The animal kingdom consists, first, of a sub-kingdom of animals which possess a spinal column, or backbone, and which are known as vertebrate animals. Such are all beasts, birds, reptiles, and fishes. There are also a variety of remotely allied marine organisms known as tunicates, sea-squirts, or ascidians (Fig. 2). There is, further, an immense group of arthropods, consisting of all insects, crab-like creatures, hundred-legs and their allies, with spiders, scorpions, tics and mites. We have also the sub-kingdom of shell-fish or molluscs, including cuttle-fishes, snails, whelks, limpets, the oyster, and a multitude of allied forms. A multitudinous sub-kingdom of worms also exists, as well as another of star-fishes and their congeners. There is yet another of zoophytes, or polyps, and another of sponges, and, finally, we have a sub-kingdom of minute creatures, or animalculæ, of very varied forms, which may make up the sub-kingdom of _Protozoa_, consisting of animals which are mostly unicellular. [Illustration: FIG. 2. A TUNICATE (_Ascidia_).] Multitudinous and varied as are the creatures which compose this immense organic world, they nevertheless exhibit a very remarkable uniformity of composition in their essential structure. Every living creature from a man to a mushroom, or even to the smallest animalcule or unicellular plant is always partly fluid, but never entirely so. Every living creature also consists in part (and that part is the most active living part) of a soft, viscid, transparent, colorless substance, termed protoplasm, which can be resolved into the four elements, oxygen, hydrogen, nitrogen and carbon. Besides these four elements, living organisms commonly contain sulphur, phosphorus, chlorine, potassium, sodium, calcium, magnesium and iron. In the fact that living creatures always consist of the four elements, oxygen, hydrogen, nitrogen and carbon, we have a fundamental character whereby the organic and inorganic (or non-living) worlds are to be distinguished, for as we have seen, inorganic bodies, instead of being thus uniformly constituted, may consist of the most diverse elements and sometimes of but two or even of only one. Again, many minerals, such as crystals, are bounded by plain surfaces, and, with very few exceptions (spathic and hematite iron and dolomite are such exceptions) none are bounded by curved lines and surfaces, while living organisms are bounded by such lines and surfaces. Yet, again, if a crystal be cut through, its internal structure will be seen to be similar throughout. But if the body of any living creature be divided, it will, at the very least, be seen to consist of a variety of minute distinct particles, called "granules," variously distributed throughout its interior.