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Animals are often described as vertebrates (animals with backbones) or invertebrates (animals without backbones). Although this difference is very important, these terms are not always used in the scientific classification of animals. Scientists have made a large number of observations of living things. The presence or absence of a backbone is only one of the observations. 238 PEARSON science NEW SOUTH WALES Using the similarities and differences these observations reveal, taxonomists have grouped all the known members of the animal kingdom into nine smaller groups. Each of the smaller groups is known as phylum (plural phyla ). The nine phyla are shown in Figure 6.2.2. Poriferans Cnidarians Echinoderms Annelids Animals Nematodes Platyhelminths Molluscs Arthropods Chordates Poriferans Figure 6.2.2 Eight of the animal phyla in this key do not have backbones. The phylum Chordates includes all the animals with backbones. Poriferans are commonly called sponges. A typical sponge is shown in Figure 6.2.3. Poriferans live in the water and most are found in marine environments. They are full of holes (pores) through which water passes, carrying their food. They filter the food out of the water, and for this reason they are known as filter feeders. The wastes along with water are pushed out through an opening at the top of the sponge. Using the similarities and differences these observations reveal, taxonomists have grouped all the known members of the animal kingdom into nine smaller groups. Each of the smaller groups is known as phylum (plural phyla ). The nine phyla