of internal organs of frog showing
complete digestive system
Intestine
Ureter
Rectum
Cloaca
Urinary
bladder
Oesophagus
Liver
Stomach
Kidney
Gall
bladder
Fat bodies
Lung
Heart
Cloacal Aperture
2024-25
82 BIOLOGY
containing digestive enzymes. Food is captured by the bilobed tongue.
Digestion of food takes place by the action of HCl and gastric juices secreted
from  the walls of the stomach. Partially digested food called chyme is passed
from stomach to the first part of the small intestine, the duodenum. The
duodenum receives bile from gall bladder and pancreatic juices from the
pancreas through a common bile duct. Bile emulsifies fat and pancreatic
juices digest carbohydrates and proteins. Final digestion takes place in the
intestine. Digested food is absorbed by the numerous finger-like folds in
the inner wall of intestine called villi and microvilli. The undigested solid
waste moves into the rectum and passes out through cloaca.
Frogs respire on land and in the water by two different methods. In
water, skin acts as aquatic respiratory organ (cutaneous respiration).
Dissolved oxygen in the water is exchanged through the skin by diffusion.
On land, the buccal cavity, skin and lungs act as the respiratory organs.
The respiration by lungs is called pulmonary respiration. The lungs are a
pair of elongated, pink coloured sac-like structures present in the upper
part of the trunk region (thorax). Air enters through the nostrils into the
buccal cavity and then to lungs. During aestivation and hibernation
gaseous exchange takes place through skin.
The vascular system of frog is well-developed closed type. Frogs have
a lymphatic system also. The blood vascular system involves heart, blood
vessels and blood. The lymphatic system consists of lymph, lymph
channels and lymph nodes. Heart is a muscular structure situated in the
upper part of the body cavity. It has three chambers, two atria and one
ventricle and is covered by a membrane called pericardium. A triangular
structure called sinus venosus joins the right atrium. It receives blood
through the major veins called vena cava. The ventricle opens into a sac-
like conus arteriosus on the ventral side of the heart. The blood from the
heart is carried to all parts of the body by the arteries (arterial system).
The veins collect blood from different parts of body to the heart and form
the venous system. Special venous connection between liver and intestine
as well as the kidney and lower parts of the body are present in frogs. The
former is called hepatic portal system and the latter is called renal portal
system. The blood is composed of plasma and cells. The blood cells are
RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes
and platelets. RBC’s are nucleated and contain red coloured pigment
namely haemoglobin. The lymph is different from blood. It lacks few
proteins and RBCs. The blood carries nutrients, gases and water to the
respective sites during the circulation. The circulation of blood is achieved
by the pumping action of the muscular heart.
The elimination of nitrogenous wastes is carried out by a well
developed excretory system. The excretory system consists of a pair of
kidneys, ureters, cloaca and urinary bladder. These are compact, dark
red and bean like structures situated a little posteriorly in the body cavity
on both sides of vertebral column. Each kidney is composed of several
structural and functional units called uriniferous tubules or nephrons.
Two ureters emerge from the kidneys in the male frogs. The ureters act as
urinogenital duct which opens into the cloaca. In females the ureters and
2024-25
Page 5


STRUCTURAL ORGANISATION IN ANIMALS 79
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ORGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Organ and Organ
System
7.2 Frogs
2024-25
80 BIOLOGY
organ in our body is made of one or more type of tissues. For example,
our heart consists of all the four types of tissues, i.e., epithelial, connective,
muscular and neural. We also notice, after some careful study that the
complexity in organ and organ systems displays certain discernable trend.
This discernable trend is called evolutionary trend (You will study the
details in class XII). In this chapter, you are being introduced to
morphology and anatomy of frog. Morphology refers to study of form or
externally visible features. In the case of plants or microbes, the term
morphology precisely means only this. In case of animals this refers to
the external appearance of the organs or parts of the body. The word
anatomy conventionally is used for the study of morphology of internal
organs in the animals. You will learn the morphology and anatomy of
frog representing vertebrates.
7.2 FROGS
Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry.  You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.
7.2.1 Morphology
Have you ever touched the skin of frog? The skin is smooth and slippery
due to the presence of mucus. The skin is always maintained in a moist
condition. The colour of dorsal side of body is
generally olive green with dark irregular spots.  On
the ventral side the skin is uniformly pale yellow.
The frog never drinks water but absorb it through
the skin.
Body of a frog is divisible into head and trunk
(Figure 7.1). A neck and tail are absent. Above the
mouth, a pair of nostrils is present.  Eyes are bulged
and covered by a nictitating membrane that
protects them while in water. On either side of eyes
a membranous tympanum (ear) receives sound
signals. The forelimbs and hind limbs help in
Figure 7.1  External features of frog
Eye
Fore limb
Hind limb
Head
Trunk
2024-25
STRUCTURAL ORGANISATION IN ANIMALS 81
swimming, walking, leaping and burrowing. The hind limbs end in five
digits and they are larger and muscular than fore limbs that end in four
digits. Feet have webbed digits that help in swimming. Frogs exhibit sexual
dimorphism. Male frogs can be distinguished by the presence of sound
producing vocal sacs and also a copulatory pad on the first digit of the
fore limbs which are absent in female frogs.
7.2.2 Anatomy
The body cavity of frogs accommodate different organ systems such as
digestive, circulatory, respiratory, nervous, excretory and reproductive
systems with well developed structures and functions  (Figure 7.2).
The digestive system consists of alimentary canal and digestive glands.
The alimentary canal is short because frogs are carnivores and hence the
length of intestine is reduced. The mouth opens into the buccal cavity that
leads to the oesophagus through pharynx. Oesophagus is a short tube
that opens into the stomach which in turn continues as the intestine, rectum
and finally opens outside by the cloaca.  Liver secretes bile that is stored in
the gall bladder. Pancreas, a digestive gland produces pancreatic juice
Figure 7.2 Diagrammatic representation of internal organs of frog showing
complete digestive system
Intestine
Ureter
Rectum
Cloaca
Urinary
bladder
Oesophagus
Liver
Stomach
Kidney
Gall
bladder
Fat bodies
Lung
Heart
Cloacal Aperture
2024-25
82 BIOLOGY
containing digestive enzymes. Food is captured by the bilobed tongue.
Digestion of food takes place by the action of HCl and gastric juices secreted
from  the walls of the stomach. Partially digested food called chyme is passed
from stomach to the first part of the small intestine, the duodenum. The
duodenum receives bile from gall bladder and pancreatic juices from the
pancreas through a common bile duct. Bile emulsifies fat and pancreatic
juices digest carbohydrates and proteins. Final digestion takes place in the
intestine. Digested food is absorbed by the numerous finger-like folds in
the inner wall of intestine called villi and microvilli. The undigested solid
waste moves into the rectum and passes out through cloaca.
Frogs respire on land and in the water by two different methods. In
water, skin acts as aquatic respiratory organ (cutaneous respiration).
Dissolved oxygen in the water is exchanged through the skin by diffusion.
On land, the buccal cavity, skin and lungs act as the respiratory organs.
The respiration by lungs is called pulmonary respiration. The lungs are a
pair of elongated, pink coloured sac-like structures present in the upper
part of the trunk region (thorax). Air enters through the nostrils into the
buccal cavity and then to lungs. During aestivation and hibernation
gaseous exchange takes place through skin.
The vascular system of frog is well-developed closed type. Frogs have
a lymphatic system also. The blood vascular system involves heart, blood
vessels and blood. The lymphatic system consists of lymph, lymph
channels and lymph nodes. Heart is a muscular structure situated in the
upper part of the body cavity. It has three chambers, two atria and one
ventricle and is covered by a membrane called pericardium. A triangular
structure called sinus venosus joins the right atrium. It receives blood
through the major veins called vena cava. The ventricle opens into a sac-
like conus arteriosus on the ventral side of the heart. The blood from the
heart is carried to all parts of the body by the arteries (arterial system).
The veins collect blood from different parts of body to the heart and form
the venous system. Special venous connection between liver and intestine
as well as the kidney and lower parts of the body are present in frogs. The
former is called hepatic portal system and the latter is called renal portal
system. The blood is composed of plasma and cells. The blood cells are
RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes
and platelets. RBC’s are nucleated and contain red coloured pigment
namely haemoglobin. The lymph is different from blood. It lacks few
proteins and RBCs. The blood carries nutrients, gases and water to the
respective sites during the circulation. The circulation of blood is achieved
by the pumping action of the muscular heart.
The elimination of nitrogenous wastes is carried out by a well
developed excretory system. The excretory system consists of a pair of
kidneys, ureters, cloaca and urinary bladder. These are compact, dark
red and bean like structures situated a little posteriorly in the body cavity
on both sides of vertebral column. Each kidney is composed of several
structural and functional units called uriniferous tubules or nephrons.
Two ureters emerge from the kidneys in the male frogs. The ureters act as
urinogenital duct which opens into the cloaca. In females the ureters and
2024-25
STRUCTURAL ORGANISATION IN ANIMALS 83
oviduct open seperately in the cloaca. The thin-walled urinary bladder is
present ventral to the rectum which also opens in the cloaca. The frog
excretes urea and thus is a ureotelic animal. Excretory wastes are carried
by blood into the kidney where it is separated and excreted.
The system for control and coordination is highly evolved in the frog. It
includes both neural system and endocrine glands. The chemical
coordination of various organs of the body is
achieved by hormones which are secreted by the
endocrine glands. The prominent endocrine
glands found in frog are pituitary, thyroid,
parathyroid, thymus, pineal body, pancreatic
islets, adrenals and gonads. The nervous system
is organised into a central nervous system (brain
and spinal cord), a peripheral nervous system
(cranial and spinal nerves) and an autonomic
nervous system (sympathetic and
parasympathetic). There are ten pairs of cranial
nerves arising from the brain. Brain is enclosed
in a bony structure called brain box (cranium).
The brain is divided into fore-brain, mid-brain
and hind-brain. Forebrain includes olfactory
lobes, paired cerebral hemispheres and unpaired
diencephalon. The midbrain is characterised by
a pair of optic lobes. Hind-brain consists of
cerebellum and medulla oblongata. The medulla
oblongata passes out through the foramen
magnum and continues into spinal cord, which
is enclosed in the vertebral column.
Frog has different types of sense organs, namely
organs of touch (sensory papillae), taste (taste
buds), smell (nasal epithelium), vision (eyes) and
hearing (tympanum with internal ears). Out of
these, eyes and internal ears are well-organised
structures and the rest are cellular aggregations
around nerve endings. Eyes in a frog are a pair of
spherical structures situated in the orbit in skull.
These are simple eyes (possessing only one unit).
External ear is absent in frogs and only tympanum
can be seen externally. The ear is an organ of
hearing as well as balancing (equilibrium).
Frogs have well organised male and female
reproductive systems. Male reproductive organs
consist of a pair of yellowish ovoid testes (Figure
7.3), which are found adhered to the upper part
of kidneys by a double fold of peritoneum called
mesorchium. Vasa efferentia are 10-12 in
number that arise from testes. They enter the
kidneys on their side and open into Bidder’s
Figure 7.3 Male reproductive system
Fat
bodies
Kidney
Urino
genital duct
Cloaca
Cloacal
aperture
Testis
Adrenal
gland
Urinary
bladder
Rectum
Vasa
efferentia
Figure 7.4 Female reproductive system
Oviduct
Ovary
Ova
Ureter
Cloaca
Cloacal aperture
Urinary
bladder
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