THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 21 



and they lie entirely within this central organ. The slow waves of contraction 

 that pass from head to tail as the worm creeps forward may be advanced from 

 segment to segment by such internuncial or association elements. 



The nervous system of the earthworm differs from that of the ccelenterate 

 in many ways, but the fundamental difference is one of centralization. In the 

 former the greater part of it has separated from the skin and become con- 

 centrated in a series of interconnected ganglia which serve as a central nervvts 

 system. These ganglia receive nerve-fibers, coming from the sense organs, and 

 give off others, going to the muscles; and the fibers are brought together and 

 grouped into nerves for convenience of passage. The neuropil within a ganglion 

 offers a variety of pathways to each incoming impulse which may accordingly 

 find its \vay out along one or more of several motor fibers. The spreading of 

 nerve impulses through the chain of ganglia is facilitated by the presence of the 

 association fibers already mentioned. Nevertheless, conduction is not diffuse 

 as in the nerve net of the medusa, but occurs along definite and more or less 

 restricted lines. This is well illustrated by the experiment cited by Parker: 

 "If an earthworm that is creeping forward over a smooth surface is suddenly 

 cut in two near the middle, the anterior portion will move onward without much 

 disturbance, whereas the posterior part will wriggle as though in convulsions. 

 This reaction, which can be repeatedly obtained on even fragments of worms, 

 shows that a single cut involves a stimulation which in a posterior direction 

 gives rise to a wholly different form of response to what it does anteriorly; in 

 other words, transmission in the nerve cord of the worm is specialized as com- 

 pared with transmission in the nervous net of the ccelenterate." In the gan- 

 glionated cord of the earthworm, as here described, we find many of the features 

 characteristic of the central nervous system of higher forms. 



The vertebrate nervous system has much in common with that of the earth- 

 worm. The central nervous system of the annelid is split off from the ectoderm 

 by a process of delamination, as will be seen by comparing the ventral nervous 

 cord of the marine worm, Sigalion, with that of the earthworm (Figs. 3, 4). 

 Through a comparable process of infolding of the ectoderm to form a neural 

 tube there is developed the central nervous system of the vertebrate (Fig. 6). 

 The dorsal position of the neural tube in vertebrates as compared with the 

 ventral position of the solid nerve cord of the annelid offers some difficulty and 

 has led to ingenious theories in explanation of their phylogenetic relationship, 

 theories which we need not consider here (Gaskell, 1908). In primitive chor- 

 dates, such as the amphioxus, we already have a simple, dorsally placed, neural 



