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| Introduction |
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| Animals can be broadly distinguished into two major groups the nonchordates (or invertebrates) and the chordates.Invertebrates account for more than 95% of the animal species that have been identified. These are animals which do not have a vertebral column or a back bone. They are distributed into nine major phyla from protozoans to echinoderms (see 2.5 for details). The chordates are animals which have a notochord on the dorsal side of the body. Most of the chordates can be described as vertebrates since the notochord in these animals is restricted to the embryonic stage and in the adults it becomes replaced by a vertebral column. |
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| Nonchordates and Chordates |
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| A very distinct evolutionary trend in the general organisation of animals can be noticed as one proceeds from protozoans to chordates. From an acellular level of organisation to cellular level and then tissue level, organ level and organ system level of organization can be recognised. In addition, one can recognise the gradual appearance of distinct features such as symmetry, body wall and body cavity. There are further improvements in the form of appearance of segmentation, body divisions and even formation of a distinct head (cephalistion). The culmination of all these evolutionary advances, is seen in the mammals which represent the most recent group of animals to have evolved and at the same time the most advanced group of animals. |
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| In order to gain an insight into the evolutionary advances among animals, it become necessary to study the salient features of representative animals from the group of invertebrates and vertebrates. |
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