ANATOMY OF ARACHNIDS 13 



PART II 

 THE GANGLIA OF THE ARACHNID 



If the cephalothorax of an arachnid such as the scorpion or 

 Thelyphonus be sectioned sagitally through the median line, the 

 gangUa composing the large suboesophageal ganglion can be 

 clearly distinguished, the suboesophageal ganglion being mapped 

 out into a number of neuromeres, separated from each other by 

 a small artery whose course can be very readily determined. 

 Each of these neuromeres represents one of the individual gan- 

 glia of which the whole is composed. In the same way also it 

 can sometimes be recognized that the abdominal ganglia are 

 composed of more than one individual ganglion. By enu- 

 merating the ganglia in this way it is determined that there are 

 eighteen ganglia in each of the four orders which go to form 

 group I of the arachnids: scorpions, spiders, Uropygi and Ambly- 

 pygi. One of these eighteen gaiiglia (the cheliceral) has moved 

 up and fused with the supraoesophageal ganglion; the other 

 seventeen being found in the suboesophageal ganglion and in 

 the abdomen. 



GROUP I. IV. SCORPIONS 



The nervous system of the scorpions is less concentrated than 

 that of the other arachnids. The suboesophageal ganglion con- 

 sists of nine neuromeres, the ninth neuromere supplying the 

 abdomen as far as the first lung (fig. 5). In the abdomen itself 

 there are three single ganglia supplj'ing lungs 2, 3 and 4. In 

 the post-abdomen are four ganglia, of which the last is double, 

 having two neuromeres. The entire ventral nerve chain, there- 

 fore, consists of seventeen ganglia which, with the cheliceral 

 ganglion, fused with the brain, would make eighteen. 



