172 



GENERAL ZOOLOGY 



and the prostomium, an incomplete segment overhanging the 

 mouth. Its large size is correlated with the higher development 

 of the sensory field at the anterior or head end of the animal as 

 compared with other portions of the body. The cephalic sensory 

 field in some annelids, such as Nereis, a marine polychaete worm, 

 is more highly developed by the presence of eyes, palps, etc. 

 (Fig. 242). The ventral nerve cord with its ganglionic enlarge- 

 ments is primarily a segmental structure, in which correlation 

 between segments is effected by longitudinal connections 

 between the ventral ganglia (Fig. 113). 



A pair of superior ganglia is a constant feature of the nervous 

 system of invertebrate animals provided with a ganglionic 

 nervous system. Where the segmentation is of the heteronymous 



Fig. 112. — The nervous system of earthworm, diagrammatic, c, nerve com- 

 missure encircling pharynx; i, inferior ganglion; s, superior ganglion. 



type, such as occurs in insects, the ganglia of the ventral chain 

 become reduced in number and varied in size, in accordance with 

 the extent of the modification of the primary segmentation. 

 Thus in the grasshopper the head contains a superior ganglion 

 (supraesophageal ganglion) and an infraesophageal ganglion, 

 connected by nerve cords on either side of the esophagus. The 

 infraesophageal ganglion is the anterior unit of the ventral nerve 

 cord which is provided with three ganglia in the thorax and five in 

 the abdomen (Fig. 53). The cord connecting these ganglia is 

 double in the thorax and single in the abdomen. The total 

 number of ganglia in the ventral chain is considerably less than 

 the number of primary body segments of the region supplied by 

 the ganglion. In such insects there is then a definite tendency 

 toward centralization of nervous control. In other arthropods, 

 such as certain crabs, this tendency is carried even farther, the 

 entire ventral chain being represented by a single large ganglion. 



