ORIGIN OF TEE NERVOUS SYSTEM 253 



THE ORIGIN OF THE NERVOUS SYSTEM AND ITS 

 APPROPRIATION OF EFFECTORS 



Bt G. H. PARKER 



PEOFESSOB OF ZOOLOGY, HAEVAED UNIVEESITY 



III. Central Nervous Organs 



IN dealing with the differentiation of nervous organs, the earth- 

 worm affords a good example of a simple type of well-centralized 

 nervous system. The central nervous organs in this animal (Fig. 1) 

 consist of a brain or cerebral ganglion situated anteriorly and dorsal to 

 the buccal cavity, right and left oesophageal connectives extending from 

 the brain ventrally to the ventral nerve-cord which stretches as a seg- 

 mented organ from near the anterior end of the worm over its ventral 

 line posteriorly to the tail. The segments in the ventral cord agree in 



Fig. 1. Head of an Eabthwobm in longitudinal Section, b, brain ; m, 

 mouth ; o s oesophagus ; vn, ventral nerve-cord. 



number and position with those of the worm's body and from each seg- 

 ment three pairs of nerves pass out to the integument and muscles of 

 the adjacent region. 



The essential nervous elements of the ventral cord can be made out 

 in transverse sections (Fig. 2). In such sections the integument will 

 be seen to be filled with sense-cells, each of which ends peripherally in a 

 sensory bristle and gives rise centrally, in addition to a few subepithelial 

 processes, to a single nerve-fiber which passes inward between the 

 muscles and enters the ventral ganglion by one of its three nerves; 

 finally this fiber spreads out in the fibrillar substance or neuropile of 

 the ganglion. This cell-body in the integument with its processes 

 including the nerve-fiber constitutes a primary sensory neurone. These 

 neurones usually do not spread beyond the ganglion with which they 

 are directly connected, but in exceptional cases they may extend into 

 the ganglion anterior or posterior to this one. 



In the ventral and lateral portions of each ganglion are numerous 

 large nerve-cells from which coarse processes extend through the neuro- 



