Animal Kingdom

Divided into three great series, the Protozoa, Mesozoa, and Metazoa. The last has these sub-kingdoms: Porifer a, Cælentera, Vermes, Arthropod a, Mollusca, Molluscoid a, Echinoderm at a, Tunic at a (eight groups known as Invertebrates), and Vertebrata. The Invertebrates are often considered by themselves, and divided into the Acælomata and the Cœlomata. Some authors establish a group Chord at a, including the Vertebrates and Tunicates. Cuvier, followed by Aga iz, recognized four types: the Radiates, Articulates, Molluses and Vertebrates, but this division is more artificial than natural, and is now no longer retained. The group Vermes is composed of several phyla, each of which is separated from the others by as strong characters as divide most of the sub-kingdoms. Ali these groups were fully evolved when the earliest strata, which show organic remains, were laid down. As the previous strata are sometimes termed azoic, it simply means that they contain no fo ils, not that at that period of geological history there existed no animals. The earliest ancestors of each sub-kingdom almost certainly po e ed no firm skeleton, and they were besides of microscopic microscopic size. size. The sub kingdoms, as is true of any group of Taxonomy, are not to be considered as following each other in a series, although it is po ible to place them in such a sequence that the simplest or "lowest" forms stand first enumerated and the most complex or "highest" last. They were probably simultaneously evolved by simple phylogenetic differentiation, which principle operated at once on the earliest differentiated groups to produce fu ther differentiation. In a certain sense all animals are equally old, but doubtle swimming forms precedəd land forms, and flying animals were last evolved.