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Chapter Thirty-Two: An Overview of Animal Diversity Preface Biologists have discovered 1.3 million ?extant? (living) species of animals with estimates running of the actual number of species running much higher. Phylogeny: The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms. Section 32.1 Animals are multi-cellular (have many cells), heterotrophic (capable of using only organic materials as a source of food), eukaryotes (organism with bound cell organelles and nucleus) that develop from embryonic layers. Animals have several distinguishing characteristics:
