An invertebrate is any animal without a backbone. Invertebrates make up 95% of all species of animals on the earth, and the variety of invertebrates is enormous. Scientists group or “classify” all of these different types of animals into broad categories called phyla, on the basis of their patterns of symmetry and on the basis of their overall body plan. There are 5 particularly important invertebrate phyla (and about another 23 less important phyla). The major invertebrate groups are classified as: Phylum Cnidaria, Phylum Annelida, Phylum Mollusca, Phylum Arthropoda, Phylum Echinodermata.