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Animal kingdom
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Point to RememberBasis of Classification :
Animals are classified on the basis of following few fundamental features—
1. Levels of Organisation :
(1) Cellular level : Cells are arranged as loose cell aggregates, e.g.,sponges.
(2) Tissue level : The cells performing the same function are arranged into
tissues, e.g., Coelenterates.
(3) Organ level : Tissues are grouped together to form organs, each
specialised for a particular function. e.g., platyhelminthes.
(4) Organ system level : organs are associated to form functional systems
e.g., Annelids, Arthropods, Molluscs, Echinoderms and Chordates.
Example : Circulatory System.
Open type : Blood pumped out through heart. Not confined to blood vessels.
Cells and tissues are directly bathed in it.
Closed types : Blood is circulated through blood vessels (arteries,veins and
capillaries)
2. Symmetry :
Asymmetrical : Cannot be divided into equal halves through median plane
e.g., Sponges.
Radial symmetry : Any plane passing through central axis can divide organism
into identical halves. e.g., coelentrates,Ctenophores and echinoderms.
Bilateral symmetry : Only one plane can divide the organism into two identical
left and right halves e.g., Annelids and Arthropods.
3. Germinal Layers :
Diploblastic : Cells arranged in two embryonic layers i.e., external ectoderm
and internal endoderm. (Mesoglea may be present in between ectoderm and
endoderm) e.g., porifers and Coelentrates. (Cnidarians)
Triploblastic : Three layers present in developing embryo i.e., ectoderm,
mesoderm and endoderm. e.g., Platyhelminthes to Chordates.
Animal Kingdom 33
, 4. Coelom (Body cavity which is lined by mesoderm)
Coelomates : Have coelom e.g., Annelids, Arthropods, molluscs, Echinoderms,
Chordates etc.
Pseudocoelomates : No true coelem as mesoderm is present in scattered
pouches between ectoderm and endoderm. e.,g., Aschelminthes.
Acoelomates : Body cavity is absent e.g., Platyhelminthes.
5. Segmentation (A) True Metamerism : Found Annelida,Arthropoda,
Chordata :
Segmentation is external as well as a internal in Annelids.
Segmentation is external in Arthropods.
Segmentation is internal in chordates.
Metamerism : If body is externally and internally divided into segments
(metameres) with serial repetition of atleast some organs, then phenomenon is
called metamerism e.g., Earthworm. (B) Pseudometamerism : Found in
tapeworm. The proglottids (segments of tapeworm) budded off from neck
not embryonic in origin.
6. Notochord :
Rod-like structure formed during embryonic development on the dorsal side.
It is mesodermally derived e.g., Chordates.
Non-chordates do not have notochord .e.g., porifera to echinoderms.
Phylum Porifera :
Also called sponges.
Are usually marine and asymmetrical.
Have cellular level of organisation and diploblastic animals.
Food gathering, respiratory exchange and removal of wastes occurs through
water canal system. Digestion intracellular.
Ostia (minute pores on body), spongocoel (body cavity) and osculum help
in water transport. They are lined by choanocytes (collar cells).
Body wall has skeleton of spicules or spongin fibres.
Animals are hermaphrodite. Fertilisation internal. Development is indirect
(i.e., has a larval stage distinct from adult stage) e.g., Sycon, Euspongia.
Spongilla (Fresh water sponge)
Phylum Coelenterata :
Also called Cnidarians.
Are usually marine and radially symmetrical.
Sessile or free swimming.
34 Biology Class - 11
, Animal Kingdom 35
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