204 



curve, the ventral outline is very sharply curved, or may even 

 form a cusp, thus reducing the ventral surface of the com- 

 missure to a minimum. 



Both dorsal and ventral surfaces of this ganglion present 

 longitudinal, shallow depressions, more conspicuous at the an- 

 terior than at the posterior end, and less pronounced above than 

 below. Further traces of the real nature of this double 

 (i-ano-lion are sometimes observed in the behavior of the main 

 nerve-trunk which arises from its posterior end, inasmuch as 

 this trunk may be reduced to zero by an early division into its 

 lateral halves. Thus the notch at the anterior end becomes 

 continuous with the dorsal and ventral furrows, and, through 

 them, with the posterior notch which separates these two nerve 

 stems. In this manner the consolidation of the lateral halves 

 of the ganglion often appears far from complete. The lateral 

 outlines of this ganglion ])ass, by a gentle curve, into those of 

 the main nerve. The latter sooner or later bifurcates, the two 

 branches diverge and undergo further sub-division, in their 

 course backward, to be distributed to the organs of the posterior 

 part of the abdomen. In addition to this main trunk there 

 arise from the lateral margins of the posterior ganglion otlier 

 nerves, which are not always symmetrically placed. As many 

 as four such nerves, at least, may arise from each side of this 

 ganglion. The commissures between this and the first sub- 

 oesophageal ganglion are short, rounded cords, which are 

 slightly flattened side-wise, and lie somewhat nearer the dorsal 

 than the ventral surfaces of the ganglia which they connect. 

 Their narrowest measurements indicate a thickness of about 

 one-half or one-third that of the ganglion just described, and a 

 breadth of one-fourth that of the same ganglion. As seen from 

 above, the opening embraced by the commissures and the gang- 

 lia which they join is an oval with its antero-posterior diameter 

 equal to the breadth of one of the commissures, and its trans- 

 verse diameter about half as long. 



The first suboesophageal pair presents a flattened, rounded 

 mass, in width one-third less than the posterior ganglion, 

 and of a length equal to its own breadth, and a thickness not 

 varying much from three-fourths of the same. The longitud- 



