But "the labor we delight in physics pain," and patient labor is but the measure of hoped-for rewards. In this case the realization exceeded all reasonable expectations; for, if he may be counted fortunate who, by chance, finds some soothsaying relic of early life, how much more than fortunate, how exceedingly blest, is the naturalist who is permitted to lift with his own hands from some ancient storehouse of the earth a long-sought-for treasure, and to awake in it, with the chisel's kiss a spirit from the childhood era of the world!
VI. A Critical Period in Organic Evolution
The net result of this fortunate find was to show that the ostracoderms, as had been predicted, were neither vertebrates nor invertebrates, but a class intermediate between the two. They were, in fact, the real missing links in the animal kingdom. The posterior part of the body was membranous and decidedly fish-like in shape; but the contour of the whole animal, especially the head, the nature of the appendages, the eyes and the mode of locomotion, were more like those of the marine scorpions. The gill, or atrial, chamber, and the structure of the dermal skeleton were intermediate in character (Figs. 2 and 6).
But the most important features of all were the long sought for mouth parts, or jaws. They were paired, consisting of four separate jaws, which in chewing, or biting, moved to and from the median line, like the jaws of all known anthropods (Fig. 6, A). They were not unpaired arches moving forward and backward, as they do in all true vertebrates.
To realize the significance of this fact, it must be understood that one of the greatest differences between a vertebrate and an arachnid, or arthropod, is the position and character of the jaws and mouth. In all arthropods and arachnids, the mouth and jaws are primarily located on the same side of the body as the nervous system; food enters the alimentary canal by a passage-way in the floor of the brain; and there may be several pairs of jaw-like legs, which, in chewing, work in a lateral direction to and from the median line. In the adult vertebrates the jaws lie on the opposite side of the body from that on which the brain and nerve cord are located; the food that enters the mouth passes directly into the alimentary canal without going through a passage-way in the floor of the brain; and the jaws are two unpaired arches that work against each other in a forward and backward direction (Fig. 7, A and B).
It is evident that either we are not dealing with the same things in the two classes, or that there has been some change in their relative locations. As a matter of fact, it is part one and part the other; for we have been able to demonstrate: (1) That the nervous system of the
