I have once or twice mentioned the words Mollusc and Crustacean, to which the reader unfamiliar with the language of Natural History will have attached but vague ideas; and although I wanted to explain these, and convey a distinct conception of the general facts of Classification, it would have then been too great an interruption. So I will here make an opportunity, and finish the chapter with an indication of the five Types, or plans of structure, under one of which every animal is classed. Without being versed in science, you discern at once whether the book before you is mathematical, physical, chemical, botanical, or physiological. In like manner, without being versed in Natural History, you ought to know whether the animal before you belongs to the Vertebrata, Mollusca, Articulata, Radiata, or Protozoa. [Illustration: Fig. 17. MALE TRITON, or WATER-NEWT.] A glance at the contents of our glass vases will yield us samples of each of these five divisions of the animal kingdom. We begin with this Triton. It is a representative of the VERTEBRATE division, or sub-kingdom. You have merely to remember that it possesses a backbone and an internal skeleton, and you will at once recognize the cardinal character which makes this Triton range under the same general head as men, elephants, whales, birds, reptiles, or fishes. All these, in spite of their manifold differences, have this one character in common:—they are all backboned; they have all an internal skeleton; they are all formed according to one general type. In all vertebrate animals the skeleton is found to be identical in plan. Every bone in the body of a triton has its corresponding bone in the body of a man, or of a mouse; and every bone preserves the same connection with other bones, no matter how unlike may be the various limbs in which we detect its presence. Thus, widely as the arm of a man differs from the fin of a whale, or the wing of a bird, or the wing of a bat, or the leg of a horse, the same number of bones, and the same connections of the bones, are found in each. A fin is one modified form of the typical limb; an arm is another; a wing another. That which is true of the limbs, is also true of all the organs; and it is on this ground that we speak of the vertebrate type. From fish to man one common plan of structure prevails; and the presence of a backbone is the index by which to recognize this plan.