NCERT Book Class 11 Biology Chapter 4 Animal Kingdom

ANIMAL KINGDOM 73
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C HAPTER 4
ANIMAL KINGDOM
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,