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1 FollowerAnimals have cells with a nucleus and no cell wall, and they usually consume organic material, breathe oxygen, and move with muscles. Animals can reproduce sexually, and their embryos develop from a hollow ball of cells called a blastula. Animals are very diverse, with millions of living species that range in size, shape, and habitat. Animals are studied by zoology and ethology, and their evolutionary history dates back to the Precambrian era.