834 



Pomona College Journal op Entomology 



I 



shown in the diaf^raiii, some of these at least seem to be confined to the 

 cephalothorax. 



Extending up from the great cephalothoracic ganglion is the supras- 

 sophageal mass which partly surrounds a portion of the digestive tract. Its 

 center is fibrous, nerve cells are above and on its sides. (Figures 261 and 263) . 

 The most conspicuous nerves from this portion are those of the ehelicenv and 

 the ocular branches. The first of these pairs is the larger. Each of this pair 

 divides into three branches, two of which are distributed to the head region 

 and the base of the chelicera, the other enters the chelicera and divides into 

 three, the lower of these goes in deeply and reaches nearly to the chitin and 

 seems to be largely a sensory branch. The upper is probably a muscular 

 branch and the middle one was traced up to near the tip of the appendage. 



In dissections and sections there was some evidence of visceral nerves 

 coming off from the suproesophageal ganglion. Other small nerves were also 

 found but from surface views and sections there was little indication of more 



Figure 262 



A, palpu.s; B, leg with the ceplialic side up from the right of a tarantula. B is 

 the first leg and it and the palpus is shown as opened up from above. Some ot 

 the muscles and chief nerves are shown, X2. 



than seven distinct parts of the nervous system; one for each pair of nerves. 

 The fusion of all parts was so complete in the adult as to almost obliterate 

 all subdivisions. However, there is some indication of segments in the adult 

 beyond the last leg division. 



In a general study of the structure of tlie nervous system intricate con- 

 nections were noted between all parts of the ganglionic mass, by means of 

 fibers connecting the seven easily recognized regions. Adjoining segments 

 were found fused by many fibers, more distant segments had po.ssibly fewer 

 communicating strands with the exception of the supru'sophageal, whicli 

 seems to have very extensive connections with all lower portions. The larger 

 nerve trunks are probably all mixed except the optic. 



The nerve cells of the cephalo-thoracic-abdominal ganglion are mostly 

 ventral in position. Those of the supra?so])liageal mass dor.sal and lateral. 

 As compared with insects there seem to be fewer very dcn.se masses of fibers 

 in the central portions of the nervous system. 



