Introduction

Kingdom Animalia is vast: its members are eukaryotic, multicellular and heterotrophic, and together they span roughly 35 phyla, of which about 11 are considered major phyla. New species are still being discovered constantly, so classification is essential simply to be able to identify, name and assign a proper systematic position to each newly found species — the Animal Kingdom is classified mainly on the basis of closely resembling characteristic features. Almost 99 percent of all animals are invertebrates, lacking a backbone; the remaining animals are vertebrates, possessing one. A more fundamental split, based on the presence or absence of a notochord, separates every animal into two major groups: non-chordates and chordates.