Classification of Animal Kingdom. The first seven groups of the above classification are back-boneless animals or Invertebrata; the eighth division alone comprising the animals with a vertebra or backbone. =Characters of the Several Phyla.--= In the first group are placed those animals which, when living, consist of only one cell, or a series of similar cells, but where the cells were never combined to form tissues having special functions, as in the higher groups. PROTOZOA.-- The _Amoeba_ of freshwater ponds is an example of such, but owing to its skin or cortex being soft, and its consequent inability to be preserved, it does not concern us here. There are, however, certain marine animals of this simple type of the Protozoa which secrete carbonate of lime to form a chambered shell (Foraminifera); or silica to form a netted and concentrically coated shell held together with radial rods (Radiolaria); and both of these types are found abundantly as fossils. They are mainly microscopic, except in the case of the nummulites and a few other kinds of foraminifera, which are occasionally as large as a crown piece. COELENTERATA.-- The second group, the Coelenterata, shows a decided advance in organisation, for the body is multicellular, and provided with a body-cavity which serves for circulation and digestion. The important divisions of this group, in which the organisms have hard parts capable of being fossilised, are the limy and flinty Sponges, the Corals, and allied groups, as well as the delicate Graptolites which often cover the surface of the older slates with their serrated, linear forms, resembling pieces of fret-saws. ECHINODERMATA.-- The third group, Echinodermata, comprises the Sea-lilies (Crinoids), Starfishes and Sea-urchins, besides a few other less important types; and all these mentioned are found living at the present day. Their bodies are arranged in a radial manner, the skin being strengthened by spicules and hardened by limy deposits ultimately forming plates. They have a digestive canal and a circulatory system, and are thus one remove higher than the preceding group. VERMES.-- The fourth group, Vermes (Worms), are animals with a bilateral or two-sided body, which is sometimes divided into segments, but without jointed appendages. Those which concern the student of fossils are the tube-making worms, the errant or wandering worms which form casts like the lob-worm, and the burrowing kinds whose crypts or dwellings become filled with solid material derived from the surrounding mud. MOLLUSCOIDEA.--