130 University of California Publications in Zoology [VOL. 18 



more or less distant parts of the ganglion and to interpolate them- 

 selves between the sensory and motor elements. Many of these are 

 homolateral and some are contralateral. The greater number of these 

 association cells are intraganglionic, i.e., never leaving the segment; 

 but a few in the anterior and posterior groups send processes into the 

 next ganglion and so connect up the ganglia segment to segment. 



The most interesting feature is that in this nervous system there 

 are no long nerve tracts, the giant fibers excepted. Impulses that 

 run the length of the cord must find their way over a complex route 

 and be necessarily slow. We have then a nervous system made up of 

 many short units. Each ganglion is a complete relay station capable 

 of receiving sensory and giving out the motor impulses necessary for 

 the functions of each particular segment. The only connections 

 between the succeeding segments are association fibers in the nerve 

 cord and a few motor fibers which Dechant (1906) shows. These 

 motor fibers take their origin from a nerve arising from the posterior 

 root and pass laterally around the muscular wall near the interseg- 

 mental furrow and at intervals give off five branches which pass into 

 the segment behind. Without these two connections, one in the cord 

 and one peripheral, there would be no nervous connection from seg- 

 ment to segment of the worm. 



Friedlander (1894) laid particular emphasis on the "pull" of one 

 segment on the succeeding ones and that coordination was accom- 

 plished even though the nerve cord were cut. The experiment of 

 cutting a worm in two and attaching a string to each part resulting 

 in coordinated movements indicates that pull certainly does play an 

 important part. Undoubtedly the tension or stretching stimulates 

 the nerve and starts the reflex movement. The succeeding movements 

 then are due to both pull and nerve impulse. If part is etherized, it 

 ceases contractions although it responds to direct stimulus. The nerve 

 reflex has been broken. Again, if tension be eliminated by pinning 

 experiments, coordinated movement proceeds ; but if now the nerve be 

 cut, coordination ceases. So while tension is important in supplying 

 a stimulus to the nerve mechanism, it is not wholly sufficient. 



Biedermann (1904) showed that these reflexes can travel consider- 

 able distances in the cord. The interpretation of this might demand 

 that there be present in the nerve cord longer systems of neurones 

 than had been previously reported. However, it can be shown that 

 no such supposition is necessary. The present knowledge of th^ 

 neurones can be used to explain the facts at hand?; 



