Take any articulate animal, for example, a shrimp, a centiped, or a beetle. Cut it across the body, and look at the end (Fig. 26). We see a ring of bone (chitin) inclosing all the organs (nervous system _n_, blood system _b_, and visceral system _v_), and a pair of jointed appendages, perhaps legs, on each side. Now imagine these parts repeated in a linear series. The rings repeated make a hollow, jointed tube or barrel, the appendages repeated make a continuous row of appendages on each side. Now this is exactly what we actually find. The whole articulate skeleton is ideally made up of a series of such repeated rings and appendages, modified according to the position in the series, and the uses to which they are put. And then the whole articulate department is made up of such articulate animals again modified according to place in the scale of articulates. The modification in the lower forms is slight, and therefore the identity of the repeated parts is obvious; but as we go up the scale, and the number and complexity of the functions increase, the adaptive modification becomes greater and greater, until finally it so obscures the essential identity, that it requires the most extensive comparison in the taxonomic series and in the ontogenic series, to pick up the intermediate links and establish the fact of common origin. In a word, whether they so originated or not, it is certain that the structure of articulate animals is exactly such as would be the case if all these animals were genetically connected, and came originally from a primal form something like one of the lower crustaceans, or, perhaps, a marine worm. [Illustration: FIG. 27.--Shrimp (Palæmonetes vulgaris).] [Illustration: FIG. 28.--External anatomy of the lobster (after Kingsley).]