Of Articulates we find only two classes, Worms and Crustacea.  Insects there were none,—­for, as we have seen, this early world was wholly marine.  There is little to be said of the Worms, for their soft bodies, unprotected by any hard covering, could hardly be preserved; but, like the marine Worms of our own times, they were in the habit of constructing envelopes for themselves, built of sand, or sometimes from a secretion of their own bodies, and these cases we find in the earliest deposits, giving us assurance that the Worms were represented there.  I should add, however, that many impressions described as produced by Worms are more likely to have been the tracks of Crustacea.

But by far the most characteristic class of Articulates in ancient times were the Crustaceans.  The Trilobites stand in the same relation to the modern Crustacea as the Crinoids do to the modern Echinoderms.  They were then the sole representatives of the class, and the variety and richness of the type are most extraordinary.  They were of nearly equal breadth for the whole length of the body, and rounded at the two ends, so as to form an oval outline.  To give any adequate idea of the number and variety of species would fill a volume, but I may enumerate some of the more striking differences:  as, for instance, the greater or less prominence of the anterior shield,—­the preponderance of the posterior end in some, while in others the two ends are nearly equal,—­the presence or absence of prongs on the shield and of spines along the sides of the body,—­appendages on the head in some species, of which others are entirely destitute,—­and the smooth outline of some, while in others the surface is broken by a variety of external ornamentation.  Such are a few of the more prominent differences among them.  But the general structural features are the same in all.  The middle region of the body is always divided in uniform rings, lobed in the middle so as to make a ridge along the back with a slight depression on either side of it.  It is from this three-lobed division that they receive their name.  The subjoined wood-cut represents a characteristic Silurian Trilobite.

[Illustration]

There is no group more prominent in the earliest creations than this one of the Trilobites, and so exclusively do they belong to them, that, as we shall see, in proportion as the later representatives of the class come in, these old-world Crustaceans drop out of the ranks, fall behind, as it were, in the long procession of animals, and are left in the ancient deposits.  Even in the Carboniferous period but few are to be found:  they had their day in the Silurian and Devonian ages.  In consequence of their solid exterior, the preservation of these animals is very complete; and their attitudes are often so natural, and the condition of all their parts so perfect, that one would say they had died yesterday rather than countless centuries ago.