But a muscular fibril contracts only under the stimulus of a nervous impulse.  More nerve-cells are necessary to control these more numerous muscular fibrils.  The animal now moves with one end foremost, and that end first comes in contact with food, hindrances, or injurious surroundings.  Here the sensory cells of feeling and their nerve fibrils multiply.  Remember that these neuro-epithelial sensory cells are suited to respond not merely to pressure, but to a variety of the stimuli, chemical, molecular, and of vibration, which excite our organs of smell, taste, and hearing.  Such organs and the directive eyes appear mainly at this anterior end.  But a ganglion cell sends an impulse to a muscle because it has received one along a sensory nerve from one or more of these sensory cells.  Hence the ganglion cells will increase in number.  The old cobweb-like plexus condenses into a little knot, the supra-oesophageal ganglion.  This ganglion cannot do much, if any, thinking; it is rather a steering organ to control the muscles and guide the animal.  It is the servant of the locomotive system.  Yet it is the beginning of the brain of higher animals, and probably still persists as an infinitesimal portion of our human brain.  And all this is the prophecy of a head soon to be developed.  An excretory system has appeared to carry off the waste of the muscles and nerves.

In the schematic worm and annelid the reproductive system is simpler, though perhaps equally effective.  It takes the excess of nutriment of the body.  The muscular system has taken the form of a sack composed of longitudinal and transverse fibres.  The perivisceral cavity, formed perhaps by cutting off and enlarging the lateral pouches of the turbellarian digestive system, serves as a very simple but serviceable circulatory system.  But in the annelid and all higher forms a special system of tubes has developed to carry the nutriment, and usually oxygen also, needed to keep up the combustion required to furnish the energy in these active organs.  The digestive system has attained its definite form with the appearance of an anal opening and the accompanying division of labor and differentiation into fore-, mid-, and hind-intestine.

The digestive and reproductive systems have thus nearly attained their final form.  From the higher worms upward the digestive system will improve greatly.  Its lining will fold and flex and vastly increase the digestive and absorptive surfaces.  The layer of cells which now secrete the digestive fluids will in part be replaced by massive glands.  Far better means of grasping food than the horny teeth of annelids will yet appear.  But all these changes are inconsiderable compared with the vast advance made by the muscular and nervous systems.  Reproduction and digestion are losing their supremacy in the animal body.  Their advance and improvement will require but little further attention.