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Introduction
The animal kingdom includes eukaryotic multicellular organisms which exhibit heterotrophic nutrition. They are commonly known as animals and differ characteristically from the other major groups of plants, in their capacity to be able to move.
Animal Diversity
Among the members of the animal kingdom, about 95% do not posses a backbone (or vertebral column) and hence are described as invertebrates. The remaining animals represent a group called chordates, which posses a solid supporting structure on the dorsal side of the body called notochord, at some stage of their life cycle.
Animal Organisation
Animals exhibit a complex body organisation which shows a very clear evolutionary trend. The complexity of animal organisation is seen with reference to features like cellularity, germ layers in the body wall, symmetry of the body parts and the nature of body cavity.
Animal Classification
The animal kingdom is classified into distinct groups based on the body organisation discussed above. The following scheme provides an outline classification of the animal kingdom into the ten major phyla.
Major Phyla of the Animal Kingdom
There are 10 major phyla in the animal kingdom from Protozoa to Chordata. Of these, phylum Protozoa is now placed in the kingdom Protista.
Phylum Porifera
Primitive multicellular animals with cellular grade of organisation. Free living aquatic, mostly marine, sedentary forms either solitary or colonial.
Phylum Coelenterata
Radially symmetrical, diploblastic multicellular animals with a tissue grade of organisation. Aquatic, freshwater or marine solitary or colonial forms which may be free swimming or sedentary.
Phylum Platyhelminthes
Bilaterally symmetrical, triploblastic true multi-cellular animals with organ grade of organisation. Free living (aquatic or terrestrial) or parasitic (ecto- or endoparasites).
Phylum Aschelminthes
Mostly parasitic (in animals and plants), a few free living called as flukes. Body is long, cylindrical, fusiform (pointed at both the ends).
Phylum Annelida
Free-living, terrestrial or aquatic form (freshwater or marine). Body is long, cylindrical and metamerically segmented.
Phylum Arthropoda
This phylum is the largest in the animal kingdom comprising of more than 75% of the animal species that have been identified.
Phylum Mollusca
Free living aquatic forms (freshwater or marine), some amphibious. Body is soft and unsegmented enclosed in a glandular mantle covered by usually a shell.
Phylum Echinodermata
Free living exclusively marine forms. Adults are radially symmetrical while larvae are bilaterally symmetrical.
Phylum Chordata
Presence of a solid supporting structure on the dorsal side of the body called notochord. Presence of a dorsal hollow, tubular nerve cords.
Summary
All multicellular animals are now placed in the kingdom of Metazoa. The multicellular animals show a very specific body organisation which ranges from cellular grade to organ system grade.