ANIMAL KINGDOM | MCAT Notes | UCAT notes | COMPLETE CLASS NOTES , Tricks & PYQ All Concp
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Basis of Classification , Classification of Animal , Levels of Organisation , Diploblastic and Triploblastic
Organisation , Coelom , CLASSIFICATION OF ANIMALS , All phylum covers here with example , long question .
ANIMAL KINGDOM complete class module with best explanation, examples and
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4.1 Basis of When you look around, you will observe different animals with different
Classification structures and forms. As over a million species of animals have been
described till now, the need for classification becomes all the more
4.2 Classification of
important. The classification also helps in assigning a systematic position
Animals to newly described species.
4.1 B ASIS 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,
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ANIMAL KINGDOM
ANIMAL KINGDOM
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
Figure 4.1 (a) Radial symmetry
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. Figure 4.1 (b) Bilateral symmetry
Coelenterates, ctenophores and echinoderms
have this kind of body plan (Figure 4.1a).
Animals like annelids, arthropods, etc., where Ectoderm
Mesoglea
the body can be divided into identical left and Endoderm
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
Mesoderm
an internal endoderm, are called diploblastic (a) (b)
animals, e.g., coelenterates. An undifferentiated
layer, mesoglea, is present in between the Figure 4.2 Showing germinal layers :
ectoderm and the endoderm (Figure 4.2a). (a) Diploblastic (b) Triploblastic
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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).
4.1.4 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
Figure 4.3 Diagrammatic sectional view of : which the body cavity is absent are called
(a) Coelomate (b) Pseudocoelomate acoelomates, e.g., platyhelminthes (Figure 4.3c).
(c) Acoelomate
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.
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