82 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



two-headed specimen. If the heads were far enough 

 apart, they no longer moved synchronically in the 



same direction, 

 in which case 

 the pulling in 

 opposite direc- 

 tions (Fig. 25) 

 was so strong 

 that the animal 

 was torn asun- 



FIG. 25. PLANARIAN WITH TWO HEADS THAT ARE , 

 ATTEMPTING TO MOVE IN OPPOSITE DIRECTIONS, 



AND IN SO DOING ARE TEARING THE COMMON ? Inthe Art- 



BODY. (After van Duyne.) nelids we find 



a segmental arrangement of the central nervous sys- 

 tem. This type of structure is also found in Arthro- 

 pods and in Vertebrates. It will perhaps make our 

 task easier if we conceive the segmented animal to be 

 a colony of as many individuals or animals as there 

 are segments (or ganglia) present in the body. Each 

 segment is then comparable to an Ascidian in which 

 the central nervous system consists of but one gan- 

 glion. The fibres and cells of each ganglion form 

 for the corresponding segment a protoplasmic bridge 

 between the skin and muscles. A stimulation be- 

 ginning, however, in one segment is not confined to 

 that segment, for the single ganglia of the various 

 segments are connected with each other by means of 

 nerve-fibres, the so called longitudinal commissures. 

 By means of these, a stimulation which originates in 

 one segment is transmitted also to the neighbouring 



