298 



THE EARTHWORM AND OTHER ANNULATA 



segment is like that of every other. Upon first examination, the 

 ventral cord seems to be a single structure with right and left sides 

 like the spinal cord of a vertebrate animal. More careful study 

 shows, however, that it is really double, since it is formed by the 

 fusion of right and left cords on the median line. In some other 

 members of the phylum these cords are separated, so that the 

 ventral cord presents a ladder-like arrangement. As in other 

 invertebrate animals, the term ganglion is applied to a region having 

 a special collection of nerve cells, while connectives are parts that 

 unite regions that are consecutive longitudinally, and com- 

 missures are the unions between symmetrical parts of the two 



Fig. 144. — Ventral nerve cord of earthworm showing neurones of rellex arc. 



ep., epidermis or surface epithelium; g.c.2, adjuster neurone; g.c.2, motor or efferent 

 neurone; m.c, muscle cell (effector); s.c, sensory cell (receptor). 



sides. The commissures are represented in the earthworm by 

 the fusions that have occurred, but sections of the ventral cord 

 show the right and left divisions. Anteriorly, the double con- 

 dition becomes evident in the separation of the two sides to 

 form the circum-pharyngeal connectives and the two cerebral 

 ganglia. 



To understand the functions of the nervous system, it is neces- 

 sary to examine its cellular organization. In each segment, 

 the ventral cord contains numerous nerve cells, the neurones, 

 located principally in the ganglia (Fig. 144). In the epithelium 

 of the body surface there are sense-organs consisting of groups 



