NCERT Textbook: Animal Kingdom | Biology Class 11 - NEET PDF Download

 Page 1


ANIMAL KINGDOM 37 37
When you look around, you will observe different animals with different
structures and forms.  As over a million species of animals have been
described till now, the need for classification becomes all the more
important. The classification also helps in assigning a systematic position
to newly described species.
4.1 BASIS OF CLASSIFICATION
Inspite of differences in structure and form of different animals, there are
fundamental features common to various individuals in relation to the
arrangement of cells, body symmetry, nature of coelom, patterns of
digestive, circulatory or reproductive systems. These features are used
as the basis of animal classification and some of them are discussed here.
4.1.1 Levels of Organisation
Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in sponges,
the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among
the cells. In coelenterates, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation. A still higher level of organisation, i.e.,
organ level is exhibited by members of Platyhelminthes and other higher
phyla where tissues are grouped together to form organs, each specialised
for a particular function. In animals like Annelids, Arthropods, Molluscs,
ANIMAL KINGDOM
CHAPTER  4
4.1 Basis of
Classification
4.2 Classification of
Animals
2024-25
Page 2


ANIMAL KINGDOM 37 37
When you look around, you will observe different animals with different
structures and forms.  As over a million species of animals have been
described till now, the need for classification becomes all the more
important. The classification also helps in assigning a systematic position
to newly described species.
4.1 BASIS OF CLASSIFICATION
Inspite of differences in structure and form of different animals, there are
fundamental features common to various individuals in relation to the
arrangement of cells, body symmetry, nature of coelom, patterns of
digestive, circulatory or reproductive systems. These features are used
as the basis of animal classification and some of them are discussed here.
4.1.1 Levels of Organisation
Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in sponges,
the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among
the cells. In coelenterates, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation. A still higher level of organisation, i.e.,
organ level is exhibited by members of Platyhelminthes and other higher
phyla where tissues are grouped together to form organs, each specialised
for a particular function. In animals like Annelids, Arthropods, Molluscs,
ANIMAL KINGDOM
CHAPTER  4
4.1 Basis of
Classification
4.2 Classification of
Animals
2024-25
38 BIOLOGY
Echinoderms and Chordates, organs have
associated to form functional systems, each
system concerned with a specific physiological
function. This pattern is called organ system
level of organisation. Organ systems in different
groups of animals exhibit various patterns of
complexities. For example, the digestive system
in Platyhelminthes has only a single opening
to the outside of the body that serves as both
mouth and anus, and is hence called
incomplete. A complete digestive system has
two openings, mouth and anus. Similarly, the
circulatory system may be of two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues are
directly bathed in it and
(ii) closed type in which the blood is circulated
through a series of vessels of varying diameters
(arteries, veins and capillaries).
4.1.2 Symmetry
Animals can be categorised on the basis of their
symmetry. Sponges are mostly  asymmetrical,
i.e., any plane that passes through the centre
does not divide them into equal halves. When
any plane passing through the central axis of
the body divides the organism into two identical
halves, it is called radial symmetry .
Coelenterates, ctenophores and echinoderms
have this kind of body plan (Figure 4.1a).
Animals like annelids, arthropods, etc., where
the body can be divided into identical left and
right halves in only one plane, exhibit bilateral
symmetry (Figure 4.1b).
4.1.3 Diploblastic and Triploblastic
Organisation
Animals in which the cells are arranged in two
embryonic layers, an external ectoderm and
an internal endoderm, are called diploblastic
animals, e.g., coelenterates. An undifferentiated
layer, mesoglea, is present in between the
ectoderm and the endoderm (Figure 4.2a).
Figure 4.2 Showing germinal layers :
(a) Diploblastic (b) Triploblastic
(a)
(b)
Ectoderm
Mesoglea
Endoderm
Mesoderm
Figure 4.1 (b) Bilateral symmetry
Figure 4.1 (a)  Radial symmetry
2024-25
Page 3


ANIMAL KINGDOM 37 37
When you look around, you will observe different animals with different
structures and forms.  As over a million species of animals have been
described till now, the need for classification becomes all the more
important. The classification also helps in assigning a systematic position
to newly described species.
4.1 BASIS OF CLASSIFICATION
Inspite of differences in structure and form of different animals, there are
fundamental features common to various individuals in relation to the
arrangement of cells, body symmetry, nature of coelom, patterns of
digestive, circulatory or reproductive systems. These features are used
as the basis of animal classification and some of them are discussed here.
4.1.1 Levels of Organisation
Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in sponges,
the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among
the cells. In coelenterates, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation. A still higher level of organisation, i.e.,
organ level is exhibited by members of Platyhelminthes and other higher
phyla where tissues are grouped together to form organs, each specialised
for a particular function. In animals like Annelids, Arthropods, Molluscs,
ANIMAL KINGDOM
CHAPTER  4
4.1 Basis of
Classification
4.2 Classification of
Animals
2024-25
38 BIOLOGY
Echinoderms and Chordates, organs have
associated to form functional systems, each
system concerned with a specific physiological
function. This pattern is called organ system
level of organisation. Organ systems in different
groups of animals exhibit various patterns of
complexities. For example, the digestive system
in Platyhelminthes has only a single opening
to the outside of the body that serves as both
mouth and anus, and is hence called
incomplete. A complete digestive system has
two openings, mouth and anus. Similarly, the
circulatory system may be of two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues are
directly bathed in it and
(ii) closed type in which the blood is circulated
through a series of vessels of varying diameters
(arteries, veins and capillaries).
4.1.2 Symmetry
Animals can be categorised on the basis of their
symmetry. Sponges are mostly  asymmetrical,
i.e., any plane that passes through the centre
does not divide them into equal halves. When
any plane passing through the central axis of
the body divides the organism into two identical
halves, it is called radial symmetry .
Coelenterates, ctenophores and echinoderms
have this kind of body plan (Figure 4.1a).
Animals like annelids, arthropods, etc., where
the body can be divided into identical left and
right halves in only one plane, exhibit bilateral
symmetry (Figure 4.1b).
4.1.3 Diploblastic and Triploblastic
Organisation
Animals in which the cells are arranged in two
embryonic layers, an external ectoderm and
an internal endoderm, are called diploblastic
animals, e.g., coelenterates. An undifferentiated
layer, mesoglea, is present in between the
ectoderm and the endoderm (Figure 4.2a).
Figure 4.2 Showing germinal layers :
(a) Diploblastic (b) Triploblastic
(a)
(b)
Ectoderm
Mesoglea
Endoderm
Mesoderm
Figure 4.1 (b) Bilateral symmetry
Figure 4.1 (a)  Radial symmetry
2024-25
ANIMAL KINGDOM 39 39
4.1.4 Coelom
Presence or absence of a cavity between the body
wall and the gut wall is very important in
classification. The body cavity, which is lined
by mesoderm is called coelom. Animals
possessing coelom are called coelomates, e.g.,
annelids, molluscs, arthropods, echinoderms,
hemichordates and chordates (Figure 4.3a). In
some animals, the body cavity is not lined by
mesoderm, instead, the mesoderm is present as
scattered pouches in between the ectoderm and
endoderm. Such a body cavity is called
pseudocoelom and the animals possessing them
are called pseudocoelomates, e.g.,
aschelminthes (Figure 4.3b). The animals in
which the body cavity is absent are called
acoelomates, e.g., platyhelminthes (Figure 4.3c).
Figure 4.3 Diagrammatic sectional view of :
(a) Coelomate (b) Pseudocoelomate
(c) Acoelomate
Those animals in which the developing embryo has a third germinal layer,
mesoderm, in between the ectoderm and endoderm, are called
triploblastic animals (platyhelminthes to chordates, Figure 4.2b).
4.1.5 Segmentation
In some animals, the body is externally and internally divided into
segments with a serial repetition of at least some organs. For example, in
earthworm, the body shows this pattern called metameric segmentation
and the phenomenon is known as metamerism.
4.1.6 Notochord
Notochord is a mesodermally derived rod-like structure formed on the
dorsal side during embryonic development in some animals. Animals with
notochord are called chordates and those animals which do not form this
structure are called non-chordates, e.g., porifera to echinoderms.
4.2 CLASSIFICATION OF ANIMALS
The broad classification of Animalia based on common fundamental
features as mentioned in the preceding sections is given in Figure  4.4.
2024-25
Page 4


ANIMAL KINGDOM 37 37
When you look around, you will observe different animals with different
structures and forms.  As over a million species of animals have been
described till now, the need for classification becomes all the more
important. The classification also helps in assigning a systematic position
to newly described species.
4.1 BASIS OF CLASSIFICATION
Inspite of differences in structure and form of different animals, there are
fundamental features common to various individuals in relation to the
arrangement of cells, body symmetry, nature of coelom, patterns of
digestive, circulatory or reproductive systems. These features are used
as the basis of animal classification and some of them are discussed here.
4.1.1 Levels of Organisation
Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in sponges,
the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among
the cells. In coelenterates, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation. A still higher level of organisation, i.e.,
organ level is exhibited by members of Platyhelminthes and other higher
phyla where tissues are grouped together to form organs, each specialised
for a particular function. In animals like Annelids, Arthropods, Molluscs,
ANIMAL KINGDOM
CHAPTER  4
4.1 Basis of
Classification
4.2 Classification of
Animals
2024-25
38 BIOLOGY
Echinoderms and Chordates, organs have
associated to form functional systems, each
system concerned with a specific physiological
function. This pattern is called organ system
level of organisation. Organ systems in different
groups of animals exhibit various patterns of
complexities. For example, the digestive system
in Platyhelminthes has only a single opening
to the outside of the body that serves as both
mouth and anus, and is hence called
incomplete. A complete digestive system has
two openings, mouth and anus. Similarly, the
circulatory system may be of two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues are
directly bathed in it and
(ii) closed type in which the blood is circulated
through a series of vessels of varying diameters
(arteries, veins and capillaries).
4.1.2 Symmetry
Animals can be categorised on the basis of their
symmetry. Sponges are mostly  asymmetrical,
i.e., any plane that passes through the centre
does not divide them into equal halves. When
any plane passing through the central axis of
the body divides the organism into two identical
halves, it is called radial symmetry .
Coelenterates, ctenophores and echinoderms
have this kind of body plan (Figure 4.1a).
Animals like annelids, arthropods, etc., where
the body can be divided into identical left and
right halves in only one plane, exhibit bilateral
symmetry (Figure 4.1b).
4.1.3 Diploblastic and Triploblastic
Organisation
Animals in which the cells are arranged in two
embryonic layers, an external ectoderm and
an internal endoderm, are called diploblastic
animals, e.g., coelenterates. An undifferentiated
layer, mesoglea, is present in between the
ectoderm and the endoderm (Figure 4.2a).
Figure 4.2 Showing germinal layers :
(a) Diploblastic (b) Triploblastic
(a)
(b)
Ectoderm
Mesoglea
Endoderm
Mesoderm
Figure 4.1 (b) Bilateral symmetry
Figure 4.1 (a)  Radial symmetry
2024-25
ANIMAL KINGDOM 39 39
4.1.4 Coelom
Presence or absence of a cavity between the body
wall and the gut wall is very important in
classification. The body cavity, which is lined
by mesoderm is called coelom. Animals
possessing coelom are called coelomates, e.g.,
annelids, molluscs, arthropods, echinoderms,
hemichordates and chordates (Figure 4.3a). In
some animals, the body cavity is not lined by
mesoderm, instead, the mesoderm is present as
scattered pouches in between the ectoderm and
endoderm. Such a body cavity is called
pseudocoelom and the animals possessing them
are called pseudocoelomates, e.g.,
aschelminthes (Figure 4.3b). The animals in
which the body cavity is absent are called
acoelomates, e.g., platyhelminthes (Figure 4.3c).
Figure 4.3 Diagrammatic sectional view of :
(a) Coelomate (b) Pseudocoelomate
(c) Acoelomate
Those animals in which the developing embryo has a third germinal layer,
mesoderm, in between the ectoderm and endoderm, are called
triploblastic animals (platyhelminthes to chordates, Figure 4.2b).
4.1.5 Segmentation
In some animals, the body is externally and internally divided into
segments with a serial repetition of at least some organs. For example, in
earthworm, the body shows this pattern called metameric segmentation
and the phenomenon is known as metamerism.
4.1.6 Notochord
Notochord is a mesodermally derived rod-like structure formed on the
dorsal side during embryonic development in some animals. Animals with
notochord are called chordates and those animals which do not form this
structure are called non-chordates, e.g., porifera to echinoderms.
4.2 CLASSIFICATION OF ANIMALS
The broad classification of Animalia based on common fundamental
features as mentioned in the preceding sections is given in Figure  4.4.
2024-25
40 BIOLOGY
The important characteristic features of the
different phyla are described.
4.2.1 Phylum – Porifera
Members of this phylum are commonly known
as sponges. They are generally marine and mostly
asymmetrical animals (Figure 4.5). These are
primitive multicellular animals and have cellular
level of organisation. Sponges have a water
transport or canal system. Water enters through
minute pores (ostia) in the body wall into a central
cavity, spongocoel, from where it goes out
through the osculum. This pathway of water
transport is helpful in food gathering, respiratory
exchange and removal of waste. Choanocytes
or collar cells line the spongocoel and the canals.
Digestion is intracellular. The body is supported
by a skeleton made up of spicules or spongin
fibres. Sexes are not separate (hermaphrodite),
i.e., eggs and sperms are produced by the same
individual. Sponges reproduce asexually by
fragmentation and sexually by formation of
gametes. Fertilisation is internal and development
is indirect having a larval stage which is
morphologically distinct from the adult.
*Echinodermata exhibits radial or bilateral symmetry depending on the stage.
Figure 4.4  Broad classification of Kingdom Animalia based on common fundamental features
(a)
(b)
(c)
Figure 4.5 Examples of Porifera : (a) Sycon
(b) Euspongia  (c) Spongilla
2024-25
Page 5


ANIMAL KINGDOM 37 37
When you look around, you will observe different animals with different
structures and forms.  As over a million species of animals have been
described till now, the need for classification becomes all the more
important. The classification also helps in assigning a systematic position
to newly described species.
4.1 BASIS OF CLASSIFICATION
Inspite of differences in structure and form of different animals, there are
fundamental features common to various individuals in relation to the
arrangement of cells, body symmetry, nature of coelom, patterns of
digestive, circulatory or reproductive systems. These features are used
as the basis of animal classification and some of them are discussed here.
4.1.1 Levels of Organisation
Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in sponges,
the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among
the cells. In coelenterates, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation. A still higher level of organisation, i.e.,
organ level is exhibited by members of Platyhelminthes and other higher
phyla where tissues are grouped together to form organs, each specialised
for a particular function. In animals like Annelids, Arthropods, Molluscs,
ANIMAL KINGDOM
CHAPTER  4
4.1 Basis of
Classification
4.2 Classification of
Animals
2024-25
38 BIOLOGY
Echinoderms and Chordates, organs have
associated to form functional systems, each
system concerned with a specific physiological
function. This pattern is called organ system
level of organisation. Organ systems in different
groups of animals exhibit various patterns of
complexities. For example, the digestive system
in Platyhelminthes has only a single opening
to the outside of the body that serves as both
mouth and anus, and is hence called
incomplete. A complete digestive system has
two openings, mouth and anus. Similarly, the
circulatory system may be of two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues are
directly bathed in it and
(ii) closed type in which the blood is circulated
through a series of vessels of varying diameters
(arteries, veins and capillaries).
4.1.2 Symmetry
Animals can be categorised on the basis of their
symmetry. Sponges are mostly  asymmetrical,
i.e., any plane that passes through the centre
does not divide them into equal halves. When
any plane passing through the central axis of
the body divides the organism into two identical
halves, it is called radial symmetry .
Coelenterates, ctenophores and echinoderms
have this kind of body plan (Figure 4.1a).
Animals like annelids, arthropods, etc., where
the body can be divided into identical left and
right halves in only one plane, exhibit bilateral
symmetry (Figure 4.1b).
4.1.3 Diploblastic and Triploblastic
Organisation
Animals in which the cells are arranged in two
embryonic layers, an external ectoderm and
an internal endoderm, are called diploblastic
animals, e.g., coelenterates. An undifferentiated
layer, mesoglea, is present in between the
ectoderm and the endoderm (Figure 4.2a).
Figure 4.2 Showing germinal layers :
(a) Diploblastic (b) Triploblastic
(a)
(b)
Ectoderm
Mesoglea
Endoderm
Mesoderm
Figure 4.1 (b) Bilateral symmetry
Figure 4.1 (a)  Radial symmetry
2024-25
ANIMAL KINGDOM 39 39
4.1.4 Coelom
Presence or absence of a cavity between the body
wall and the gut wall is very important in
classification. The body cavity, which is lined
by mesoderm is called coelom. Animals
possessing coelom are called coelomates, e.g.,
annelids, molluscs, arthropods, echinoderms,
hemichordates and chordates (Figure 4.3a). In
some animals, the body cavity is not lined by
mesoderm, instead, the mesoderm is present as
scattered pouches in between the ectoderm and
endoderm. Such a body cavity is called
pseudocoelom and the animals possessing them
are called pseudocoelomates, e.g.,
aschelminthes (Figure 4.3b). The animals in
which the body cavity is absent are called
acoelomates, e.g., platyhelminthes (Figure 4.3c).
Figure 4.3 Diagrammatic sectional view of :
(a) Coelomate (b) Pseudocoelomate
(c) Acoelomate
Those animals in which the developing embryo has a third germinal layer,
mesoderm, in between the ectoderm and endoderm, are called
triploblastic animals (platyhelminthes to chordates, Figure 4.2b).
4.1.5 Segmentation
In some animals, the body is externally and internally divided into
segments with a serial repetition of at least some organs. For example, in
earthworm, the body shows this pattern called metameric segmentation
and the phenomenon is known as metamerism.
4.1.6 Notochord
Notochord is a mesodermally derived rod-like structure formed on the
dorsal side during embryonic development in some animals. Animals with
notochord are called chordates and those animals which do not form this
structure are called non-chordates, e.g., porifera to echinoderms.
4.2 CLASSIFICATION OF ANIMALS
The broad classification of Animalia based on common fundamental
features as mentioned in the preceding sections is given in Figure  4.4.
2024-25
40 BIOLOGY
The important characteristic features of the
different phyla are described.
4.2.1 Phylum – Porifera
Members of this phylum are commonly known
as sponges. They are generally marine and mostly
asymmetrical animals (Figure 4.5). These are
primitive multicellular animals and have cellular
level of organisation. Sponges have a water
transport or canal system. Water enters through
minute pores (ostia) in the body wall into a central
cavity, spongocoel, from where it goes out
through the osculum. This pathway of water
transport is helpful in food gathering, respiratory
exchange and removal of waste. Choanocytes
or collar cells line the spongocoel and the canals.
Digestion is intracellular. The body is supported
by a skeleton made up of spicules or spongin
fibres. Sexes are not separate (hermaphrodite),
i.e., eggs and sperms are produced by the same
individual. Sponges reproduce asexually by
fragmentation and sexually by formation of
gametes. Fertilisation is internal and development
is indirect having a larval stage which is
morphologically distinct from the adult.
*Echinodermata exhibits radial or bilateral symmetry depending on the stage.
Figure 4.4  Broad classification of Kingdom Animalia based on common fundamental features
(a)
(b)
(c)
Figure 4.5 Examples of Porifera : (a) Sycon
(b) Euspongia  (c) Spongilla
2024-25
ANIMAL KINGDOM 41 41
cnidoblasts or cnidocytes (which contain the stinging capsules or
nematocysts) present on the tentacles and the body. Cnidoblasts are used
for anchorage, defense and for the capture of prey (Figure 4.7). Cnidarians
exhibit tissue level of organisation and are diploblastic. They have a central
gastro-vascular cavity with a single opening, mouth on hypostome.
Digestion is extracellular and intracellular. Some of the cnidarians, e.g.,
corals have a skeleton composed of calcium carbonate. Cnidarians exhibit
two basic body forms called polyp and medusa (Figure 4.6). The former
is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the
latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.
Those cnidarians which exist in both forms exhibit alternation of
generation (Metagenesis), i.e., polyps produce medusae asexually and
medusae form the polyps sexually (e.g., Obelia).
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone),
Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).
Figure 4.7
Diagrammatic view of
Cnidoblast
Figure 4.6 Examples of  Coelenterata indicating outline of their body form :
(a) Aurelia (Medusa) (b) Adamsia (Polyp)
(b) (a)
Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia
(Bath sponge).
4.2.2 Phylum – Coelenterata (Cnidaria)
They are aquatic, mostly marine, sessile or free-swimming, radially
symmetrical animals (Figure 4.6). The name cnidaria is derived from the
2024-25