ORIGIN OF THE NERVOUS SYSTEM 



2 55 



organs. The middle and largest of these fibers extends almost the 

 whole length of the ventral cord and, according to Friedlander (1894), 

 has unquestionable connections with ganglion-cells. The two lateral 

 fibers, though smaller, have much the same extent as the median one 

 and are also directly connected with cells. Both sets of fibers connect 

 by branches with the neuropile of the successive segments. Thus the 

 ventral cord of the earthworm may be described as composed of three 

 long neurones and an immense number of overlapping short neurones. 

 This peculiarity in the structure of the cord makes itself manifest 

 in the movements of the worm. Undoubtedly the slow waves of mus- 

 cular activity that move over the worm from head to tail as it creeps 

 along are dependent upon the interlocked short neurones, whereas the 



Fig. 3. Transverse Section of the Ventral Nervous Cord of Segalion 

 (modified from Hatschek). ~bm, basement membrane; e, cuticula ; e, epidermis; gc, 

 ganglion-cells ; n, nerve-fibers and neuropile ; s, space occupied by vacuolated sup- 

 porting tissue. 



sudden drawing together of the worm as a whole, when it is vigorously 

 stimulated, is very probably the result of impulses spread through the 

 long neurones. 



The absence of degenerated fiber-tracts in the ventral cords of 

 earthworms that have been cut in two and the rapidity with which 

 nervous regeneration takes place in these worms are conditions that 

 very likely depend upon the almost entire formation of the cord from 

 systems of short neurones. 



At first sight the central nervous apparatus of the earthworm seems 

 to be widely different from the neuromuscular mechanism of the ccelen- 

 terates, but the difference in reality is not so pronounced. To begin 

 with, the whole nervous mechanism of the ccelenterate is within an 



