More than a million extant species of animals are known, and at least as many more will probably be identified by future generations of biologists. Based largely on anatomical and embryological criteria,animals are grouped into about 35 phyla,the exact number depending on the views of different systematists.most of these phyla consist mainly of aquatic animals. Living as we do on land, our sense of animal diversity is biased in favor of vertebrates, the animals with backbones,which are well represented in terrestrial environments. But vertebrates make up one subphylum within the phylum chordata, less than 5% of all animal species. If we were to sample the animals inhabiting a tidepool (as in the photograph on this page) or a coral reef, we would find ourselves in the realm of invertebrates, the animals without backbones.