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| Animal Diversity |
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| 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. Among chordates, the major group is that of vertebrates in which the notochord is replaced in the adult stage by a vertebral column. |
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| At present, there are 33 distinct groups or phyla of invertebrates and 4 distinct groups or phyla of chordates which have been identified. However, as new groups are being continuously discovered, the classification of animals in general and invertebrates in particular, is being constantly revised. However, one can recognise nine major phyla. Since they constitute nearly 90% of the known invertebrates, all the chordates are placed in one major phylum. |
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| An evaluation of the present day success of animal groups involves a consideration of their diversity (known number of species) and their abundance (number of individuals). The incredible array of animals is the result of hundreds of million years of organic evolution. |
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| The following is a list of the approximate number of species known in the animal groups regarded as major phyla. |
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 Protozoa 35,000 |
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 Porifera 9,000 |
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 Coelenterates 9,000 |
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 Platyhelminthes 20,000 |
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 Aschelminthes 12,000 |
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 Annelids 15,000 |
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 Arthropods 9,60,000 |
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 Molluscs 50,000 |
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 Echinodermata 7,000 |
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 Chordates 50,000 |
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