We must remember, too, the great age of this group.  They are present in highly modified forms in the very oldest palaeozoic strata, and probably therefore came into existence as the first traces of continental areas were beginning to rise above the primeval ocean.  They are literally “older than the hills.”  They were exposed to a host of rapidly changing conditions, very different in different areas.  This prepares us for the fact that the worms represent a stage in animal life corresponding fairly well to the Tower of Babel in biblical history.  The animal kingdom seems almost to explode into a host of fragments.  Our genealogical tree fairly bristles with branches, but the branches do not seem to form any regular whorls or spirals.  Few of them have developed into more than feeble growths.  They now contain generally but few species.  Many of them are largely or entirely parasitic, and in connection with this mode of life have undergone modifications and degeneration which make it exceedingly difficult to decipher their descent or relationships.

Four of these branches have reached great prominence in numbers and importance.  One or two others were formerly equally numerous and have since become almost extinct; so the brachiopoda, which have been almost entirely replaced by mollusks.  The same may very possibly be true of others.  For of the amount of extinction of larger groups we have generally but an exceedingly faint conception.  Indeed in this respect the worms have been well compared to the relics which fill the shelves of one of our grandmother’s china-closets.

The four great branches are the echinoderms, mollusks, articulates, and vertebrates.  The echinoderms, including starfishes, sea-urchins, and others straggled early from the great army.  We know as yet almost nothing of their history; when deciphered it will be as strange as any romance.  The vertebrates are of course the most important line, as including the ancestors of man.  But we must take a little glance at mollusks, including our clams, snails, and cuttle-fishes; and at the articulates, including annelids and culminating in insects.  The molluscan and articulate lines, though divergent, are of great importance to us as throwing a certain amount of light on vertebrate development; and still more as showing how a certain line of development may seem, and at first really be, advantageous, and still lead to degeneration, or at best to but partial success.

When we compare the forms which represent fairly well the direction of development of these three lines, a snail or a clam with an insect and a fish, we find clearly, I think, that the fundamental anatomical difference lies in the skeleton; and that this resulted from, and almost irrevocably fixed, certain habits of life.