Answer:
Vertebrates - animals with a backbone.
The animals have been divided into two groups based on the presence or absence of a backbone
EXAMPLES
Fish, reptiles, birds, amphibians and mammals are different sub-groups of vertebrates - they all have internal skeletons and backbones
EXPLANATION
Mammals differ from other vertebrate animals in that their young are nourished with milk from special mammary glands of the mother.
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