THE HIGHER ORGANISMS 151 



Thus the interval between the protozoan, whose sub- 

 stance is irritable, and the metazoan with specialized 

 receptive or sensory cells, nervous or controlling cells, 

 and communicating fibres, is quickly spanned and the 

 foundation of the nervous system laid. 



It is important to note that the purpose of the primi- 

 tive nervous system seems to be to correlate exter- 

 nal impressions with movements directed toward the 

 capture of food or escape from enemies. When such 

 movements embrace the cooperation of various members, 

 they can only be successfully performed when appropriate 

 impulses are sent to them. It is, therefore, imperative 

 that some portion of the developing nervous system 

 develop disproportionately to the rest and become the 

 centralizing and coordinating organ or brain. The 

 brain is not only the centre from which impulses proceed, 

 but also that in which they are received, analyzed, co- 

 ordinated, and utilized. The analysis and utilization 

 of impressions is to us synonymous with consciousness, 

 but it is only after the coordinating centre arrives at a 

 certain degree of complexity and becomes the seat of 

 multifarious impressions and responses that anything 

 meriting the term consciousness can be attributed to the 

 animal. 



The nervous system of an unsegmented worm con- 

 sists of a brain in the form of an aggregation of nerve 

 cells at the anterior end. From it two lateral nerve 

 trunks extend to the tail, becoming smaller as they 

 recede from the brain, evidently through the loss of 

 fibres that are given off to the muscular tissue in due 

 course. 



The segmented worms differ in that there is a pair of 

 nervous ganglia for each segment,, connecting with one 

 another and with those of the adjoining segments by 

 delicate bundles of fibres. In addition to these ganglia 

 nerve cells are found here and there. In such animals 

 each segment may be said to possess its own brains, 

 though the anterior brains or ganglia, being the largest 



