deserve the name of elementary nervous organs.  Its many activities are simple responses to stimuli that reach it from without, and its reactions to such stimuli are called reflex processes.  Should the light become too strong, it will slowly crawl to a shady place; should the water in which it lives become warmer, it responds by displaying greater activity.  It exhibits, in a word, the property of irritability—­that is, simply the power of receiving and reacting to stimuli; and being only a single cell this property is held in common by all of its parts.

We come next to a simple many-celled animal like the polyp Hydra, or a jellyfish.  In such an animal the body is composed of numerous cells which are not all alike either in their make-up or in their functions.  Some of them are concerned primarily with digestion, others with protection, while still others are exempt from these tasks and as sense-cells they devote all their energies to the reception of stimuli from without, or, beneath the outer sheet of cells of the two-layered body, they conduct impulses from one part of the animal to another, and thus serve as coordinating members of the community.  For the first time, then, a nervous system as such is set apart and specialized to devote itself to the two tasks of sensation and coordination that are performed by nervous systems throughout the entire range of organisms higher in the scale.  But the activities of Hydra, like those of Amoeba, are reflex and mechanical,—­that is to say, given similar stimuli and similar physiological states of the animal, the reactions will be the same.  A little water-crustacean like Daphnia may swim against the tentacles of Hydra; it is stung to death by the minute cell-batteries which the animal possesses, and then in a mechanical way the tentacles transport the food to the mouth, through which it is passed inward to the digestive cavity.  There is nothing that can be called “mentality” throughout these processes, but the series of activities is much more complex than in Amoeba because the whole organism is constructed more elaborately, and because the special and peculiar mechanism directing the activities has advanced to a far higher condition.

Passing to the jointed animals like worms and insects, we find nervous mechanisms that are still more intricate, and with their advance in structural respects there is a corresponding and correlated progress in their functions.  Because the whole organism has developed more highly differentiated groups of organs to perform the several biological tasks, such as eating and respiring and moving, it is necessary for the nervous structures concerned with the direction of these actions to become more efficient.  An earthworm avoids the light of day and digs its burrow and seeks its food by wonderfully cooerdinated activities of its muscles and other parts, which are controlled by a double chain of ganglia along