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In the farther development of this division, the ganglia about the oesophagus increase in number and size, and become the controlling nervous centres. Of this character are the nervous systems of the cuttlefish and of the Aeolidae or Nudibranchiates. The head thus begins to take its final relation to the nervous system. It becomes the centre of the special senses, and the chief source of stimuli. This accumulation first takes place about the mouth, the leading organ of nutrition. As the attainment of food is a primary necessity, this effort is supported by all the senses. In the vertebrates, the mouth, though it is no longer surrounded by the great ganglia, is still supported by the sense of taste, touch, smell, and less directly, by sight and hearing. While the higher life rises out of the lower, it is united to it by the old bond of service.

In the lower Articulates, as larvae, many proximately equal ganglia corresponding to the several divisions of the body are united to each other by a longitudinal cord. The ganglia of the head only slightly predominate. To this bilateral symmetry the nervous system begins to conform, and the nervous cord and ganglia become double. In the higher Articulates, as insects, the ganglia show more subordination, and are frequently, as in the bee and the fly, gathered into the thorax and the head, the two seats of action and perception. It is here that instinctive life finds its highest development, yet, as in the white-ant, the ganglia are often markedly subdivided.

In the Vertebrates, cephilization becomes far more complete. If we start with brainless-fish, and close the series with man, we have traveled from a uniform spinal cord to a complete centralization of life, and subordination of it in all its forms to consciousness.

When we observe the great complexity and wonderful harmony of action in the nervous system of man, and also