As we ascend to the higher members of the Articulate type, the specialization of function increases, but not sufficiently to obliterate all the individuality of the segments. The tendency in the Arthropods is toward a continuous welding of the segments. Thus, in the Crustaceans we find twenty or twenty-one segments compacted into three body sections. In the higher families these three are reduced to two; and, in the highest crabs, the abdominal section becomes so reduced that all the body functions are performed by a single section.
At the same time the chitinous armor of the segments becomes a continuous cortical armor; and the chain of nerve-ganglia is reduced to a single large ganglion, which supplies nerve-fibers to all the body.
In this manner the Crustacean reaches its most specialized condition, but only by a loss of its longitudinal extension, and a return to the compact, slow-moving, armored type of animal. Thus, its highest evolution has produced an organization antagonistic to any advanced degree of development.
Of the air-breathing Arthropods the Arachnidæ seem to be closely allied to the Crustaceans. They have the same compactness of organization; are not, as a rule, adapted to swift motion; and are inferior to the high Crustaceans from the fact that their tendency is toward development of the abdomen, instead of the head section, as in the crabs.
The insects and Myriapods do not possess the relations to the Crustaceans shown by the spiders and their allies. On the contrary, their larval form seems to indicate a separate line of descent from the worms. Different as insects and Myriapods are in their mature forms, they appear to have had a common origin—the embryo of the Myriapods passing through a stage that resembles the larval stage of insects. They seem, indeed, to have developed from their primitive form in opposite directions, the segments being multiplied in the Myriapods and reduced in the insects. The embryo of the Myriapod has at first but three pairs of legs. At a later period posterior legs bud successively from the new-formed segments. There seems to be no fixed limit to the number of segments, since they continue to increase throughout life. And their individuality is strongly declared, each segment possessing the organs necessary to a separate life, as a nerve ganglion and fibers, breathing organs, muscles, an intestine, and a vascular space. These organs, if redeveloped from their partly aborted condition, might well suffice to sustain life in separate animals.
Even in the highest of the Arthropods—the insects—this hereditary individual organization of the segments continues manifest; these organically independent members of the society stubbornly resist the cession of their primitive functions, only partly yielding to the common needs, and thus retaining a generalization of function which is repressive of any high development.
The animal best suited for progression is one which has all its