THE NERVOUS FUNCTIONS 29 



of the body. Each of these ganglia is connected by means of 

 peripheral nerves with the skin and muscles of its own segment, 

 and each joint of the body with its contained ganglion (ventral 

 ganglion) has a certain measure of physiological independence so 

 that it can act as a unit. This is a typical segmented nervous 

 system. At the head end of the body the ventral ganglionic 

 chain divides around the pharynx and mouth, and there are 

 enlarged ganglia above and below the pharynx. The superior 

 ganglia (supra-esophageal ganglia) are sometimes called the 

 brain, and this organ dominates the local activities of the several 

 segments, enabling the animal to react as a whole to external 

 influences. 



The nervous systems of crustaceans (crabs and their allies), 

 spiders, and insects have been derived from the type just 

 described. In these animals the segments of the body are more 

 or less united in three groups, constituting respectively the head, 

 thorax, and abdomen, and the ganglia of the central nervous 

 system are modified in a characteristic way in each of these 

 regions. Figure 3 illustrates the nervous systems of four species 

 of flies, showing different degrees of concentration of the ganglia. 

 In all cases the head part (brain) is greatly enlarged, and is 

 arranged, as in worms, in ganglia above and below the mouth 

 and esophagus. The other ganglia are diversely arranged, from 

 the simple condition (A} where there are three thoracic ganglia, 

 one for each pair of legs, and six abdominal ganglia, through in- 

 termediate stages (B and (7), to the highest form (D), where all 

 of the ganglia of both thorax and abdomen are united in a single 

 thoracic mass. 



The type of nervous system just described is found throughout 

 the highest groups of invertebrate animals, as in insects and 

 spiders, and is constructed on a totally different plan from that 

 of all of the vertebrate or back-boned animals. In this latter 

 group we have, instead of a segmented chain of ventrally placed 

 solid ganglia, a hollow tube of nervous tissue which extends 

 along the back or dorsal wall of the body and constitutes the 

 spinal cord and brain. The cavity or lumen of this tube extends 

 throughout the entire length of the central nervous system, 

 forming the ventricles of the brain and the central canal of the 

 spinal cord. The details of the invertebrate nervous systems 



