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Biology LibreTexts

28.0: Introduction

Figure 28.0.1: Nearly 97 percent of animal species are invertebrates, including this sea star (Astropecten articulates) common to the eastern and southern coasts of the United States (credit: modification of work by Mark Walz)

A brief look at any magazine pertaining to our natural world, such as National Geographic, would show a rich variety of vertebrates, especially mammals and birds. To most people, these are the animals that attract our attention. Concentrating on vertebrates, however, gives us a rather biased and limited view of biodiversity, because it ignores nearly 97 percent of the animal kingdom, namely the invertebrates. Invertebrate animals are those without a cranium and defined vertebral column or spine. In addition to lacking a spine, most invertebrates also lack an endoskeleton. A large number of invertebrates are aquatic animals, and scientific research suggests that many of the world’s species are aquatic invertebrates that have not yet been documented.

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