THE NERVOUS SYSTEM 171 



with the discussion of the excretory and reproductive systems of 

 this form. Save for those at the anterior and posterior ends, the 

 segments of the earthworm are practically alike. Some light on 

 the significance of segmentation may be obtained from the 

 manner in which segments originate. The first step consists in 

 the constriction of the unsegmented embryo into anterior and 

 posterior parts, of which the latter represents the posterior seg- 

 ment of the fully developed worm. New segments are budded 

 from the posterior border of the anterior part until the full 

 number is formed, which in Lumbricis terrestris is between 140 

 and 150. If segmentation be regarded as a sort of imperfect 

 fission, a segmented animal really represents a string of individ- 

 uals united end to end, an idea that is born out by the fact that 

 each segment is provided with similar organs. Actually, of 

 course, the segments are parts of a whole in which the indi- 

 viduality or independence of the segments is lost or obscured. 

 That segmentation among annelids is a relatively primitive char- 

 acter is indicated by the fact that segments are strikingly similar. 

 In an insect, such as the grasshopper, the primitive segmentation 

 is obscured by a regrouping to form larger body divisions such as 

 head, thorax, and abdomen. The type of segmentation found in 

 annelids is called homonymous or homodynamous segmentation; 

 that in insects heteronymous segmentation. Within limits, the 

 greater the difference between the segments of the body of an 

 animal, the less independence is retained by individual segments. 

 Thus if an earthworm is cut in two through the middle, the 

 anterior half is capable of living by itself and is able to regenerate 

 some of the missing parts. If an insect is cut in two, both parts 



die. 



The nervous system of the earthworm consists of two general 

 regions, (1) a fused pair of relatively large superior ganglia 

 located in segment 3 on the dorsal side of the pharynx and, (2) 

 ventral ganglionated nerve cord extending backward from segment 

 4 through the length of the body (Fig. 112). The ventral nerve 

 cord really consists of a series of ganglia, one to a body segment, 

 joined together by a double nerve cord, partially fused. The 

 superior ganglia are connected with the anterior ganglion of the 

 ventral chain by a nerve cord on either side of the pharynx. 

 The pharynx at this point is thus encircled by a nervous ring. 

 The superior ganglion innervates the anterior three segments 



