The fibers in a nerve have no necessary functional relations one with another, but are brought together chiefly by convenience of passage. They are characteristic of those animals in which sense organs and muscles have become well differentiated and widely separated from the central organs, and are not to be confused with elongated bundles of nervous elements such as are to be met with in some cœlenterates and many echinoderms, for though these may represent early steps in the evolution of nerves, they still retain so many evidences of functional interrelation among their elements that they are to be classed rather with nervous nets than with nerves.
The differentiation of nerves as thus defined implies an increased interrelation of neurones in the central apparatus as compared with the condition in the more primitive nervous net. The nature of this growing interrelation has been well expressed by Sherrington (1906) in his principle of the common path. This principle implies that each sense organ may be connected through the central organ with every effector and conversely any effector may receive through the central organ impulses from any sense-organ. In consequence the central organ must contain many common paths which are momentarily used, now for this, now for that combination of particular receptors and effectors. This condition without doubt obtains in earthworms as it does in higher animals, and is a feature that can hardly be said to exist in the nervous nets of the cœlenterates.
It is also probable that the nervous mechanism in cœlenterates differs from that in the earthworm in its capacity as a nervous transmitter. Attention has already been called to the fact that transmission in the nervous net of a cœlenterate may occur in almost any direction and that in the central nervous organs of vertebrates it is very definitely limited and may in fact flow in only one of two apparently possible directions. So definite a restriction can not be asserted for the earthworm but, as Norman (1900) has shown, significant differences do obtain. If an earthworm that is creeping forward over a smooth surface is suddenly cut in two near the middle, the anterior portion will move onward without much disturbance whereas the posterior part will wriggle as though in convulsions. This reaction, which can be repeatedly obtained on even fragments of worms, shows that a single cut involves a stimulation which in a posterior direction gives rise to a wholly different form of response to what it does anteriorly; in other words, transmission in the nerve-cord of the worm is specialized as compared with transmission in the nervous net of the cœlenterate.
There is good reason to believe that the cerebral ganglion or brain of the earthworm is in a measure degenerate. Certainly if we turn to such an annelid as Nereis we find in place of the small mass of ganglionic cells and fibers that represent the brain in the earthworm a much
