Page 7 - e-book Animal
P. 7
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Invertebrates
The first animals to evolve, as far back as a billion
years ago, invertebrates are characterized by their lack of
backbones and internal skeletons as well as their relatively
simple anatomy and behavior, at least compared with those
of most vertebrates. Today, invertebrates account for a
whopping 97 percent of all animal species, a widely varied
group that includes insects, worms, arthropods, sponges,
mollusks, octopuses, and countless other families.
