VERTEBRATES: BACKBONE AND BRAIN 103 



sary 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. Kem ember 

 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 an- 

 terior 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-cesophageal 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 begin- 

 ning 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 mus- 

 cular 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 sys- 

 tem, serves as a very simple but serviceable circulatory 

 system. But in the annelid and all higher forms a 



